Abstract: Eight remarkably preserved specimens of ichthyosaurs from the lower Toarcian (Lower Jurassic) of Strawberry Bank (Ilminster, Somerset, England) are described fully for the first time. Whereas previously these ichthyosaurs were assigned to one species, Stenopterygius hauffianus, our study shows there are two, Stenopterygius triscissus and Hauffiopteryx typicus. S. triscissus is a small- to medium-sized ichthyosaur up to 3.5 m long, characterised by three apomorphies: long and slender rostrum, large elliptical supratemporal fenestra and bipartite pelvis. H. typicus is a small ichthyosaur up to 2.95 m long, with five apomorphies: short and extremely slender rostrum, very large orbit, small rounded supratemporal fenestra and tripartite pelvis, which is fused distally. Cladistic analysis is equivocal about their relationships, suggesting either that Hauffiopteryx and perhaps also Stenopterygius are members of a clade Eurhinosauria or that this clade does not exist, and both genera are members of a wider clade Thunnosauria. Further, the clade Stenopterygiidae, in which Hauffiopteryx had been located, is not identified. Most striking is that the specimens are all juveniles (five specimens) or infants (three specimens), ranging from one-tenth to one-half the normal adult length of the species.
T he Ichthyosauria are a diverse group of extinct diapsid reptiles of uncertain affinity (Motani 1999) that were adapted to active life in a marine habitat. Ichthyosaurs range in age from the Early Triassic to the Late Cretaceous (Motani 2005) and were most diverse in the Early Jurassic (Motani 1999; Sander 2000), with 24 valid species (McGowan and Motani 2003). Early Jurassic species were prolific in the Lower Lias of England and in the Lower and Upper Lias of continental Europe, but Upper Lias ichthyosaurs are rare in England, being known only from the Whitby region in Yorkshire (Benton and Taylor 1984; McGowan and Motani 2003), with four species: Eurhinosaurus longirostris, Temnodontosaurus crassimanus, T. acutirostris and Stenopterygius longifrons.
Lower Jurassic ichthyosaurian taxonomy is problematic. Taxa are hard to distinguish, perhaps because their shared adaptations to underwater life caused convergence. Numerous ambiguous species names have been proposed for fragmentary or inadequate material, and diagnostic characters of the species are sometimes poorly defined (McGowan and Motani 2003). Furthermore, species may vary as a result of sexual dimorphism, ontogenetic stage, individual and geographical variation, evolution, unusual preservation and pathological abnormalities (McGowan and Motani 2003).
Here, we describe the ichthyosaurs of Strawberry Bank, Ilminster, Somerset, UK (Text-fig. S1). These fossils are part of the Charles Moore collection, housed at the Bath Royal Literary and Scientific Institution, which comprises an assemblage of exceptionally preserved ichthyosaurs, marine crocodilians, fishes, ammonites, belemnites, teuthoids, crustaceans and insects (Moore 1866; Hallam 1967). There are eight ichthyosaur skeletons preserved in three dimensions, some with soft tissues, and they represent individuals at various stages of maturity. These remarkable specimens have only lately been ‘rediscovered’, following an account of the small metriorhynchid crocodilian Pelagosaurus in the same collection (Pierce and Benton 2006).
The Strawberry Bank ichthyosaurs were announced by Moore (1866), who named them all Ichthyosaurus acutirostris, but they were subsequently reidentified by McGowan (1978) as Stenopterygius hauffianus. These papers, however, do not provide detailed anatomical accounts and do not include all material. Our aims are to (1) describe and illustrate the material, (2) determine whether one or more species are present, (3) determine whether one or more may be classified as Stenopterygius hauffianus or whether they belong to another taxon and (4) seek to understand the role of the ichthyosaurs in the Strawberry Bank ecosystem.
Institutional abbreviations. BRLSI, Bath Royal Literary and Scientific Institution, Bath, UK; GPIT, Geologisch-Paläontologische Institut und Museum, Universität Tübingen, Germany; MHH, Museum Hauff, Holzmaden, Germany; MNHNL, Muséum National d’Histoire Naturelle, Luxembourg; NHMUK, Natural History Museum, London, UK; NMO, Naturmuseum Olten, Switzerland; SMNS, Staatliches Museum für Naturkunde Stuttgart, Germany.
Anatomical abbreviations. 2, 3, 4, distal carpals/tarsals; ii, iii, iv, v, metacarpals/metatarsals; a, angular; ap, aperture; at, atlas; at–ax, atlas–axis complex; ax, axis; bo, basioccipital; bo-pg, basioccipital peg; ca.v, caudal vertebrae, neural spines and ribs; cl, clavicle; cn, condyle; co, coracoid; co.l, left coracoid; co.r, right coracoid; c.r, cervical bicipital rib fragment (proximal end); c.v, cervical neural spines and ribs; d, dentary; dp, diapophysis; d.r, dorsal ribs; d.v, dorsal vertebrae, neural spines and ribs; eco, extracondylar area; eo, exoccipital; exn, external naris; f, frontal; fe, femur; fi, fibula; fb, fibulare; gl, glenoid; gl.c, glenoid contribution; h, humerus; hd, humurus head; i, intermedium; icl, interclavicle; ico.f, intercoracoid facet; is, ischium; j, jugal; l, lacrimal; m, maxilla; ma, mandible; n, nasal; na, neural arch; n.a, anterior notch; na.f, facet for neural arch; nc.fl, floor of neural canal; no, notch; ns, neural spine; o, obturator foramen; op, opisthotic; p, parietal; pb, pubis; p.f, parietal foramen; pm, premaxilla; po, postorbital; pp, parapophysis; prf, prefrontal; ps, pisiform; pt, pterygoid; ptf, postfrontal; pv, pelvis; pv.l, left pelvis; pv.r, right pelvis; q, quadrate; qj, quadratojugal; r, radius; ra, radiale; sa, surangular; sc, scapula; sc.f, scapular facet; sc.pl, sclerotic plate; sc.r, sclerotic ring; s.f, single rib facet; so, supraoccipital; sp, splenial; sq, squamosal; st, supratemporal; st.f, supratemporal fenestra; stp, stapes; tb, tibiale; ti, tibia; u, ulna; ul, ulnare; zp.a, anterior zygapophyses; zp.p, posterior zygapophyses; ?, tentative assignment.
During the early Toarcian, north-western Europe was covered by an extensive epeiric sea, and this shelf zone protected the region from strong tides or storm influences (Benton and Spencer 1995; Bailey et al. 2003). England was close to the tropics, and Ilminster was 40–50 km west of the London–Brabant platform, an island that extended from western Germany to southern England. The Strawberry Bank fauna is almost entirely marine, but the plant and insect fossils suggest that land was close. Moore (1866, pp. 174) reported this suggestion ‘that the saurian and fish bed was deposited in an estuary there can be little doubt, since thousands of insects of this remote age were driven from the adjoining land, and settling upon the muddy surface, left by the ebbing tide of the surrounding ocean, were entombed, and their remains preserved through succeeding ages to this time, and with them are occasional fruits and vegetables which point to the same conclusion’.
The Strawberry Bank locality is now inaccessible, so an account of taphonomy can only be reconstructed from specimens and historical accounts (Duffin 1978, 1979). The Ilminster fossil bed yielded well-preserved and diverse vertebrate and invertebrate fossils in a horizon a few centimetres thick. The ichthyosaur skeletons are generally articulated, with varying degrees of disarticulation of the skull and girdles. These probably became detached following microbial scavenging and storm activity, which might also explain the absence of other skeletal elements, particularly the tail, snout tip, and distal paddle elements (cf. Martill 1987, 1993). The bones show a range of preservation quality, from immaculate with striations and capillary canals to poorly preserved eroded surfaces (although some surface damage might have been produced by crude hammer-and-chisel preparation work in the nineteenth century). In the fish specimens, the scales and fin spines are in pristine condition, with surface texture and lustre still preserved. A greyish to black amorphous material adheres to and outlines areas of the ichthyosaur skeletons and may represent the remains of soft tissue.
The high degree of articulation of the ichthyosaur skeletons suggests that (1) post-mortem drifting was minimal and individuals died where they lived; otherwise, bones would have been dispersed by the activity of scavengers, predators and/or currents; (2) carcasses reached the sea floor soon after death, prior to the onset of decay; (3) once on the sea floor, carcasses were rapidly buried in sediment and/or sank completely into the soupy bottom muds; and (4) after initial burial, the sea bed conditions represented a low-energy setting. These observations suggest that the Strawberry Bank deposit is an in situ accumulation (Konservat Lagerstätte) rather than a site of concentration of skeletons from a wider area (Konzentrat Lagerstätte).
Burial was probably fast, as suggested by the absence of encrusters and burrowers on the bone surfaces (Martill 1987, 1993). In addition, the pristine condition of the scales and tail spines of the fish specimens also indicates rapid burial. The carbonate concretions around the skeletons formed relatively early before sediment compaction, as the original sediment layering is preserved. These concretions provided protection against compression from overburden pressures during diagenesis and thus preserved the fossils in three dimensions.
Materials and methods
All specimens are preserved in three dimensions and enclosed in limestone concretions. Small specimens (BRLSI M1403–M1408) are in a single concretion, and larger individuals (BRLSI M1399–M1401, M1409) in four or more (17 for the largest skeleton). The materials are summarised in Table 1, and fuller details given in the Electronic Supplement.
Table 1. Listing of the eight ichthyosaurs from Strawberry Bank, Ilminster.
Details of main elements preserved, illustration, and estimated body length are given; fuller information is available in the Supplementary file.
BRLSI M1399. Articulated juvenile skull and postcranium that is incomplete beyond vertebra 51, lacking the rostrum tip, distal flipper elements and tail, preserved on its left side, with the underside of the pectoral girdle and right-hand dorsal ribs visible (Text-fig. 9). Estimated total length is 1.08 m
BRLSI M1400. An articulated juvenile postcranium that is incomplete beyond vertebra 62, with some disarticulated skull elements (Text-fig. 10). It lacks the skull and most of the pectoral girdle, pelvic girdle and tail. Estimated total length is 1.33 m (postcranial length, c. 900 mm)
BRLSI M1401. An articulated juvenile skull and postcranium that is incomplete beyond vertebra 77, lacking the anterior part of the rostrum, the pelvic girdle, coracoids, interclavicles, distal flipper elements and tail (Text-fig. 11). Total length of vertebral column, 1595 mm
BRLSI M1403, M1404, M1406. An articulated partial skull consisting of two sclerotic rings supported by a limestone cast of the eyeball and the posterior section of an articulated juvenile skull roof exposed ventrally (Text-fig. 1A–G)
BRLSI M1405. An articulated infant with left laterally exposed postcranium, with a total length of 267 mm (Text-fig. 2A, B)
BRLSI M1407. An articulated infant pectoral girdle in ventral view (Text-fig. 2C, D), including both humeri, a fragment of right radius, left radius, ulna, ulnare, metacarpal V, distal carpal 4 and a phalanx
BRLSI M1408. An articulated infant with a posterolaterally compressed skull that is preserved in three dimensions and the right laterally exposed postcranium, which is incomplete beyond vertebra 51 (Text-fig. 3A, B). The skull is 165 mm long and vertebral column is 367 mm in length, giving an estimated total length of 0.53 m
BRLSI M1409. An articulated juvenile with an undistorted skull that is preserved three dimensionally with its postcranium, which is incomplete beyond vertebra 71 and is visible in left lateral view (Text-fig. 3C, D). This specimen lacks most of the snout, its distal flipper elements and tail. The skull is 171 mm long with a vertebral column length of 725 mm, giving an estimated total length of 0.9 m
Charles Moore presumably carried out preparation of the specimens when they were found, and there is no documentation to suggest any further work. Mechanical preparation of two skulls was carried out at Bristol University: in BRLSI M1399, the right side of the posterodorsal skull roof was revealed, and in BRLSI M1409, matrix was removed from the posterodorsal and left lateral region behind the skull.
Measurements and terminology
Measurements were made to the nearest 1 mm for those over 160 mm and to the nearest 0.1 mm for those below. Skull length is measured from the tip of the snout to the posterior end of the quadrate. In most cases, the length of the axial skeleton was measured along the vertebral column, but in BRLSI M1399, the measurements follow the curvature of the articulated neural spines, because the vertebrae are concealed in matrix.
Anatomical terminology follows McGowan and Motani (2003). In ichthyosaurs, presacral vertebrae are counted as those anterior to the ilium and caudals as those behind. The first sacral centrum is identified by fusion of the diapophysis and parapophysis into a common ventrolateral facet (McGowan and Motani 2003). The pelvic girdle is preserved only in BRLSI M1399, M1409 and M1405: in the others, the estimated position of the caudals is somewhat arbitrary. However, it corresponds to the position at which Massare et al. (2006) noted a morphological transition in Ophthalmosaurus, with a change in centrum dimensions, and the position where the two articulations merge into a single facet on the centrum.
Taxonomic assignments of ichthyosaurs can rely on comparative ratios of the lengths of various skull elements (McGowan 1974, 1976, 1986). However, the Strawberry Bank specimens lack many key skull elements, and the orbital length and snout length ratios can be obtained only from specimens BRLSI M1399 and M1401 (Table S1). Other ratios are hard to establish: McGowan (1978) provided prenarial length and premaxillary length ratios of BRLSI M1401, but the specimens are too incomplete to be sure.
Holotype. GPIT 12/0224-2, a nearly complete skeleton of a young adult, 2.19 m in length.
Geographical distribution. The area around Holzmaden, Frittlingen, Schömberg, Württemberg, Germany; Altdorf, Switzerland; Curcy, Normandy; Dudelange, Bascharage, Esch-sur-Alzette, Luxembourg; Whitby and Ilminster, England (McGowan and Motani 2003).
Horizon. All ichthyosaur material is contained within limestone concretions that were collected from a 102- to 140-mm-thick laminated limestone band of the Junction bed (‘Saurian and fish’ bed: Moore 1866), Harpoceras falciferum Zone, lower Toarcian, Lower Jurassic (Moore 1866; Duffin 1978, 1979; Pierce and Benton 2006).
Locality. Strawberry Bank quarry (now built over), north of the High Street, in an area called the Triangle, Ilminster, Somerset, England (National Grid Reference ST 361148: Duffin 1979).
Referred specimens. BRLSI M1405, BRLSI M1407, BRLSI M1408 and BRLSI M1409 (Text-figs 1–7).
Diagnosis. Small to medium-sized ichthyosaur, maximum total length approximately 3.5 m; 44 presacral vertebrae; usually <85 preflexural vertebrae, but >80. Possesses seven apomorphies diagnosing Stenopterygius (Maisch and Matzke 2000; McGowan and Motani 2003): moderately large orbit; elongate elliptical supratemporal fenestra; four primary manus digits and one postaxial accessory digit; forefin with two notches in the first digit; hindfins greatly reduced, being half the size of the forefins; bipartite pelvis consisting of fused pubis and ischium; and atlas and axis fused into a single massive element. Possesses apomorphies of S. triscissus: frontals very narrow; nasals reach the parietal; postfrontal and frontal not in contact; slender and elongate snout (always greater than two-thirds of total jaw and skull length).
Lectotype. GPIT 1491/4, a complete skeleton with a soft tissue outline (Text-fig. S2).
Geographical distribution. Holzmaden, Baden-Württemberg, Germany; Dudelange, Luxembourg; Ilminster, England (McGowan and Motani 2003).
Stratigraphic range, horizon and locality. Same as Stenopterygius triscissus (see above).
Referred specimens. BRLSI M1399, BRLSI M1400, BRLSI M1401, BRLSI M1403, BRLSI M1404 and BRLSI M1406 (Text-figs 8–16).
Diagnosis. Small-sized ichthyosaur, maximum total length approximately 2.95 m; skull in immature individuals is small in comparison with preflexural length; <46 presacral vertebrae; preflexural vertebrae usually <39, but >34. Shares 14 apomorphies with German Hauffiopteryx: short snout; small overbite; anterior end of snout extremely delicate and pointed; short nasals that do not contact parietals; broad frontals that largely enclose parietal foramen; large orbit; cheek region narrow and faces backward; basioccipital has extensive extracondylar surface (Godefroit 1994); teeth decrease in size anteriorly; four-fingered forefins with five notches; tripartite pelvic girdle with pubis and ischium fused distally; femur slender with well-developed posterior facet; and tibia significantly smaller than fibula; and three primary hindfin digits with three notches.
Identification of the Ilminster ichthyosaurs
There are two morphological groups among the Strawberry Bank ichthyosaurs, not one, as previously assumed (e.g. Moore 1866; Duffin 1978; McGowan 1978). The skulls of BRLSI M1408 and M1409 share a large, elliptical temporal fenestra, which differs from the skulls of BRLSI M1399, M1401, and M1406 where the fenestra is significantly smaller. The coracoids of BRLSI M1408 and M1407 are similar in form. The bipartite pelvic girdle in BRLSI M1409 matches that of BRLSI M1405, but differs from the tripartite pelvis in BRLSI M1399. Specimen BRLSI M1400 is united with BRLSI M1399 and M1401 by similarities in the forefin elements, scapula morphology and presacral count. Comparison with material from England and Germany shows that these two forms are (1) Stenopterygius triscissus (BRLSI M1405, M1407, M1408, M1409) and (2) Hauffiopteryx typicus (BRLSI M1399, M1400, M1401, M1403, M1404, M1406).
Specimens of the first group show all seven synapomorphies of Stenopterygius (Maisch and Matzke 2000; McGowan and Motani 2003; see above). Further, the large, triangular and laterally exposed squamosal that is excluded from the supratemporal fenestra by the supratemporal is also a characteristic of Stenopterygius. The estimated presacral vertebral count of 44 is within the observed range for Stenopterygius (44–46: Maisch and Matzke 2000). The Ilminster material is closest to S. triscissus, sharing several apomorphies (see ‘Diagnosis’ above).
The second Ilminster taxon is probably Hauffiopteryx. This assignment is problematic because the specimens also share features with Leptonectes, including (Maisch and Matzke 2000; McGowan and Motani 2003; Maisch 2008): small temporal fenestra; large orbit relative to skull length; surface enamel of teeth unornamented; delicate snout; small overbite; forefin with four digits; hindfin less than two-thirds length of forefin; and humerus widely expanded distally. The Ilminster specimens resemble L. tenuirostris in particular (see ‘Discussion’ below), but the skull, forefin, hindfin, and pelvis morphology indicate assignment to Hauffiopteryx. The skull shares five features with German Hauffiopteryx specimens (MHH ‘9’, Lectotype GPIT 1491/4; Text-fig. S2A–D): short nasals that do not contact parietals; broad frontals that largely enclose parietal foramen; short snout; small overbite; and large orbit. In the Ilminster skull, the prefrontal extends posteriorly between the postfrontal and frontals, a feature not seen in the German specimens, but the difference could result from compression of the skull. Both the Ilminster material and German Hauffiopteryx share a presacral count of 46. The flippers are remarkably similar to those of Hauffiopteryx (GPIT 1491/4; Text-fig. S2C, D), in possessing four-fingered forefins with five notches, and three primary hindfin digits with three notches. In two Hauffiopteryx specimens (GPIT 1491/4; SMNS 51552), a preaxial accessory digit is located posterior to the third hindfin toe, but this is not seen in the Ilminster material. The unusual structure of the Ilminster pelvis matches Hauffiopteryx (SMNS 81962; Text-fig. S2E) perfectly. The present material shows further features of Hauffiopteryx (Maisch 2008; see ‘Diagnosis’ above).
Description of Stenopterygius triscissus
Skull. The description of the skull is based mainly on BRLSI M1409 (Text-fig. 3), with supplementary comments from BRLSI M1408 (Text-fig. 2A, B).
The premaxilla is an elongate rectangular bone, dividing posteriorly around the anterior margin of the external narial fossa. The supranarial process meets the nasal, which separates it from the narial margin, and the subnarial process borders the naris and meets the lacrimal posteriorly, also contacting the maxilla along its posteroventral margin.
The maxilla is a low, elongate triangular element that is overlapped for half its length by the premaxilla, along a straight suture. The dorsal apex of the maxilla has a short contact with the lacrimal, while the posterior contacts with the lacrimal and jugal are unclear because of poor preservation. Posteriorly, the broken anterior process of the jugal overlaps the maxilla. The ventral maxillary border is almost straight and is inclined posteroventrally.
The paired nasals are narrow elements that form the roof of the snout, terminating in at mid-orbit length. The ventral and dorsal margins converge anteriorly, forming a narrow strip with the anteriormost tip missing. The ventral margins are straight anteriorly and curve round the dorsal margin of the external naris. Posteriorly, the nasal overlaps the prefrontal then becomes constricted, extending into a long posterior process that just touches the parietal and forms a serrated suture with the frontal.
The lacrimal is triradiate, forming the posteroventral margin of the external naris and the anterior margin of the orbit. It sends a narrow anterior process beneath the premaxilla and a broader posterior process that meets the prefrontal and nasal at mid-orbital height.
The prefrontal is a crescentic, slender and sculpted element that forms the anterodorsal orbital margin as a thin ridge that is steeply inclined anteriorly, and which flattens out posteriorly. The anterior margin is overlapped by the nasal, with the lacrimal constricted between. On the skull roof, the medial margin is overlapped medially by the nasal for its entire length, the posterior tip contacts the parietal, and the posterior region is substantially overlapped by the postfrontal.
The jugal is an elongate, slender strut of bone that forms the ventral margin of the orbit. The anterior contact with the maxilla is not preserved, and the element flares laterally and meets the postorbital, terminating in a slender strip of bone that overlaps the quadratojugal.
The postorbital is a wide crescentic bone that forms the entire posterior orbital margin. The postorbital contacts the jugal, quadratojugal and squamosal and makes a point contact with the postfrontal dorsally.
The postfrontal is an elongate narrow bone forming the posterodorsal orbital margin. It overlaps the prefrontal anteriorly, is bounded by the nasal medially and overlaps the parietal. The posterior process curves around the supratemporal fenestra and overlaps the supratemporal.
The frontal is a slender rectangular plate that bulges upwards and is overlapped by all surrounding elements. It has three radiating processes: the anterior process extends between the nasals, the posterior process is narrow and meets the parietal, and the posteromedial process is short and forms the anterior margin of the parietal foramen. The parietal foramen (Text-fig. 3C, D) is a small teardrop-shaped depression that is widest anteriorly and tapers back to a point.
The paired parietals form a broad, rectangular plate in the posterior portion of the skull roof and enter the occiput, with the highest point in the midline. The parietal meets the prefrontal, is bounded by nasals and postfrontal anteriorly and forms the medial margin of the supratemporal fenestra, meeting the supratemporal posteriorly. The medial margins are elevated and curve round the parietal foramen to meet the frontals.
The supratemporal is a large, sculpted element with three processes. The medial process is broad and borders the supratemporal fenestra and overlaps the parietal anteriorly. On the lateral surface thin striations diverge anteriorly. The lateral process contacts the dorsal margin of the squamosal in a complex suture, overlapping it anteriorly, and lying beneath it posteriorly. Two short, conical posterior processes descend into the occiput, but do not contact other elements.
The squamosal is a thin, triangular sheet of bone with two processes. The anterodorsal process overlaps the postorbital and supratemporal, but is overlapped by the supratemporal. The posteroventral process overlaps the postorbital and quadratojugal. The lateral surface has fine striations radiating from a posterior point.
The quadratojugal is an elongate rectangular bone with an anteroventral ‘V’-shaped process. In BRLSI M1408, the quadratojugal is compressed into a broader element. It borders and is overlapped by the posterior portion of the postorbital. Dorsally, it contacts the overlapping squamosal, and ventrally, the quadrate along an oblique suture.
The quadrate is a slender, curved bone with curved lateral and medial margins, and it is set back from most of the occiput. In lateral view, the quadrate appears almost rectangular, but with concave anterior and convex posterior margins. The dorsal margin is straight and overlain by the quadratojugal. The mandibular condyle is positioned ventrally, but is covered by matrix and the mandible.
The left stapes in BRLSI M1408 has drifted from its normal position below and behind the paraoccipital process to lie isolated between the quadrate and first cervical vertebra. The stapes is a robust element composed of a massive head with a rugose surface and stout rod-like shaft.
The sclerotic ring is almost circular and preserved in articulation. The internal aperture is widest in the anterodorsal–posteroventral direction. There are 16–17 sclerotic plates with individual plates overlapping each other without any pattern. They show striations that extend from a point close to the external margin of the sclerotic ring, widening towards the inner margin. The peripheral surface is smooth with no striations.
Mandible and dentition. The mandible (Text-fig. 3A–B) is a straight, narrow element that curves upwards below the cheek region and has a long retroarticular process. All elements may be seen in lateral view, except the articular, but the medial views of the dentary, splenial and angular, and the entire prearticular are concealed by matrix.
The dentary is a broad, elongate bone with straight margins. It forms about one-third of the mandible and is widest anteriorly, tapering to a point posteriorly where it terminates at mid-orbital level. The anterior section is absent. The ventral margin slopes obliquely, overlapping the splenial, angular and surangular. The surface is finely striated.
The angular is a thin, elongate bone with straight margins. It is overlapped by the dentary, tapering far forwards in the floor of the Meckelian canal. The angular is overlapped above by the surangular for its entire length, widening and curving up in the retroarticular process.
The surangular is elongate and slender but wider than the angular. It tapers anteriorly to a narrow strip that underlies the dentary, and the posterior margin is curved, giving the mandible a convex profile. Ventrally, the surangular curves round beneath the articular and forms the bulk of the retroarticular process.
The splenial is thin and elongate in lateral view, forming the ventral margin of the lower jaw, in contact mainly with the dentary, and then the angular further back.
The dentition consists of numerous closely packed teeth, filling the entire length of the jaw, and terminating at a level posterior to the anterior orbital margin. Most tooth surfaces are not preserved: where present, the enamel is smooth with no visible ornamentation. All teeth are slender, pointed and elongate (maximum length of 15 mm), ranging from straight to recurved.
Axial skeleton. The postcranial skeleton comprises 44 presacral and 27 caudal vertebrae, with a total vertebral count of 71 and a maximum column length of 725 mm.
There are seven cervical vertebrae (Text-fig. 2). The centra are flattened dorsally and extended ventrally, and the articular surfaces are heart-shaped and with rounded edges. The atlas and axis (Text-fig. 4A–B) are fused, but the line of fusion is visible ventrally. The atlas is larger than the other cervicals, and the neural spine is a tall, slender, rectangular plate that is inclined slightly posteriorly. The anterior cervical neural spines are shorter and broader than those of cervicals 5–7. The diapophyses and parapophyses are poorly defined, appearing as a single bony swelling on the lateral surface of the centra, merging with the facet for the neural arch. The neural canal is a square midline cavity framed by the neural arch.
There are 38 dorsal vertebrae (Text-fig. 2). The margins of the centra are more rounded than in the cervicals, and widths and heights are almost equal. Centrum diameter increases gradually to presacral 38, after which the diameter increases more rapidly and then gradually decreases after 44. The neural spine is a tall, slender, rectangular plate that is inclined slightly posteriorly. The anterior dorsal neural spines have robust zygapophyses. Further back, the neural spines become broader and square-shaped, and then irregular polygons (Text-fig. 4C–D) with less well-developed zygapophyses just before the pelvic area. The anterior dorsals have well-separated diapophyses and parapophyses close to the anterior margin of the centrum. Along the trunk, the diapophysis moves ventrally and fuses with the parapophysis at vertebra 45, the last dorsal or first sacral (Text-fig. 4C–D). Unicipital ribs replace bicipital ribs from vertebra 46. Ribs are long and slender and curve posteroventrally. Anterior dorsal ribs are thickest and their length increases posteriorly and then decreases from dorsal 35. The broad proximal end of the anterior ribs is ‘Y’-shaped and consists of two widely separated facets: the anterior is narrower and longer, the posterior somewhat compressed and thicker. Ventrally, the ribs are more circular and thinner.
The exact number of caudal vertebrae is unknown: only 27 are preserved (Text-fig. 2C–D). Posteriorly, the centra become progressively more rounded and deeper than wide. Centrum width reduces dramatically, while centrum height decreases more moderately, producing laterally compressed vertebrae. The anteriormost caudal neural spines have irregular polygonal outlines, and further posteriorly these reduce to posteriorly inclined rod-shaped spines. Caudal vertebrae have only one ventral rounded rib articulation. Ribs are unicipital and much shorter than dorsal ribs. Anterior caudal ribs are short and broad with flared proximal and distal ends, while posterior caudal ribs are not preserved.
Shoulder girdle and forelimb. The scapula (Text-fig. 5) is a short, robust element consisting of a strap-like shaft and flared ends. The ventral end is curved and broadly expanded, while the dorsal end is only slightly flared. The proximal contact with the coracoid is extensive. The shaft is narrow with straight parallel sides. The scapula is slightly smaller than the clavicle.
The coracoid (Text-figs 1C–D, 6) is a broad plate-like, fan-shaped element, with a prominent anterior emargination. Anteriorly, the scapular facet is thick, with a rugose texture. The wide medial edge is overlain by the interclavicle. The anterior and posterior margins are triangular in outline and of similar width. The dorsal and ventral surfaces bear fine striations radiating towards the edges.
The interclavicle (Text-fig. 1C–D) is ‘T’-shaped. The anterior processes narrow laterally, terminating in a blunt point below the clavicle. The posterior margins are gently concave and the anterior margin is gently convex. The posterior process is an elongate, wide strip of bone that widens slightly distally, before converging and terminating in a blunt point. The ventral surface is elevated, with height increasing towards the lateral ends of the anterior bar.
The clavicle (Text-figs 1C–D, 5) is a robust, curved element. The medial portion is broad and rectangular, and the lateral portion is a slender strip that narrows to a point. The anterior and posterior margins extend parallel to each other. In BRLSI M1409 (Text-fig. 5A–B), the lateral process overlies the lateral margin of the scapula. The clavicle narrows posteriorly and terminates in a blunt point.
The humerus (Text-fig. 6) has a short, narrow shaft that is concave fore and aft, widens distally and is longer than broad. The proximal humeral articular head extends back into the broad deltopectoral crest, which is a low ridge extending halfway along the shaft. The dorsal surface lies close to the anterior margin just below the humeral head; it slopes towards the anterior margin and reduces in height distally. Distally, the radial and ulnar facets are subequal.
The radius (Text-fig. 6) is a hexagonal element that is broader than long and contacts the ulna for its entire length. The proximal end is slightly wider than the distal. There is an emargination halfway down the anterior margin, but in BRLSI M1407, the notch is missing from both radius and radiale (Text-fig. 6C–D). The proximal articular facet matches the radial facet of the humerus. The two distal facets of the radius, for radiale and intermedium, meet at an obtuse angle.
The ulna (Text-fig. 6) is of similar size to the radius and is hexagonal in outline. The proximal end is markedly wider than the distal and bears a prominent humeral articular facet. The two distal facets of the ulna, for intermedium and ulnare, meet at an obtuse angle.
Of the forearm mesopodials (Text-fig. 6), the radiale is hexagonal, with a notch at the midpoint of the anterior margin. The symmetrical and pentagonal intermedium lies between the distal facets of radius and ulna, and the pentagonal ulnare lies distal to the ulna. The pisiform, lying close to ulna and ulnare, has straight proximal, anterior and distal borders, and a rounded posterior border. Three distal carpals (2–4) support the first three digits. They are trapezoidal, with their long axes at right angles to the long axis of the forefin. Distal carpals 2–3 are marginally larger than 4, but all are of similar diameter to the proximal carpals.
The forefin (Text-fig. 6) has four primary digits and one postaxial accessory digit, with two narrow notches along the leading edge. Notches are absent in the infant forefin (Text-fig. 1C–D), but this is not unusual. In some species, the development of notching can occur at a later ontogenetic stage, but this applies only if notching is characteristic of mature individuals (Johnson 1977). The paddle elements are closely packed and polygonal in the proximal part of the paddle. Metacarpal II is pentagonal, metacarpals III and IV trapezoidal and metacarpal V elliptical with its long axis parallel to the forefin. Metacarpal V is smaller than the primary digits and contacts the ulnare and lies behind distal carpal 4. The phalanges are also polygonal: distally, they reduce in size and become more rounded or subrectangular. The postaxial accessory digit is composed of four discoidal elements and lies posterior to the third element on the fifth digit.
Pelvis and hindlimb. The pelvis (Text-fig. 7A–B) is a tiny bipartite element, an ischiopubic complex with the distal end more than twice as broad as the proximal. The proximal pelvic area is rectangular and robust, with convex anterior and posterior margins and no trace of original separation, except for the obturator foramen, an elliptical narrow hole near the middle of the proximal pelvic plate. The ischium is the larger of the two elements, and the pubis is higher than the ischium, giving the proximal end a posterior sloping profile.
The hindfin (Text-fig. 7C–D) has three digits with three notches along the anterior margin. The hindfin is half the length of the forefin.
The femur has a short and narrow shaft, widened distally, and is longer than it is wide. The convex proximal surface is rugose and slopes gently posteriorly. The anterior and posterior margins are concave in profile. The dorsal process slopes towards the anterior margin and reduces in height distally, terminating just before midshaft. Distally, the concave tibial and fibular facets are subequal.
The tibia is a pentagonal element, wider than long, and with a broad articular facect for the femur. The anterior margin has a notch midway along its length. The distal intermedium and tibiale facets meet at an obtuse angle. Lying beside the tibia is the fibula, a hexagonal element, slightly wider than long, slightly larger than the tibia and with a broad contact with the femur. The distal intermedium and fibulare facets meet at an obtuse angle.
Of the hindlimb mesopodials, the tibiale is smaller than the fibulare and intermedium. It is a notched bean-shaped element that contacts the tibia proximally and distal tarsal 2 distally. The pentagonal intermedium, the largest of the three elements, lies between fibula and tibia and contacts distal tarsal 3 distally. The square fibulare, with subrounded corners and a notch at the midpoint of the posterior margin, is slightly wider than long and contacts the fibula proximally and distal tarsal 4 distally.
The three distal tarsals support the posterior three digits. They are pentagonal, with the long axes at right angles to the hindfin. Distal tarsal 4 is similar in size to distal tarsal 3 and lies posterior to it. Proximally, distal tarsals 3 and 4 contact the intermedium and fibulare, respectively. In BRLSI M1409, distal tarsal 2 is notched and contacts the tibiale proximally and distal tarsal 3 posteriorly.
Description of Hauffiopteryx typicus
Skull. The description of the skull (Text-fig. 12) is based mainly on BRLSI M1399, with supplementary comments from BRLSI M1401. The dorsal surface of the skull is based mainly on BRLSI M1401, with supplementary comments from BRLSI M1399. The description is abbreviated where features are the same as in Stenopterygius triscissus.
The premaxilla is a low, elongate and slender bone that narrows anteriorly. Approximately 5 mm is missing from its anterior tip, and it is incomplete posteriorly. The maxilla is a low, elongate triangular element that forms the entire ventral margin of the naris and contacts the lacrimal and jugal posteriorly.
The nasals are long, narrow elements whose margins converge anteriorly, forming a narrow strip, but the anteriormost tip is missing. The ventral margin is straight anteriorly and curves back round the external naris, and there is a lateral protruding lobe here in BRLSI M1401 that matches a lobe on the dorsal margin of the maxilla. Behind the naris, the nasal runs back to a ‘V’-shaped point between the prefrontal and frontal. The nasal-frontal suture is serrated and terminates at the midline.
The lacrimal is ‘T’-shaped and extends from the external naris to the orbit. It meets maxilla, prefrontal and jugal. The prefrontal is a broad, crescentic, vertical element, with two surfaces separated by a thin pronounced laterally directed ridge between the snout and the orbital margin. It enters the posterior narial margin and contacts lacrimal and nasal, running back between the postfrontal and frontal to touch the parietal and postorbital. The jugal is an elongate, slender element in the ventral border of the orbit that contacts the lacrimal, postorbital and, medially, the pterygoid. In BRLSI M1401, the anterior spur lies next to, but does not contact, the posterior margin of the maxilla.
The postorbital is transversely compressed and crescentic and meets the postfrontal, jugal, quadratojugal and squamosal. The postfrontal is a broad, thin, elongate sheet of bone. The anterior, ‘V’-shaped process has a complex contact with the prefrontal and meets the postorbital in a point. The posterior process overlaps the parietal, and the supratemporal in a meandering suture.
The frontals are broad and long, occupying approximately one-quarter of the length of the skull roof, and meeting nasal, prefrontal and parietal. The medial margin is a straight midline suture with the other frontal, extending back around the anterior and lateral margins of the heart-shaped parietal foramen. Fine striations are present with capillary holes covering the dorsal surface.
The parietals are broad, highly sculpted elements that meet in the midline, forming a narrow pronounced ridge with a rough surface, which decreases in height anteriorly. The parietal contacts the prefrontal, frontal and postfrontal, and posteriorly, it pinches between the supratemporals and enters the occiput.
The supratemporal is a short, wide, sculpted element that contacts the parietal, postfrontal, postorbital and squamosal. In the occiput, a ventral, slender, cone-shaped process does not contact any of the surrounding skull elements. The squamosal is a thin triangular plate with two processes, one contacting the postorbital, quadratojugal and supratemporal, and the other extending into the occiput above the quadrate.
The quadratojugal is broad and rectangular. In BRLSI M1401, it is positioned on the back of the skull and the medial margin curves gently mediodorsally, with the ventral and medial corner forming a conical process extending medially to a point. It contacts the postorbital, squamosal and quadrate. The posterior margin is straight. The quadrate is a poorly preserved, slender, curved bone that widens ventrally, with curved anterior and posterior margins, and a bulbous process above the articular condyle, which is encased in matrix.
The pterygoid is incomplete and in occipital view (Text-fig. 12E, F) appears as a broad compressed element. Laterally, it is concave and contacts the jugal, postorbital and quadrate.
The sclerotic ring is preserved in articulation and is elliptical in shape, with the major axis parallel to the longitudinal axis of the skull (Text-fig. 8). There are 17 sclerotic plates that overlap in a nonuniform manner. The sutures between the plates are serrated and show strong overlap. Individual plates become wider to the periphery of the ring. They show clear striations, which extend from a point close to the external margin of the sclerotic ring, widening towards the inner margin, but the peripheral surface is smooth.
Braincase. The braincase is visible (in part) in occipital view in BRLSI M1399 (Text-fig. 12E, F), in dorsal view in BRLSI M1400 (Text-fig. 10: disarticulated elements), and in dorsal view, but slightly disarticulated, in BRLSI M1401 (Text-fig. 13A, B).
The basioccipital is a rounded, robust element. In posterior view (Text-fig. 12E, F), it consists of a smooth, extensive extracondylar area and a pronounced condyle. The condyle is bulbous, widest dorsally and has a rugose surface. The extracondylar area is furrowed, with posteriorly elevated sections. The dorsal surface is marked by a concave groove that extends from the condyle to the anterior margin. On either side of this structure is a rounded depression for union with the exoccipital. In dorsal view (Text-fig. 10), the anterior surface is elongated to form an extensive conical structure, the basioccipital peg. This narrows to a point halfway along its length before the margins diverge again, to give it an hourglass shape.
The supraoccipital is a large arched bone. The posterior surface is gently convex, with the lateral regions slightly depressed and there is a broad emargination midway along the ventral margin.
The exoccipitals are not preserved in articulation (Text-fig. 13A, B), but their relationship with the supraoccipital is still evident. The exoccipital is a short, stout bone that is curved medially, forming a support between the basioccipital and supraoccipital.
The opisthotics are incomplete (Text-fig. 13A, B), with only the posterior surface visible. They lie in close proximity to other bones of the basicranium. The opisthotic is a large, broad and solidly build bone with a rugose ventral surface.
The stapes (Text-fig. 13A, B) has broken away but remains in contact with the opisthotic dorsally. It is a robust bone composed of a short, stout shaft with a greatly enlarged medial end that is a bulbous and massive, with a rugose surface.
Mandible and dentition. The mandible and dentition are best seen in BRLSI M1399, in lateral view (Text-fig. 12A, B), but the anterior portion is missing. When the jaws are shut, the posterior portion of the mandible is partly concealed behind the massively expanded orbital ventral margin. There is a long retroarticular process. All elements are visible in lateral view, except the articular, but the medial views of the dentary, splenial and angular, and the entire prearticular, are concealed by matrix.
The dentary is a narrow, elongate bone with straight margins that tapers to a rounded and blunt anterior process that terminates just short of the snout tip. The splenial is an elongate narrow element that extends back to approximately orbital midlength. It tapers anteriorly and posteriorly and contacts the dentary and angular dorsally.
The angular is an extremely thin, elongate bone with straight margins, less than half the width of the surangular. Anteriorly, it tapers to an extremely narrow splint that terminates beneath the external naris, and it broadens backwards beneath the surangular. The surangular is elongate and slender, tapering anteriorly beneath the dentary.
Teeth are present along the entire length of the jaw. Preservation is poor in both specimens, and most tooth surfaces are not preserved: where present the enamel is smooth. Teeth are slender and conical, with crowns that are sharply pointed. Teeth are long (maximum length, 11.5 mm) and the majority of crowns are straight, but some are slightly recurved. In BRLSI M1399 (Text-fig. 12A, B), average crown length ranges from 6.2 mm in the posterior region of the jaw to 4.1 mm at the anterior end.
Axial skeleton. The description of the vertebral column is based primarily on BRLSI M1401 (Text-fig. 11), with supplementary comments from BRLSI M1399 (Text-fig. 9) and M1400 (Text-fig. 10). There are 46 presacral and 30 caudal vertebrae, for a total vertebral count of 76 and a column length of 1.95 m.
There are six cervical vertebrae, whose centra increase in diameter posteriorly. The atlas and axis (Text-fig. 13C–D) are fused, and the line of fusion is visible. The atlas has a long centrum and a low neural spine, while the axis has a shorter centrum and taller and longer neural spine. Other features of the cervical vertebrae are as in Stenopterygius triscissus.
There are 40 dorsal vertebrae. Centrum widths and heights are almost equal, and the diameter increases gradually back to vertebra 46, after which it decreases. Anterior neural spines are slender, and they become broader and shorter further back. The diapophysis fuses with the parapophysis in vertebra 47, the putative first sacral. Dorsal ribs are long and slender, and the anterior ribs are thickest. Proximally, the rib is compressed, with a shallow groove along both anterior and posterior surfaces, and then becomes more circular in cross-section distally. Anterior bicipital ribs are replaced by unicipital ribs from vertebra 47 back.
The exact number of postsacral vertebrae is unknown, with only 30 caudal vertebrae preserved in BRLSI M1401 (Text-fig. 11). The caudal centra become more rounded and then somewhat compressed posteriorly. Anterior neural spines are square with rounded corners, and they reduce in size, grading into compressed fan-shaped spines with slight posterior curvature. Towards the end of the tail, the neural spines are posterodorsally inclined rods. Anterior caudal ribs flare proximally with ‘V’-shaped elongate distal ends. Further back, the ribs reduce in size, becoming rectangular with subequal proximal and distal ends. The posterior caudal ribs are not present but facets are seen on all centra.
The scapula shaft has a straight anterior margin and a curved posterior margin. The coracoid is thickest on the dorsal and anterior margins, thinning towards the centre, and bears a notch on the anterior edge. The dorsal surface bears fine striations radiating towards the edges, and the lateral border is rugose. The interclavicle is ‘T’-shaped, but lacks part of the right anterior process. The posterior process is covered by fine striations. The clavicle (Text-fig. 11) is a robust element, but is only partially preserved.
The humerus (Text-fig. 15) bears a prominent flange along the anterior margin, but no obvious dorsal process. The radius is a pentagonal element with a narrow notch midway down the anterior margin. It contacts the ulna, which is slightly wider, along its entire length. In the mesopodial series, the radiale is pentagonal, with a narrow notch at the midpoint of the anterior margin. The pentagonal intermedium is asymmetrical with the proximal edge inclined anteriorly. Distal to the ulna, a pentagonal ulnare is preserved. Three hexagonal distal carpals (2, 3, 4) are present and support the first three digits. The anterior margin of distal carpal 2 is notched. Distal carpal 3 is marginally larger than the 2 or 4. There are two forms of metacarpals, hexagonal metacarpals II–IV and pentagonal metacarpal V. Metacarpal V is smaller than the primary digits and lies between the ulnare and distal carpal 4. The phalanges, like the metacarpals, are polygonal in outline.
Pelvis and hindlimb. The hindfin is less than two-thirds the length of the forefin, and it has three digits, with two visible notches along the anterior margin. Descriptions of the pelvic girdle and hindlimb are based mainly on BRLSI M1399 (Text-fig. 16).
The pelvis is bipartite, with the pubis and ischium fused proximally. The pubis is a slender, rod-like element, which curves posteriorly, so that the distal end lies close to the distal end of the ischium. The ischium is similar in shape, but its distal end is flared. Proximally, the puboischiadic contact is unclear, and both elements seem to merge into one another.
The femur has a short and narrow shaft, which flares distally. The distal end has a prominent posterior facet and two articular facets, but these have been damaged by poor surface preservation. The tibia is a rectangular element with a narrow notch midway down the anterior margin. The fibula is a pentagonal element and is larger than the tibia. In the mesopodial series, the tibiale lies near the anterior margin, the fibulare posteriorly, with the intermedium between them. The elliptical tibiale has a notch at the midpoint of the anterior margin and it contacts distal tarsal 2 distally. The polygonal intermedium lies between the distal ends of the tibia and fibula, and it contacts distal tarsal 3 distally. The fibulare is a square element with rounded corners, the largest of the three, and it contacts the fibula proximally and distal tarsal 4 distally. Distal tarsal 2 is tiny and ovoid, whereas distal tarsals 3 and 4 are subrounded. Distally, distal tarsal 3 contacts the subrounded metatarsal III, whereas distal tarsal 4 meets the subrectangular metatarsal IV. Metatarsal IV is shorter than metatarsal III.
Reconstruction and juvenile characteristics
The two Strawberry Bank ichthyosaurs were similar in size (Text-fig. 17), but they differ in proportions: relative size of the head, relative size of the orbit and length of snout, relative extent and depth of the rib cage, and relative limb sizes.
The Strawberry Bank ichthyosaurs include three infants and the others are immature individuals of various sizes, ranging up to a maximum length of 1.96 m. The five larger specimens are judged to be juveniles because they are all much smaller than examples of their species from the Germanic Basin (Stenopterygius triscissus, 2.0–3.5 m long, Ilminster specimens, 0.9 m; Hauffiopteryx typicus, up to 2.95 m, Ilminster specimens, 1.05–1.96 m). Of the three supposed infants (BRLSI M1405, M1407, M1408), estimated lengths of the first and last are 0.27 and 0.53 m, half the lengths of the supposed juveniles just mentioned, and one-sixth that of the type materials from Germany. Further evidence that the Ilminster specimens are juveniles is that the sutures are not fused; in particular, the cranial sutures are loose, showing incomplete ossification between skull elements. Furthermore, specimens of these species with a mandibular length less than 400 mm are considered immature (McGowan 1978).
The discovery of infants and juveniles, but no adults, could be a chance finding, or a sedimentary sorting effect, or it might indicate something palaeobiological. Ichthyosaurs were viviparous, giving birth to one to eleven young in the water, much as whales do (Motani 2005). It may be that, like some modern cetaceans, ichthyosaurs periodically moved to shallow marine environments to give birth (Massare 1988), as suggested also for plesiosaurs (Wiffen et al. 1995). The Ilminster site was located on the continental shelf, perhaps close to land, as indicated by the abundant insect and plant remains. The circumstantial evidence might then suggest that the Strawberry Bank ichthyosaurs are sampled from a birthing ground where the young were somewhat protected from predators, and where juvenile mortality was high.
The diet of the ichthyosaurs consisted of small fishes, belemnites and squid-like cephalopods (Motani 2005), all of which are found at Strawberry Bank. Prey preferences of marine reptiles can be deduced from tooth morphology (Massare 1987): the small, slender pointed teeth in Hauffiopteryx typicus correspond to the pierce guild, ichthyosaurs that impaled soft-bodied prey, whereas Stenopterygius triscissus has larger, more curved teeth, indicating the smash guild, ichthyosaurs that grasped and punctured hard-shelled prey such as cephalopods.
Cladistic studies (e.g. Motani 1999; Maisch and Matzke 2000; Sander 2000) have established general patterns in ichthyosaur phylogeny. The pattern of branching events approximates to stratigraphic occurence, with Early, Middle and Late Triassic forms in succession, then a mass of Early Jurassic taxa, followed by Late Jurassic and Cretaceous forms in the most derived positions. Hardest to resolve have been the relationships of Early Jurassic taxa, not least because of problems in defining the terminal taxa.
The assignment of the Ilminster ichthyosaurs to Stenopterygius triscissus (Quenstedt, 1856) and Hauffiopteryx typicus (Huene, 1931) is explored first in light of recent debate on these species, and we then attempt two cladistic analyses to determine their phylogenetic positions.
Taxonomic issues concerning Stenopterygius
Several of the synapomorphies of Stenopterygius, noted earlier, are highly variable. For example, the anterior notching of the limb elements and the supernumerary digits vary between individuals and even between the right and left paddles of a single individual. In some specimens, there is a supernumerary digit between S1–S2 and/or S3–S4. To further complicate matters, juvenile specimens appear more similar in form to other juveniles than to mature individuals of their own species. In addition, poor preservation or lack of exposure of the skull roof in the majority of specimens hinders detailed comparisons of cranial suture patterns. These factors led Maisch (2008) to propose that differences between many supposed species of Stenopterygius are consistent with ontogenetic variation in growth series of three valid species, S. quadriscissus, S. triscissus and S. uniter.
The first Ilminster ichthyosaur is identical to German and British materials of Stenopterygius triscissus. German S. triscissus specimens of Toarcian age (Holotype GPIT 12/0224-2; SMNS 54027; SMNS 14846) show a relatively narrow rostrum, reduced frontals, parietals touching nasals, ventral process of premaxilla contacting the lacrimal, short and wide fins and a slender body. Only in specimen SMNS 14846 can the relationships between temporal cranial sutures be observed clearly and, like the Ilminster material, the postfrontals and frontals are not in contact, being separated by the posterior process of the nasals.
Maisch (2008) identified the English species S. longifrons (holotype, NHMUK 33157; Text-fig. S3A–B) as a synonym of S. triscissus based on identical skull morphology. An autapomorphic feature of S. longifrons is that the maxilla forms the ventral narial margin (Godefroit 1993; McGowan and Motani 2003). However, a second specimen (NHMUK 32681; Text-fig. S3C), referred to S. longifrons by Lydekker (1889), has exactly the same skull configuration as the Ilminster material, including the exclusion of the maxilla from the naris by the lacrimal. This character is probably variable among individuals of the species, as is evident in S. quadriscissus (Maisch and Matzke 2000). Furthermore, the ventral process of the premaxilla and the anterior process of the lacrimal are not preserved in the holotype, giving the impression that the maxilla is in contact with the naris. The articulated pectoral girdle of BRLSI M1407 is similar to a specimen of S. longifrons (MNHNL TU332) where the coracoid is anteroposteriorly elongate and the posterior process of the interclavicle extends the entire length of the coracoid (Godefroit 1994, pl. 2B, fig. 17B).
Taxonomic issues concerning Hauffiopteryx
Stenopterygius hauffianusHuene, 1922 is a nomen dubium, as the majority of specimens, including the lectotype GPIT 18387 (Huene 1922; McGowan 1979, McGowan and Motani 2003), are actually representatives of S. quadriscissus (Maisch 2008). The remaining material, assigned to the subspecies S. hauffianus typica (Huene 1931), has several autapomorphies that distinguish it from all other genera (notably Stenopterygius), and thus Maisch (2008) created the new taxon Hauffiopteryx. Until the Ilminster material was assigned to this taxon, H. typicus was known from only seven specimens from Germany, Belgium and Luxembourg.
There is uncertainty about overlap in characters between Hauffiopteryx and Leptonectes, as noted earlier. The genus Leptonectes consists of three valid species, L. solei (McGowan 1993), L. moorei (McGowan and Milner 1999) and L. tenuirostris (Conybeare 1822). Of these, L. solei is readily distinguished from the other species by its larger size, with the skull length exceeding 100 mm, a smaller orbit (orbital ratio of 0.15), a high presacral vertebral count (50 in the holotype), a larger, more elongate temporal fenestra, and only the radiale emarginated. It could be argued that L. moorei shows that the typical ‘short-snouted’ morphology of Hauffiopteryx is also present in a taxon currently recognised as a leptonectid, which would support Maisch’s (2008) conclusion that Hauffiopteryx might be a leptonectid. Indeed, L. moorei is similar to the Ilminster skulls in sharing a large orbit: the orbit ratio is 0.31, compared with 0.30 and ≤ 0.34 in BRLSI M1399 and M1401, respectively. The snout is marginally longer in the Ilminster specimens, the snout/skull length ratio being 0.62 in BRLSI M1399 and 0.59 in L. moorei. L. moorei also differs from the Ilminster material in the absence of forefin notching, the deeper snout, and a protruding rounded bump on the anterodistal end of the ventral surface of the humerus. The species differs further in having a larger humerus, 73 mm long, compared with 64.1 mm in the largest specimen BRLSI M1401 (skull length 369 mm).
Leptonectes tenuirostris is characterised by a small overbite, narrow postorbital region, dentition small compared with skull width, and quadratojugal extensively exposed on the lateral skull surface, all similar to the Ilminster specimens. Although the wing-like process on the dorsal margin of the external naris of BRLSI M1401 is not universally present in L. tenuirostris, it is seen in two individuals (Maisch and Matzke 2003; Maisch and Reisdorf 2006). The front paddles differ from L. tenuirostris in the absence of a foramen between the ulna and radius, but the foramen is a primitive character largely restricted to Triassic forms (McGowan 1996). Many skull features are shared between the Ilminster specimens and L. tenuirostris, particularly a recently described small individual, NMO 26575, with a skull length of 328 mm, from the upper Pliensbachian of Switzerland (Maisch and Reisdorf 2006). The snout ratio for the Ilminster material is ≥0.62, comparable to NMO 26575 with a value of 0.64, but less than for English specimens of L. tenuirostris (skull lengths between 530 and 950 mm) with values exceeding 0.70 (McGowan 1989, 1996; McGowan and Motani 2003). The orbit ratio in the Ilminster material (0.30–0.34) is slightly larger than in the Swiss specimen (0.28; Maisch and Reisdorf 2006) but far exceeds that of larger individuals of L. tenuirostris (<0.25, McGowan 1989, 1996; McGowan and Motani 2003). If the Ilminster specimens are juveniles, then allometric growth could explain these differences from the larger L. tenuirostris specimens: snout length increases and orbit diameter decreases relative to skull length.
Some unusual features of the skull, pelvic and pectoral girdles in the second Ilminster taxon differ from L. tenuirostris but are consistent with Hauffiopteryx (see below). In the skull roof, the frontals are broadly exposed and contact the prefrontals laterally. The nasals are short, extending back for a third of the orbital length. Unlike L. tenuirostris, where notching is restricted to one forefin element in the first digit, five are seen in the Ilminster material. The pelvis is unusual, showing a very slender pubis and ischium that diverge widely proximally and have ossified distal ends (McGowan 1978), unlike L. tenuirostris where the ischiopubic complex is much wider (McGowan and Motani 2003).
Phylogenetic analysis of Stenopterygiusand Hauffiopteryx
In previous cladistic analyses, Stenopterygius has been assigned different positions. Mazin (1982) placed it as the sister-taxon to an unresolved group of other Jurassic and Cretaceous ichthyosaurs, but more derived than Ichthyosaurus and Ophthalmosaurus. Motani (1999) found that Stenopterygius was basal member of the new clade Thunnosauria, and outgroup to the more derived forms Ichthyosaurus, Brachyptergius, Ophthalmosaurus, Caypullisaurus and Platypterygius. Sander (2000) interpreted Stenopterygius as a much more derived taxon and sister to Ophthalmosaurus. Maisch and Matzke (2000) recovered a result similar to that of Motani (1999), with Stenopterygius in a more basal position in the cladogram, but more derived than Leptonectes, Eurhinosaurus, Suevoleviathan and Ichthyosaurus.
Hauffiopteryx was not included in these earlier studies. Maisch (2008) identified this taxon as the most basal member of the Family Stenopterygiidae, which includes also Stenopterygius and Ophthalmosaurus. Characters cited as diagnostic of Stenopterygiidae are bicipital dorsal ribs, tail shorter than body length, quadratojugal only slightly exposed on the lateral surface of the skull, and squamosal a thin, delicate plate, which is not firmly integrated within the skull. Hauffiopteryx lacks apomorphies of the Stenopterygius–Ophthalmosaurus clade, including temporal fenestra less than half the size of orbit, surfaces of tooth crowns are smooth, quadratojugal is positioned more posteriorly, small overbite, and partially fused ischiopubis. Note, however, that a reduced temporal fenestra is also plesiomorphic for the clade Leptonectidae (Leptonectes, Excalibosaurus and Eurhinosaurus). Furthermore, with the exception of the pelvis, all the apomorphic traits of Hauffiopteryx are shared with Leptonectidae. Therefore, we question Maisch’s (2008) conclusion that Hauffiopteryx is a stenopterygiid.
We ran cladistic analyses based on the data matrices of Motani (1999) and Maisch and Matzke (2000). In both cases, we used the taxa and codings employed in the original analysis, and added Hauffiopteryx, scored according to their character definitions (Table S2). We ran four analyses, two based on all taxa included in the original studies (see Electronic Supplement; Text-fig. S4), and two based on the post-Triassic forms. The analyses of Jurassic taxa were executed using PAUP version 4.0b10 (Swofford 2001), with a branch-and-bound tree search, and using Shastasaurus from the Late Triassic as outgroup. In both cases, all characters were treated as unordered and of equal weight. Bootstrap values on all nodes were assessed with 1000 replicates, and decay indices (Bremer values) were calculated in PAUP by re-running the branch-and-bound analyses for progressively higher tree lengths, retaining all trees, computing the strict consensus, and noting when each node disappeared.
The Motani (1999) reanalysis yielded a single tree, with Stenopterygius as a basal member of Eurhinosauria, and Temnodontosaurus as outgroup to Eurhinosauria + Thunnosauria. However, bootstrap support for these relationships is very weak (<50 per cent), and they collapse when only values >50 per cent are considered (Text-fig. 18A). Of the remaining nodes, only two are robust, with strong bootstrap support (>85 per cent) and decay indices (= 4): Parvipelvia and Brachypterygius + Ophthalmosauria. Motani (1999) also reported these clades, but he found a different resolution of Jurassic ichthyosaurian taxa. Whereas he reported an unresolved polytomy among (1) Temnodontosaurus, (2) the clade (Leptonectes + Excalibosaurus + Eurhinosaurus, unresolved) and (3) the clade (Stenopterygius + Ophthalmosauria), we find that Stenopterygius is removed from clade (3), and becomes sister group to clade (2), the Eurhinosauria. Our analysis provides weak support (bootstrap 69 per cent, decay index 2) for Motani’s clade Eurhinosauria (= Leptonectes + Excalibosaurus + Eurhinosaurus), with Hauffiopteryx as a possible basal member. The position of Stenopterygius, whether as a yet more basal member of this clade or in an unresolved position (Text-fig. 18A), is not clear. These differences from the Motani (1999, fig. 1) result relate mainly to the insertion of Hauffiopteryx into our analysis, and the instability confirms the low robustness of many nodes. The mapping of character transitions (Table S3) shows relatively few unequivocal apomorphies, and many nodes are founded on character transitions that are repeated elsewhere in the cladogram, reversals, and uncertain transitions (because of incomplete coding and acctran/deltran differences). The placement of Hauffiopteryx and Stenopterygius (Text-fig. 18A) in the larger clade with Eurhinosauria and Thunnosauria is based on six characters (Table S3) of which only one (79*: forelimb longer than hindlimb, but less than twice the length) is unequivocal. There are no unique characters linking either Hauffiopteryx or Stenopterygius with Eurhinosauria or Thunnosauria, and so they retain an unresolved basal position.
The Maisch and Matzke (2000) reanalysis (Text-fig. 18B) is similar to their result, but shows less resolution in the Jurassic portions. Only three clades are reasonably well supported: Parvipelvia minus Callawayia (bootstrap 95 per cent, decay index 3), Thunnosauria (bootstrap 85 per cent, decay index 2), and the subclade Brachyptergius to Caypullisaurus (bootstrap 95 per cent, decay index 3). The Leptonectidae is not recognised as a clade, but as an unresolved polytomy below a well-resolved Thunnosauria, consisting largely of Jurassic forms. Here, Hauffiopteryx is basal outgroup to a clade of Ichthyosaurus and more derived forms (bootstrap 64 per cent, decay index 1), and Ichthyosaurus is more derived than Stenopterygius. Neither analysis supports Maisch’s (2008) claim that Hauffiopteryx is a stenopterygiid nor recovers that family as a clade. The mapping of character transitions (Table S3) shows more unique apomorphies associated with clades than in the Motani (1999) analysis. Further, Hauffiopteryx is placed as immediate outgroup to Thunnosauria by three unique apomorphies (39*: basioccipital peg present; 53*: rib articulation in thoracic series exclusively bicipital; 65*: tail distinctly shorter than the rest of the body), and Stenopterygius is basal to ‘Ophthalmosauria’ on the basis of two unequivocal apomorphies (38*: processus postpalatinalis pterygoidei present; 103*: pelvis bipartite with ischiopubic plate).
Acknowledgements. We are most grateful to Matt Williams (curator of the BRLSI), for allowing access to the Strawberry Bank collection and encouraging this project, and for transporting and preparing two of the specimens. We also thank Remmert Schouten (University of Bristol) for overseeing preparation and providing laboratory facilities and Simon Powell (University of Bristol) for photographing the specimens and resolving computer graphic problems. A special thanks to Michael Maisch for providing additional information. We also thank the following curators for their help when HC visited their collections: Sandra Chapman at NHMUK, Rolf Hauff at MHH, and Michael Maisch and Rainer Schoch at GPIT and SMNS. We are also grateful to the Linnean Society of London and Systematics Research funds for covering the costs of these museum visits. Thanks also to Michael Maisch, Ben Kear, Judy Massare and an anonymous referee for their very helpful comments on the MS, and Paul Barrett for editorial assistance. The Jurassic Ecosystem of Strawberry Bank, Ilminster (JESBI), project is generously supported by the Esmée Fairburn Foundation.