Derivation of name. From the greek ‘Phthi-’ (‘waned’, or ‘faded away’) and ‘pod’ (foot), as the first fossil chiton demonstrably lacking a true foot.
Type species. Helminthochiton thraivensisReed, 1911, from the Lady Burn Starfish Bed, Threave (Thraive) Glen, Girvan, Scotland (Upper Ordovician).
Diagnosis. Valves not articulated in life position, lacking articulamentum, overall shaped as a half-ellipse, folded on the shorter axis with the rounded edge at the anterior. Posterior margins pinched at the midline fold, forming a pronounced point but not a significant projecting beak. Apical angle weakly obtuse, jugal angle weakly acute. Head and tail valves slightly smaller than intermediate valves. All valves mixoperipheral with significant (ventral) apical area extending to 30 per cent of valve length. Ornament of growth lines only.
1911 Helminthochiton thraivensis Reed, pp. 337–339, pl. 15.
1932 Helminthochiton thraivensis Reed; Quenstedt, p. 85 [annotation].
1939 Helminthochiton thraivensis Reed; Berry, p. 207.
1960 Helminthochiton thraivensis Reed; Smith, p. I48, fig. 33, 2a–c.
1967 Helminthochiton thraivensis Reed; Curry and Morris, p. 423.
1977 Gotlandochiton thraivensis (Reed); Sirenko and Starobogatov, p. 31.
1981 Septemchiton thraivensis (Reed); Rolfe, p. 677.
1987 Septemchiton? thraivensis (Reed); Smith and Hoare, p. 55.
2004 Helminthochiton thraivensis Reed; Cherns, p. 452.
2004 Septemchiton? thraivensis (Reed); Vendrasco and Runnegar, pp. 679, 686.
2006 Helminthochiton thraivensis Reed; Hoare and Pojeta, p. 15.
2007 ‘Helminthochiton’ thraivensis Reed; Sigwart and Sutton, pp. 2413–2419 and electronic supplement, fig. 2.
2010 ‘Helminthochiton’ thraivensis Reed; Donovan et al., pp. 935–937, figs 2, 3.
Lectotype. Here designated as NHMUK G.47258 (Reed 1911, pl. XV, fig. 1).
Material. Specimens collected by Mrs Gray, Natural History Museum (London); Paralectotypes examined; NHMUK G.47251 (Reed 1911, pl. XV, fig. 2), NHMUK G.47247 (Reed 1911, pl. XV, fig. 4), NHMUK G.47246 (Reed 1911, pl. XV, fig. 7), G.47250, G.47253.
Additional material examined; NHMUK G.22679, NHMUK G.47260; NHMUK G.47261; NHMUK G.47265; NHMUK G.47254.
Horizon. ‘Lady Burn Starfish Bed’, Threave Glen, Girvan district, Ayrshire (Strathclyde), southwest Scotland; NGR NS 250 038. Drummuck Subgroup, South Threave Formation, Farden Member (Harper 1982a, b); Upper Ordovician, Katian; Ashgill Series, Rawtheyan Stage (= Richmondian, Cincinnatian in terms of the North American succession); see Ingham inFortey et al., 2000, fig. 24.
Description. Animals preserved in variably curved posture (Fig. 2S–T), interpreted as analogous to the post-mortem contortion typical of extant chitons and aplacophorans, curled in toward the pedal groove. Body shape long and narrow, from seven to ten times long as wide; total body length 40–99 mm, width 5–10 mm. The valves in series are preserved in presumed life position, in almost all cases separated by gaps. Within these gaps between valves, the tissue is covered in spicules; although these are rarely resolvable as individual elements, they form a distinct spicular sheet (e.g. Fig. 2Y). Spacing between valves is variable between and within individuals. Valves do not articulate but do overlap in more curved specimens (compare Fig. 2S–U); this difference is interpreted as variation following body posture.
Valves are subcarinate, keeled at posterior and rounded at anterior. Elevation (height/width) ratio in intermediate valves varies from c. 0.35 to c. 0.65; ratio does not appear to vary consistently along series, although middle valves (IV–VI) do not include any examples of low elevation. Valves slightly beaked. Valves thin, maximum observed values <0.5 mm (e.g. Fig. 2BB); thinner at dorsal median.
Head valve (Figs 1A–C, M–O, 2A–C) generally similar in outline to intermediate valves but shorter. Head valve of same width as tail valve, slightly narrower than intermediate valves. The valve is also lower, with an elevation ratio of below 0.1 in some cases (e.g. Fig. 2B). Head valve forms somewhat variable in apical angle and expression of median jugal embayment.
Intermediate valves subrectangular, lateral areas differentiated by a variably expressed diagonal depression normally more visible toward posterior apex (see e.g. Fig. 1D). Posterior margin with pronounced apex (mean apical angle 110 degrees); apical angle does not vary appreciably along valve series (including tail valve). Anterior margin with shallow median jugal embayment, median valve length at midline more than 90 per cent of maximum total valve length. Side slopes slightly convex, rounded. Shape of valve II not differentiated from other intermediate valves (in contrast to Recent Chitonida). Tail valve (Figs 1K–L, V–X, HH–JJ, 2P–R) shaped as intermediate valves, with mixoperipheral growth and without dorsal mucro. Planar valve width is slightly longer than total valve length, shape is as a ellipse folded on shorter axis on the jugal ridge. Side slopes straight. Anterior margin convex, evenly rounded. Posterior margin convex, pinched in middle, strongly subcarinate. Lateral areas separated by shallow diagonal depression, more pronounced at posterior margin.
Dorsal valve surface without sculpture other than growth lines, which are strong in many cases (e.g. Fig. 1D, S). Growth lines occasionally reflected on ventral surface (e.g. Fig. 2H) as well as on the apical area. Valves preserve no evidence for aesthetes.
Ventral valve surface smooth with posterior mixoperipheral fold. The mixoperipheral margin (ventral apical area) is shortest in the head valve; in other valves, it occupies a larger portion of more anterior valves to a maximum of 35 per cent of the length in valve II compared to a minimum of 13 per cent of the length of valve VIII. There is general decrease through the valve series, with more posterior valves having a smaller mixoperipheral area; however, there are not sufficient specimens to determine whether this is a robust trend. Valves in two of the specimens examined by XMT have evidence for a ventral posterior tunnel (NHMUK G.47251 valve VII, Fig. 1FF; NHMUK G.47254 valve VIII, Fig. 2P); these are not consistently preserved, and the two tunnels observed may represent different structural artefacts.
The girdle is preserved as a large mass or more typically as a sheet of mouldic spicules. Preservation is not sufficient to make any detailed description of the morphology of girdle elements or to determine whether there are multiple types of spicules present. Girdle extending ventrally around whole body and intruding between valves. The extent of the spicule preservation varies in different specimens, but in most, it appears as a contorted, folded sheet, impersistently preserved but in places clearly continuous ventrally. NHMUK G.47250 illustrates spicule distribution most clearly in three dimensions (Fig. 2Y, CC); in other specimens variously the contortions of specimens, impersistent preservation and presence of cracks render satisfactory three-dimensional visualization of the spicule sheet impossible. Virtual cross-sections (Fig. 2AA–CC), however, demonstrate ventral continuity in NHMUK G.47253 as well as in NHMUK G. 47250; note that the light-coloured object descending from the left of the valve in Figure 2AA represents a major crack, which we interpret as following the line of weakness left by the sheet of mouldic spicules. The spicular sheet includes erratically placed ventral disturbances; because of the asymmetric and inconsistent nature of these disturbances, we interpret them as taphonomic artefacts, where the spicules have been disturbed in process of preservation, rather than indicating the substantial gaps in the girdle armature that would be required to accommodate a polyplacophoran-like foot. The spicules appear to have contact with the ventral part of the valves (Fig. 2AA–CC), perhaps indicating direct attachment.
There is no evidence for preservation of a radula nor any softer tissues in any of the specimens examined.
The lectotype, NHMUK G.47258, preserves a series of associated shelly debris extending from valve IV to the posterior end of the tail valve (Fig. 2T). Items are arranged in a discontinuous string ventral to the midline of shell series, in a position that corresponds to the presumed placement of the gut in the living animal. Where identifiable, debris includes pieces of calcified subcircular rings with variable-sized hole and small fragments with crenulated structures; we interpret these to represent crinoids and trilobites. The crinoid material in these gut contents has been described by Donovan et al. (2010, 2011).
Remarks. Reed (1911) originally assigned Phthipodochiton thraivensis to the genus Helminthochiton essentially on the basis of its overall elongate shape. The Silurian type species of this genus, H. griffithi, was recently redescribed by one of us (Sigwart 2007). H. griffithi remains poorly characterized (known only from the holotype), but differs from the material described here in the much closer articulation of its valves, the possession of radial ornament, in many details of valve outline (such as the lack of an anterior embayment, lack of a sharp apex and lower length/width ratio). The differences are particularly apparent in the head valve, which in H. griffithi is relatively large and more elongate than the trunk valves. We hence consider the assignment of thraivensis to this genus to be untenable and assign it to the new genus Phthipodochiton. The systematics of paleoloricate ‘polyplacophorans’ is in need of further general revision, and in particular, we lack confidence in the monophyly of certain familial and higher-level taxa in use for these fossils. To establish the generic distinctiveness of Phthipodochiton, therefore, we have exhaustively compared Phthipodochiton with all other ‘chiton’ genera reported from Ordovician rocks; these are dealt with alphabetically below. None of these are sufficiently similar to warrant placement of Phthipodochiton thraivensis within any established genus, but we tentatively note that it seems morphologically most closely allied to Alastega, Robustum and Septemchiton.
AlastegaCherns, 1998a, was erected to house a Silurian-aged species; Hoare and Pojeta (2006) have since assigned other Ordovician fossils to the genus. Alastega intermediate valves differ from those of Phthipodochiton in their acute apical angle and low length/width ratio, and head valves are not closely comparable (those of Alastega are elongate and lack a clear apical area). AmblytochitonHoare and Pojeta, 2006, was placed within the same family as Alastega; it is relatively poorly characterized, but differs from Phthipodochiton at least in its head valves (elongate and subrectangular) and in the acute apical angle of its intermediate valves. CalceochitonFlower, 1968, is also poorly known but similar to Matthevia. ChelodesDavidson and King, 1874, has become something of a ‘bucket genus’ to which a variety of Ordovician species have been assigned. The Silurian type species C. bergmani was, however, redescribed by Cherns (1998a); while only one intermediate valve is known, it clearly differs from Phthipodochiton in its relatively long apical area, rounded posterior margin, lack of anterior embayment, higher length/width ratio, rounded jugum and other details besides. EochelodesMarek, 1962, is Chelodes like in possessing a long apical area and straight anterior margins, and hence not closely comparable to Phthipodochiton. GotlandochitonBergenhayn, 1955, is another genus with a Silurian type species to which Ordovician fossils have been assigned (by Smith inSmith and Toomey, 1964). The type species G. interplicatus was redescribed by Cherns (1998b), but is known only from the dorsal surface on a single intermediate valve; this differs from Phthipodochiton valves in its lower apical angle, convex anterior margin, concave posterolateral margins and lower length/width ratio. As discussed by Vendrasco and Runnegar (2004), HelminthecellaStinchcomb and Darrough, 1995, is very similar to Matthevia.
The type species of Ivoechiton Smith inSmith and Toomey, 1964, is imperfectly known, but intermediate valves possess a much lower length/width ratio, higher jugal angle and a higher apical angle than those of Phthipodochiton, which they do not closely resemble in outline. KindbladochitonVan Belle, 1975 (= Eochiton Smith inSmith and Toomey, 1964), is also imperfectly known; its intermediate valves are broader than those of Phthipodochiton and lack a sharp apex and an anterior embayment. ListrochitonHoare and Pojeta, 2006, differs from Phthipodochiton in its lower apical angle and distinctive heart-shaped valve outline in all valves, lower jugal angle and in its much longer apical area, extending to near mid-length. Valves of LitochitonHoare and Pojeta, 2006, lack a sharp jugum and straight sides of Phthipodochiton, possess parallel lateral margins and lack a true apical area. They also possess strong concentric ornament. MattheviaWalcott, 1885, is a distinctive form that occurs in both Cambrian and Ordovician rocks. It differs from Phthipodochiton, and indeed from most paleoloricates, in the extreme elongation and low apical angle of its intermediate and tail valves, in the extremely long apical area not fully flattened onto the valve surface (forming the lower part of a compressed cone shape) and in its flattened holoperipheral head valves. OrthriochitonVendrasco and Runnegar, 2004, was erected for Cambrian material, but an Ordovician genus was assigned to it by Hoare and Pojeta (2006). The genus differs significantly from Phthipodochiton in its valve outline, notably in its acute apical angle, and its deep and straight-sided anterior embayment. The putative head valve figured by Vendrasco and Runnegar (2004, fig. 12.26) is subcircular, and not like that of Phthipodochiton.
Paleochiton Smith inSmith and Toomey, 1964, is poorly characterized; the holotype and three paratypes are all fragmentary intermediate valves. These differ from those of Phthipodochiton in their outline, which is subrectangular with straight and parallel lateral margins and an apical angle near 180 degrees; they also possess shorter apical areas. PreacanthochitonBergenhayn, 1960, is similar to Phthipodochiton in valve outline although with a lower apical angle; however, Phthipodochiton valves lack the distinctive pustulose ornament of the type species Preacanthochiton cooperi and possess a much sharper jugum. PriscochitonDall, 1882, is similar to Matthevia. RobustumStinchcomb and Darrough, 1995, possesses elongate subcylindrical intermediate valves similar to those of Septemchiton. Vendrasco and Runnegar (2004, p. 686) drew attention to the apparent saddle-like form of an internal mould of Phthipodochiton thraivensis figured by Reed (1911, pl. XV, fig. 3), comparing it to similar saddle shapes in Robustum; we do not consider this observation to be indicative of any particularly close relationship between the genera. SarkachitonDzik, 1994, is poorly characterized, but differs from Phthipodochiton in possessing apparent spiny ornament, subrectangular tail valves and an acute apical angle. SeptemchitonBergenhayn, 1955, possesses elongate subcylindrical intermediate valves, with complete overlap when articulated; it also differs from Phthipodochiton in possessing a very small head valve (see Rolfe 1981). The head valve in some specimens of Phthipodochiton is reduced but not to the dramatic extent seen in Septemchiton. While some authors (e.g. Vendrasco and Runnegar 2004) have tentatively assigned Phthipodochiton thraivensis to the genus Septemchiton, we consider the differences in intermediate valve morphology to be sufficient for differential diagnosis. Note also that the genus Solenocaris is considered a synonym of Septemchiton (see Rolfe 1981). Lastly, SpiculchelodesCherns, 1998a, was erected to house a Silurian-aged species; Hoare and Pojeta (2006) have since assigned Ordovician fossils to the genus. The type species S. pilatis differs from Phthipodochiton in its distinctive pustulose ornament, low jugal angle and rounded jugum, long apical area and high length/width ratio.
In summary, none of the genera examined appear especially closely comparable to Phthipodochiton thraivensis; while many are inadequately known, in all cases, there are differences in multiple characters. It should also be noted that the valves of P. thraivensis are relatively thin. While valve thickness has rarely been described for other taxa, this may serve as another distinguishing character. The erection of the new genus Phthipodochiton is hence taxonomically warranted although we prefer not to assign this to any higher taxa beneath the level of the Paleoloricata until further revisionary work has been completed.