Chondrichthyan-like scales from the Middle Ordovician of Australia



Abstract:  Microvertebrate sampling of the Stairway Sandstone (Darriwilian, Middle Ordovician, central Australia) has yielded scales that are chondrichthyan-like in their overall construction, and Tantalepis gatehousei gen. et sp. nov. is erected here to describe these specimens. Tantalepis gatehousei gen. et sp. nov. is the stratigraphically oldest microsquamous taxon described thus far, and the ‘shark-like’ appearance of the scales may extend the chondrichthyan lineage back into the Middle Ordovician. The presence of ‘shark-like’ scales in the fossil record some 50 myr prior to the first articulated chondrichthyan body fossils and 44 myr before the first clearly identifiable chondrichthyan teeth suggests there is a considerable scope for the recovery of articulated specimens with which to document the early history of crown gnathostomes. Traditional hypotheses of phylogenetic relationships among early jawed vertebrates were recently challenged by the proposal of a radically different tree topology. However, the development of a new data set specifically addressing scale-based characters is required before taxa such as Tantalepis, that are based upon disarticulated remains alone, can be firmly placed within the emerging, revised, evolutionary narrative.

The ordovician was an interval of major diversification among the marine biota (the Great Ordovician Biodiversification Event), and there is now an increasing appreciation that vertebrates underwent a significant radiation during this time interval. The oldest known pteraspidomorphs occur in the Floian of Australia (Young 1997) and, more contentiously, Bolivia (Erdtmann et al. 2000), with arandaspids and astraspids being the dominant components of fish faunas in Gondwana and Laurentia, respectively. The earliest scales of microsquamous taxa have been recorded from the Darriwilian of Australia (Young 1997) and Sandbian of North America (Sansom et al. 1996, 2001; Sansom and Elliott 2002). These rare taxa have been referred to thelodonts (Larolepis darbyiSansom and Elliott, 2002) whilst others, AreyongalepisYoung, 2000 and scale morphology A. Sansom et al. (1996), have been compared tentatively to chondrichthyan scales. Here we report a new taxon recognized from isolated polyodontode scales, which are chondrichthyan-like in overall construction, from the Stairway Sandstone of central Australia that enlarges the range of likely early, and perhaps plesiomorphic, scale conditions for crown-group gnathostomes.

Geological setting

The Stairway Sandstone is the middle unit within the Larapinta Group that comprises much of the Late Cambrian and Ordovician record within the large intracratonic Amadeus Basin in the southern half of the Northern Territory, central Australia (Wells et al. 1970). In stratigraphic order, the Larapinta Group consists of the Tremadocian Pacoota Siltstone, the Floian–Dapingian Horn Valley Siltstone, the early Darriwilian (Da1–Da2) Stairway Sandstone and the middle Darriwilian (Da2?) to Katian Stokes Siltstone. The new vertebrate material described here was recovered from a small outcrop of fine grained siltstone from the middle part of the Stairway Sandstone exposed in a small gully on the northern flank of Maloney Hill (24°30′18″S; 133°16′41″E), to the east of the Stuart Highway where it crosses Maloney Creek (Fig. 1). This outcrop is to the east of the section that provided samples for Cooper’s (1981) study of conodonts from the underlying Horn Valley Siltstone. Conodonts from the Stairway Sandstone are uncommon and restricted to a few intervals in the lower or middle part of the formation that contain calcareous sands or silts. Reflecting the clastic nature of this unit, preservation of the conodont elements is generally poor and the majority of elements are broken. However, the recovery of Lenodus cf. L. variabilis from the same samples that yield the scales described herein suggests an early Darriwilian age for that part of the formation. Lenodus antivariabilis has a very restricted range in the upper part of the Baltoniodus norrlandicus Zone (Löfgren and Zhang 2003).

Figure 1.

 Locality map showing position of section through the middle Stairway Sandstone at Maloney Hill, Northern Territories, Australia.

Systematic palaeontology

Depository.  Illustrated specimens are deposited in the Commonwealth Palaeontological Collection (CPC) at Geoscience Australia, Canberra, Australia.

Superclass GNATHOSTOMATA Cope, 1889
Class ?CHONDRICHTHYES Huxley, 1880
Genus TANTALEPIS gen. nov.

Derivation of name.  The generic name is derived from Tantalus, a son of Zeus and was the king of Sipylos, whose name is the root of tantalize, and lepis the Latin for scale; the combination makes reference to the tantalizing glimpse that these scales provide into the early evolution of jawed vertebrates.

Type species. Tantalepis gatehousei gen. et sp. nov.

Diagnosis.  As for the type and only species.

Tantalepis gatehousei sp. nov
Figure 2

Figure 2.

 Scales of Tantalepis gatehousei gen. et sp. nov. from the Stairway Sandstone of the Amadeus Basin, Northern Territories, Australia. A–C, crown and lateral views of polyodontode scale, CPC 40513 (holotype). D, oblique lateral view of single odontode scale, CPC 40514. E, H, basal and oblique basal views of polyodontode scale, CPC 40515. F–G, crown view of polyodontode scale, CPC 40516. A, C–F, H, SEM micrographs, B, G, light micrographs. Scale bar represents 100 μm for all images except D where it represents 50 μm.

Derivation of name.  The specific epithet honours Colin Gatehouse for his work in the Amadeus Basin and invaluable assistance to IJS and NSD in the field.

Holotype.  Isolated polyodontode scale CPC 40513 (Fig. 2A–C).

Other material.  Over 150 scales have been recovered from the same sample as the type and figured suite.

Locality and horizon.  Small gully (24°30′18″S, 133°16′41″E), exposing a friable siltstone from the middle part of the Stairway Sandstone, near Maloney Creek, Northern Territory, Australia.

Diagnosis.  Fish bearing small scales with three prominent ridges on the external surface: one each in lateral and medial positions producing a broad radiating ornament to the crown and the posterior margin of the crown is prominently scalloped. The scale base is simple and distinguished from the crown by a shallow constriction forming a neck region.

Description.  In the type suite of scales, the dorsal surface of the crown has three prominent ridges that appear to have clearly differentiated cavities (pulp cavities?) underlying them, although preservation hampers illustration and clear confirmation of a polyodontode construction; these prominent ridges diverge slightly from a focal point anterior to the scale margin. The posterior edge of the scales is similarly defined by these ridges, giving a scalloped appearance to this margin. The ventral surface of the crown is striated, perhaps reflecting the edges of individual components of the polyodontode crown. The base is offset anteriorly from the crown in what are presumed flank scales, with the neck region being defined by a shallow constriction, and there appear to be no neck canals emerging from within the neck region. The base is slightly flared.

Remarks.  Acknowledging difficulties inherent to the notion of ‘odontodes’ as irreducible basic units from which teeth and scales are constructed (Donoghue 2002), material studied in the present work includes specimens most easily described as isolated individual ‘odontodes’. Each of these has a distinct neck region and flared base (Fig. 2D). Although these have been abraded, they could be interpreted as components of polyodontode scales formed by marginal accretion, much along the lines of aggregations illustrated by Zangerl (1981, fig. 1). However, like much of the fish material from the Ordovician of the Amadeus Basin, the hard tissue histology of this material has been largely obliterated by diagenetic processes, and so a study of their growth history based upon the internal structure of these specimens is not possible.

The suprageneric assignment of Tantalepis

Although the internal structure and hard tissue composition of these scales is lacking because of very limited preservation of histological detail, the overall appearance of the scales of Tantalepis falls well within the morphological range of chondrichthyan scales from the Middle Palaeozoic, and the only features lacking from the present material that would enable a firm assignment to the Chondrichthyes are neck canals. However, a number of chondrichthyan taxa also lack neck canals. These include, but are not limited to, scale-based fossil taxa attributed to Chondrichthyes such as Frigorilepis (Märss et al. 2002) and Wellingtonella (Märss et al. 2006). Moreover, Hanke and Wilson (2010) comment that the ‘putative chondrichthyans’Seretolepis and Polymerolepis exhibit neck canals only in those scales which were sufficiently deeply embedded within the skin for the interception of blood vessels. Neck canals are not ubiquitous in the scales of modern day chondrichthyans, such as those on the tail of the extant Scyliorhinus (see Johanson et al. 2008).

The considerable stratigraphic gap between the earliest described articulated chondrichthyans (Doliodus problematicus from the early Emsian, Miller et al. 2003; Maisey et al. 2009) and the oldest widely accepted chondrichthyan fossil teeth (Leonodus and Celtiberina from the early Lochkovian; Botella 2006; Botella et al. 2009) and the early Darriwilian occurrence of Tantalepis is populated sporadically by a number of other disarticulated remains that have been referred to the chondrichthyans with a greater (for example Elegestolepis and Tuvalepis; Karatajūtė-Talimaa 1973; Novitskaya and Karatajūtė-Talimaa 1986; Žigaitė and Karatajūtė-Talimaa 2008) or lesser (for example: mongolepids and sinacanthids, Karatajūtė-Talimaa et al. 1990; Karatajūtė-Talimaa and Novitskaya 1992; Karatajūtė-Talimaa 1995; Zhu 1998; Sansom et al. 2000, 2005) degrees of confidence, based largely upon the possession of neck canals by the former and their absence in the latter. Stratigraphically older scales have been recorded, such as the Ordovician taxa described by Sansom et al. (1996) as scale morphology A, from the Sandbian (Sa2) Harding Sandstone of Colorado, as being ‘shark-like’ with a polyodontode construction and possible neck canals (although these may be pulp cavity openings of small odontodes) whilst Young (1997, 2000) considered Areyongalepis, from the middle Darriwilian (Da2?) part of the lower Stokes Siltstone of the Amadeus Basin, to be a chondrichthyan based upon a comparison with the Lochkovian Polymerolepis.

The systematic affinity of these early chondrichthyan-like remains must be considered in the light of Brazeau’s (2009) iconoclastic revision of gnathostome phylogeny. In particular, Brazeau’s analysis lends considerable support to previously rejected hypotheses that have suggested a close phylogenetic relationship between at least some of the acanthodians and chondrichthyans (such as Janvier’s (1996, p. 331) ‘odd phylogeny’, and that promoted by Romer and Grove (1935, p. 845) referring to ‘recognized shark orders, such as the acanthodians and cladoselachians’– although Romer’s (1966) classic textbook clearly places acanthodians closer to osteichthyans), with his study resolving many acanthodian taxa within the chondrichthyan stem, with others falling within the osteichthyan stem; the apparent shift in position considered by Romer and Grove (1935) and Romer (1966) perhaps hinting at this dichotomy within acanthodian characters. Although Brazeau’s (2009) character set effectively diagnoses osteichthyan scale conditions (scales with peg and socket, flattened, a bulging base and the presence of fulcra scales), it is insufficiently refined to provide a clear phylogenetic signal among ‘chondrichthyan’ scale characters, be they from scale-based taxa or those based upon articulated remains. The scales of a number of the chondrichthyans that Brazeau considered, such as Orthacanthus, Pucapampella and Cladodoides, have not been described, and others, such as Akmonistion and Cladoselache (Coates and Sequeira (2001) and Coates, pers. obs. respectively) have a significantly reduced scale cover and those scales that are present exhibit highly specialized morphologies. In addition, although Brazeau’s data matrix is not configured to capture a phylogenetic signal among scale data, his analysis reveals a curious pattern whereby taxa bearing Nostolepis-type scales (sensuValiukevičius 1995) fall within both the chondrichthyan clade and the osteichthyan stem. Clearly, many scale-based taxa need to be reconsidered in the light of Brazeau’s (2009) phylogenetic hypothesis, but additional coding of scale characters from those taxa with articulated remains is required to try and develop a reliable signal of plesiomorphic versus derived characters within early crown gnathostomes.

Acknowledgements.  This work was largely funded by Natural Environment Research Council Grant NE/B503576/1 to IJS with additional funding from the American Recovery and Reinvestment Act of 2009, NSF Award number: 0917922 to MIC. IJS and NSD would like to thank Colin Gatehouse for his considerable assistance during the collection of this material in the field. Kevin Burkhill (University of Birmingham) is also thanked for drafting Fig. 1. We are grateful to two anonymous reviewers for their suggestions on improving this manuscript from the initial submission.

Editor. Philip Donoghue