The characters of Palaeozoic jawed vertebrates

Abstract

Newly discovered fossils from the Silurian and Devonian periods are beginning to challenge embedded perceptions about the origin and early diversification of jawed vertebrates (gnathostomes). Nevertheless, an explicit cladistic framework for the relationships of these fossils relative to the principal crown lineages of the jawed vertebrates (osteichthyans: bony fishes and tetrapods; chondrichthyans: sharks, batoids, and chimaeras) remains elusive. We critically review the systematics and character distributions of early gnathostomes and provide a clearly stated hierarchy of synapomorphies covering the jaw-bearing stem gnathostomes and osteichthyan and chondrichthyan stem groups. We show that character lists, designed to support the monophyly of putative groups, tend to overstate their strength and lack cladistic corroboration. By contrast, synapomorphic hierarchies are more open to refutation and must explicitly confront conflicting evidence. Our proposed synapomorphy scheme is used to evaluate the status of the problematic fossil groups Acanthodii and Placodermi, and suggest profitable avenues for future research. We interpret placoderms as a paraphyletic array of stem-group gnathostomes, and suggest what we regard as two equally plausible placements of acanthodians: exclusively on the chondrichthyan stem, or distributed on both the chondrichthyan and osteichthyan stems.

Keywords: Acanthodii; Chondrichthyes; Gnathostomata; Osteichthyes; Placodermi; morphology; palaeontology; plesiomorphy; synapomorphy.

Figure 1

Assumed phylogenetic framework for the principal extant clades of vertebrates used in this analysis.

Figure 2

Decomposition of a compound character. The proposed synapomorphy of acanthodians (Burrow & Turner, 2010) can be decomposed into separate conditions each having greater levels of generality. A, acanthodians as a monophyletic group and the compound character of Burrow & Turner yields a zero-length branch. B, collapsed Acanthodii, showing true level of support for the individual character components.

Figure 3

Pectoral fins of stem and crown gnathostomes. A, Errivaspis waynensis, NHMUK P.17477, a heterostracan lacking paired fins. B, Hemicyclaspis murchisoni, NHMUK P.8816, an osteostracan with paired fins. C, Cladoselache sp., NHMUK P.9276, a crown gnathostome and chondrichthyan with paired fins. Scale bars = 10 mm.

Figure 4

Tail geometry of stem and crown gnathostomes. A, Errivaspis waynensis, NHMUK P.17477, a heterostracan. B, Birkenia sp., NHMUK P.42020 (image reversed), an anaspid. C, ‘Cephalaspis’ powriei, NHMUK P.670, an osteostracan. D, Promesacanthus eppleri, UALVP 42652, an acanthodian. Scale bars = 10 mm.

Figure 5

Endoskeletal mineralization of gnathostomes. A, Buchanosteus confertituberculatus, NHMUK P.48675, an arthrodire placoderm. Fractured postorbital process/lateral commissure showing perichondral lining of canals, but absence of endochondral ossification. B, Griphognathus whitei, NHMUK P.52574, a crown osteichthyan and crown sarcopterygian. Ethmoid region showing perichondrally lined canals for olfactory tracts, surrounded by endochondral ossification. C, Tristychius arcuatus, NHMUK P.57305/6, a crown chondrichthyan and stem elasmobranch. Fragment of cranial skeleton showing prismatic calcified cartilage. D, Helodus simplex, NHMUK P.8212, a crown chondrichthyan and stem holocephalan. Basicranial region showing prismatic calcified cartilage. Scale bars = 5 mm.

Figure 6

External neurocranial anatomy in lateral view. A, Norselaspis, an osteostracan (after Janvier, 1981b). B, Macropetalichthys, a petalichthyid placoderm (after Stensiö, ; Young, 1980). C, Dicksonosteus, an arthrodire placoderm (after Goujet, 1984a). D, Cladodoides, a chondrichthyan (after Maisey, 2005). E, Mimpiscis, a crown osteichthyan and actinopterygian (after Gardiner, 1984b). Abbreviation: N.II, opening for the optic tract (second cranial nerve). Not drawn to scale.

Figure 7

Endocranial cavities of various gnathostomes. A, Benneviaspis, an osteostracan (after Janvier, 1985a). B, Brindabellaspis, a placoderm (after Young, 1980). C, Kujdanowiaspis, an arthrodire placoderm (after Goujet, 1984a). D, Buchanosteus, an arthrodire placoderm (after Young, 1979). E, Cladodoides, a crown gnathostome and chondrichthyan (after Maisey, 2005). A, B, in dorsal view. C, D, E, in ventral view. Abbreviations: N.VII, canal or openings for the facial nerve (seventh cranial nerve); hm, hyomandibular branch; pal, palatine branch. Not drawn to scale.

Figure 8

Gnathostome neurocrania in dorsal view. A, Dicksonosteus, an arthrodire placoderm (after Goujet, 1984a). B, Lawrenciella, a crown osteichthyan and crown actinopterygian (after Hamel & Poplin, 2008). C, cf. Cobelodus, a chondrichthyan and possible stem holocephalan (after Maisey, 2007). D, Orthacanthus, a chondrichthyan and possible stem elasmobranch (after Schaeffer, 1981).

Figure 9

Gnathostome neurocrania in ventral view. A, Acanthodes, an acanthodian (after Davis et al., 2012). B, Gogonasus, a crown osteichthyan and crown sarcopterygian (after Long, Barwick & Campbell, 1997). C, Mimipiscis, a crown osteichthyan and actinopterygian (after Gardiner, 1984b). D, Dicksonosteus, an arthrodire placoderm (after Goujet, 1984a). E, Cladodoides, a chondrichthyan and possible stem elasmobranch (after Maisey, 2005). F, Pucapampella sp. a probable stem chondrichthyan (after Maisey, 2001).

Figure 10

Gnathostome dental anatomy. A, Torosteus pulchellus, NHMUK P.50966, an arthrodire placoderm. B, Ischnacanthus gracilis, NMS 1887.35.2, an acanthodian. C, Cladoselache sp., NHMUK P.9272, a crown gnathostome and chondrichthyan. D, Onychodus jandemarrai, NHMUK P.63576 (image reversed), a crown osteichthyan and sarcopterygian. Scale bars = 10 mm.

Figure 11

Dorsal fin spines with trailing edge denticles, a potential synapomorphy of chondrichthyans. A, Brochoadmones milesi, UALVP 41495, an acanthodian. B, Tristychius arcuatus, NHMUK P.11378-79, a crown chondrichthyan and stem elasmobranch. Scale bars = 10 mm.

Figure 12

Alternative phylogenetic placements for problematic acanthodian-like taxa and their implications in light of two characters discussed in the text and assumptions of acanthodian monophyly. A, one of the hypotheses implied by placing only Kathemacanthus and Seretolepis on the chondrichthyan stem, independent of other acanthodians. B, equally parsimonious placement of Kathemacanthus and Seretolepis on osteichthyan stem. C, consensus tree showing that resolution of Kathemacanthus and Seretolepis to the chondrichthyan stem collapses if all other acanthodians are placed on the osteichthyan stem (areal scale growth is plesiomorphic). D–F, improvements to parsimony score if some or all acanthodians are moved to the chondrichthyan stem (restoration of areal scale growth as a chondrichthyan synapomorphy). D, hypothesis in which all acanthodians are stem chondrichthyans, but paraphyletic. E, hypothesis in which all acanthodians are stem chondrichthyans but monophyletic. F, hypothesis in which taxa with areally growing scales are stem chondrichthyans, whereas the remaining (assumed monophyletic) Acanthodii are stem osteichthyans. Ambiguities in character state distributions based on a soft polytomy may entail different lengths depending on their resolution. Note that loss of areal scale growth in Acanthodii and Osteichthyes reflects transitions to different, not identical, states and must therefore be treated as separate events. Asterisk indicates values derived from resolving stem chondrichthyan polytomy as a paraphylum with respect to the crown.

Figure 13

Summary cladograms of hypotheses of phylogenetic placements argued in this paper. A, cladogram depicting acanthodian genera distributed on the chondrichthyan and osteichthyan stems. B, cladogram depicting acanthodians restricted to chondrichthyan stem, but left unresolved. Character transformation labels at internal nodes correspond to those in the text. Numbers in parentheses reflect ambiguities that are resolved to their most inclusive level (i.e. ‘accelerated transformation’) and could have more restricted distributions.