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opencc-zeroAug 2022View details →

Squamation and scale morphology at the root of jawed vertebrates

Placoderms, as the earliest branching jawed vertebrates, are crucial to understanding how the characters of crown gnathostomes comprising Chondrichthyes and Osteichthyes evolved from their stem relatives. Despite the growing knowledge of the anatomy and diversity of placoderms over the past decade, the dermal scales of placoderms are predominantly known from isolated material, either morphologically or histologically, resulting in their squamation being poorly understood. Here we provide a comprehensive description of the squamation and scale morphology of a primitive taxon of Antiarcha (a clade at the root of jawed vertebrates), Parayunnanolepis xitunensis, based on the virtual restoration of an articulated specimen by using X-ray computed tomography. Thirteen morphotypes of scales are classified to exhibit how the morphology changes with their position on the body in primitive antiarchs, based on which nine areas of the post-thoracic body are distinguished to show their scale variations in the dorsal, flank, ventral, and caudal lobe regions. In this study, the histological structure of yunnanolepidoid scales is described for the first time based on disarticulated scales from the type locality and horizon of P. xitunensis. The results demonstrate that yunnanolepidoid scales are remarkably different from their dermal plates as well as euantiarch scales in lack of a well-developed middle layer. Together, our study reveals that the high regionalization of squamation and the bipartite histological structure of scales might be plesiomorphic for antiarchs, and jawed vertebrates in general.

opencc-zeroMay 2024View details →

Supplementary files for: The oldest complete jawed vertebrates from the early Silurian of China

Molecular studies suggest that the origin of jawed vertebrates was no later than the Late Ordovician period (around 450 million years ago (Ma)). Together with disarticulated micro-remains of putative chondrichthyans from the Ordovician and early Silurian period, these analyses suggest an evolutionary proliferation of jawed vertebrates before, and immediately after, the end-Ordovician mass extinction. However, until now, the earliest complete fossils of jawed fishes for which a detailed reconstruction of their morphology was possible came from late Silurian assemblages (about 425 Ma). The dearth of articulated, whole-body fossils from before the late Silurian has long rendered the earliest history of jawed vertebrates obscure. Here we report a newly discovered Konservat-Lagerstätte, which is marked by the presence of diverse, well-preserved jawed fishes with complete bodies, from the early Silurian (Telychian age, around 436 Ma) of Chongqing, South China. The dominant species, a 'placoderm' or jawed stem gnathostome, which we name Xiushanosteus mirabilis gen. et sp. nov., combines characters from major placoderm subgroups and foreshadows the transformation of the skull roof pattern from the placoderm to the osteichthyan condition. The chondrichthyan Shenacanthus vermiformis gen. et sp. nov. exhibits extensive thoracic armour plates that were previously unknown in this lineage, and include a large median dorsal plate as in placoderms, combined with a conventional chondrichthyan bauplan. Together, these species reveal a previously unseen diversification of jawed vertebrates in the early Silurian, and provide detailed insights into the whole-body morphology of the jawed vertebrates of this period.

publicMay 2024View details →

Supplementary files for: The oldest complete jawed vertebrates from the early Silurian of China

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Squamation and scale morphology at the root of jawed vertebrates

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dryad32/100

Increasing morphological disparity and decreasing optimality for jaw speed and strength during the radiation of jawed vertebrates

The Siluro-Devonian adaptive radiation of jawed vertebrates, which underpins almost all living vertebrate biodiversity, is characterised by the evolutionary innovation of the lower jaw. Multiple lines of evidence have suggested that the jaw evolved from a rostral gill arch, but when the jaw took on a feeding function remains unclear. We quantified the variety of form in the earliest jaws in the fossil record and , from which we generated a range of theoretical morphospacelogies within this morphological range, which that we then tested for their functional optimality. By drawing comparisons with the real jaw data and reconstructed ancestral forms, our results show that the earliest jaw shapes were optimised for fast closure and stress resistance, inferring a feeding rather than solely ventilation function. Jaw shapes then became less optimal for these functions during the later radiation of jawed vertebrates. Thus, the evolution of jaw morphology has continually explored new morphospace and accumulated disparity through time, laying the foundation for diverse feeding strategies and the success of jawed vertebrates.

opencc-zeroJan 2022View details →

Figure 2 in The characters of Palaeozoic jawed vertebrates

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 9 in The characters of Palaeozoic jawed vertebrates

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 1 in The characters of Palaeozoic jawed vertebrates

Figure 1. Assumed phylogenetic framework for the principal extant clades of vertebrates used in this analysis.

Figure 13 in The characters of Palaeozoic jawed vertebrates

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.

Figure 5 in The characters of Palaeozoic jawed vertebrates

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 8 in The characters of Palaeozoic jawed vertebrates

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 4 in The characters of Palaeozoic jawed vertebrates

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 7 in The characters of Palaeozoic jawed vertebrates

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 12 in The characters of Palaeozoic jawed vertebrates

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 3 in The characters of Palaeozoic jawed vertebrates

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 6 in The characters of Palaeozoic jawed vertebrates

Figure 6. External neurocranial anatomy in lateral view. A, Norselaspis, an osteostracan (after Janvier, 1981b). B, Macropetalichthys, a petalichthyid placoderm (after Stensiö, 1969; 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 10. Gnathostome dental anatomy. A in The characters of Palaeozoic jawed vertebrates

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 in The characters of Palaeozoic jawed vertebrates

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.