The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins.

Modern gnathostomes (jawed vertebrates) emerged in the early Palaeozoic era, but this event remains unclear owing to a scant early fossil record. The exclusively Palaeozoic acanthodians are possibly the earliest gnathostome group and exhibit a mosaic of shark- and bony fish-like characters that has long given them prominence in discussions of early gnathostome evolution. Their relationships with modern gnathostomes have remained mysterious, partly because their un-mineralized endoskeletons rarely fossilized. Here I present the first-known braincase of an Early Devonian (approximately 418-412 Myr bp) acanthodian, Ptomacanthus anglicus, and re-evaluate the interrelationships of basal gnathostomes. Acanthodian braincases have previously been represented by a single genus, Acanthodes, which occurs more than 100 million years later in the fossil record. The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes in exhibiting several plesiomorphic features shared with placoderms and some early chondrichthyans. Most striking is its extremely short sphenoid region and its jaw suspension, which displays features intermediate between some Palaeozoic chondrichthyans and osteichthyans. Phylogenetic analysis resolves Ptomacanthus as either the most basal chondrichthyan or as the sister group of all living gnathostomes. These new data alter earlier conceptions of basal gnathostome phylogeny and thus help to provide a more detailed picture of the acquisition of early gnathostome characters

Early Gnathostome Phylogeny Revisited: Multiple Method Consensus

This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.A series of recent studies recovered consistent phylogenetic scenarios of jawed vertebrates, such as the paraphyly of placoderms with respect to crown gnathostomes, and antiarchs as the sister group of all other jawed vertebrates. However, some of the hylogenetic relationships within the group have remained controversial, such as the positions of Entelognathus, ptyctodontids, and the Guiyu-lineage that comprises Guiyu, Psarolepis and Achoania. The revision of the dataset in a recent study reveals a modified phylogenetic hypothesis, which shows that some of these phylogenetic conflicts were sourced from a few inadvertent miscodings. The interrelationships of early gnathostomes are addressed based on a combined new dataset with 103 taxa and 335 characters, which is the most comprehensive morphological dataset constructed to date. This dataset is investigated in a phylogenetic context using maximum parsimony (MP), Bayesian inference (BI) and maximum likelihood (ML) approaches in an attempt to explore the consensus and incongruence between the hypotheses of early gnathostome interrelationships recovered from different methods. Our findings consistently corroborate the paraphyly of placoderms, all `acanthodians' as a paraphyletic stem group of chondrichthyans, Entelognathus as a stem gnathostome, and the Guiyu-lineage as stem sarcopterygians. The incongruence using different methods is less significant than the consensus, and mainly relates to the positions of the placoderm Wuttagoonaspis, the stem chondrichthyan Ramirosuarezia, and the stem osteichthyan LophosteusÐthe taxa that are either poorly known or highly specialized in character complement. Given that the different performances of each phylogenetic approach, our study provides an empirical case that the multiple phylogenetic analyses of morphological data are mutually complementary rather than redundant

A critical appraisal of appendage disparity and homology in fishes

Fishes are both extremely diverse and morphologically disparate. Part of this disparity can be observed in the numerous possible fin configurations that may differ in terms of the number of fins as well as fin shapes, sizes and relative positions on the body. Here, we thoroughly review the major patterns of disparity in fin configurations for each major group of fishes and discuss how median and paired fin homologies have been interpreted over time. When taking into account the entire span of fish diversity, including both extant and fossil taxa, the disparity in fin morphologies greatly complicates inferring homologies for individual fins. Given the phylogenetic scope of this review, structural and topological criteria appear to be the most useful indicators of fin identity. We further suggest that it may be advantageous to consider some of these fin homologies as nested within the larger framework of homologous fin‐forming morphogenetic fields. We also discuss scenarios of appendage evolution and suggest that modularity may have played a key role in appendage disparification. Fin modules re‐expressed within the boundaries of fin‐forming fields could explain how some fins may have evolved numerous times independently in separate lineages (e.g., adipose fin), or how new fins may have evolved over time (e.g., anterior and posterior dorsal fins, pectoral and pelvic fins). We favour an evolutionary scenario whereby median appendages appeared from a unique field of competence first positioned throughout the dorsal and ventral midlines, which was then redeployed laterally leading to paired appendages.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151971/1/faf12402_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151971/2/faf12402.pd

Placoderm interrelationships: a new interpretation, with a short review of placoderm classifications

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Exceptional preservation of nerve and muscle tissues in Late Devonian placoderm fish and their evolutionary implications

In this paper, we show exceptional three-dimensionally preserved fossilized muscle tissues in 380–384 Myr old placoderm fish (Late Devonian), offering new morphological evidence supporting the hypothesis that placoderms are the sister group to all other gnathostomes. We describe the oldest soft tissue discovered in gnathostomes, which includes striated muscle fibres, circulatory and nerve tissues, preserved as phosphatized structures precipitated by microbial infilling of small, protected areas under the headshield of the arthrodire, Eastmanosteus calliaspis. Muscle impressions have also been found in the ptyctodontid, Austroptyctodus gardineri. The specimens display primitive vertebrate muscle structures; in particular, shallow W-shaped muscle blocks such as those observed in lampreys. New information from fossilized soft tissues thus elucidates the affinities of the placoderms and provides new insights into the evolution and radiation of gnathostomes

Number and arrangement of extraocular muscles in primitive gnathostomes: evidence from extinct placoderm fishes

Exceptional braincase preservation in some Devonian placoderm fishes permits interpretation of muscles and cranial nerves controlling eye movement. Placoderms are the only jawed vertebrates with anterior/posterior obliques as in the jawless lamprey, but with the same function as the superior/inferior obliques of other gnathostomes. Evidence of up to seven extraocular muscles suggests that this may be the primitive number for jawed vertebrates. Two muscles innervated by cranial nerve 6 suggest homologies with lampreys and tetrapods. If the extra muscle acquired by gnathostomes was the internal rectus, Devonian fossils show that it had a similar insertion above and behind the eyestalk in both placoderms and basal osteichthyans