A three‐dimensional placoderm (stem‐group gnathostome) pharyngeal skeleton and its implications for primitive gnathostome pharyngeal architecture

Abstract

The pharyngeal skeleton is a key vertebrate anatomical system in debates on the origin of jaws and gnathostome (jawed vertebrate) feeding. Furthermore, it offers considerable potential as a source of phylogenetic data. Well‐preserved examples of pharyngeal skeletons from stem‐group gnathostomes remain poorly known. Here, we describe an articulated, nearly complete pharyngeal skeleton in an Early Devonian placoderm fish, Paraplesiobatis heinrichsi Broili, from Hunsrück Slate of Germany. Using synchrotron light tomography, we resolve and reconstruct the three‐dimensional gill arch architecture of Paraplesiobatis and compare it with other gnathostomes. The preserved pharyngeal skeleton comprises elements of the hyoid arch (probable ceratohyal) and a series of branchial arches. Limited resolution in the tomography scan causes some uncertainty in interpreting the exact number of arches preserved. However, at least four branchial arches are present. The final and penultimate arches are connected as in osteichthyans. A single median basihyal is present as in chondrichthyans. No dorsal (epibranchial or pharyngobranchial) elements are observed. The structure of the pharyngeal skeleton of Paraplesiobatis agrees well with Pseudopetalichthys from the same deposit, allowing an alternative interpretation of the latter taxon. The phylogenetic significance of Paraplesiobatis is considered. A median basihyal is likely an ancestral gnathostome character, probably with some connection to both the hyoid and the first branchial arch pair. Unpaired basibranchial bones may be independently derived in chondrichthyans and osteichthyans.

An early chondrichthyan and the evolutionary assembly of a shark body plan

“…The jaws, hyoid arch and gill skeleton are exceptionally complete (figures 1a,b and 2; electronic supplementary material, figure S5), providing ready comparison with recently described in-group [15,54] and out-group [55] examples. The mandibular arch morphology (electronic supplementary material, figure S6) is more conventional than previously understood.…”

“…The fifth arch ceratobranchials are unusually broad, nearly rectangular and keeled along the anterior margin. Remarkably, and uniquely, these resemble the posteriormost ceratobranchials of Paraplesiobatis, a Lower Devonian 'placoderm' [55] (electronic supplementary material, figure S8). The ceratobranchials of Gladbachus' fourth and fifth gill arches articulate with a large basibranchial copula, which is separated from the basihyal process by a large gap.…”

The pharynx of the stem-chondrichthyan Ptomacanthus and the early evolution of the gnathostome gill skeleton

“…Raynerius 16 ) and stem-gnathostomes (eg. Paraplesiobatis 21 ) this means that an osteichthyan-like pharynx can be placed at the gnathostome crown-node with a reasonable degree of confidence. The ostensibly osteichthyan-like condition in extant holocephalans is likely homoplasious, given the presence of an elasmobranch-like pharynx in the stem-chondrichthyan Gladbachus , and the probable stem-holocephalan Ozarcus 5,6 .…”

“…Raynerius 16 ) and stem-gnathostomes (eg. Paraplesiobatis 21 ) the arches lie mostly ventrally relative to the neurocranium. Contrastingly in the elasmobranch condition they extend well caudally (eg.…”

Was the Devonian placoderm Titanichthys a suspension feeder?

“…If Titanichthys was indeed a suspension feeder, presumably it would have fed in a roughly analogous manner to modern planktivorous fish, which separate prey from water entering the oral cavity using elaborate or ornamented gill rakers (this was also the suspension-feeding method of planktivorous pachycormids [12]). Placoderm gill arches are rarely preserved [13], so the absence of a fossil suspension-feeding structure may be an artefact of the poor fossil record [14], or it may indicate that Titanichthys was not a suspension feeder.…”