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.
Highlights
In osteichthyans, the ceratohyals attach to a median bone via hypohyals, (Allis, 1897, 1922; Grande & Bemis, 1998; Jarvik, 1954, 1972) the latter of which are usually considered absent in chondrichthyans
A comparative analysis of basal gill arch elements in early and modern gnathostomes is here used to infer some aspects of primitive branchial arch patterns
This investigation is based on the type specimen of Paraplesiobatis heinrichsi Broili (1933) from the Schlossparkmusuem (Karl-Geib-Museum), Bad Kreuznach KGM 1983/294
Summary
Around the throat and meeting at the ventral midline, where one or more medial (or paired) corpuses may connect one or more arches. The two main divisions of the gnathostome crown group (Osteichthyes and Chondrichthyes) differ significantly in the pattern of topological relationships of their pharyngeal arch segments, in the ventral elements Both chondrichthyans and osteichthyans possess one or more median basibranchial elements. In osteichthyans, the ceratohyals attach to a median bone (usually termed a basibranchial) via hypohyals, (Allis, 1897, 1922; Grande & Bemis, 1998; Jarvik, 1954, 1972) the latter of which are usually considered absent in chondrichthyans In both chondrichthyans and osteichthyans, each branchial arch terminates ventrally and medially by a hypobranchial bone or cartilage, which may connect either at the midline to its antimere or to a ventral basibranchial bone or cartilage. Partially ossified examples exist in the placoderms: an assemblage of Paleozoic jaw-bearing stem-group gnathostomes These gill skeletons are weakly mineralized and tend to be poorly preserved. A comparative analysis of basal gill arch elements in early and modern gnathostomes is here used to infer some aspects of primitive branchial arch patterns
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