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

Abstract

The gill apparatus of gnathostomes (jawed vertebrates) is fundamental to feeding and ventilation and a focal point of classic hypotheses on the origin of jaws and paired appendages. The gill skeletons of chondrichthyans (sharks, batoids, chimaeras) have often been assumed to reflect ancestral states. However, only a handful of early chondrichthyan gill skeletons are known and palaeontological work is increasingly challenging other pre-supposed shark-like aspects of ancestral gnathostomes. Here we use computed tomography scanning to image the three-dimensionally preserved branchial apparatus in Ptomacanthus , a 415 million year old stem-chondrichthyan. Ptomacanthus had an osteichthyan-like compact pharynx with a bony operculum helping constrain the origin of an elongate elasmobranch-like pharynx to the chondrichthyan stem-group, rather than it representing an ancestral condition of the crown-group. A mixture of chondrichthyan-like and plesiomorphic pharyngeal patterning in Ptomacanthus challenges the idea that the ancestral gnathostome pharynx conformed to a morphologically complete ancestral type.

Evolution of vertebrate gill covers via shifts in an ancient Pou3f3 enhancer

“…We propose that in an ancestral gnathostome, co-option of the neural gene Pou3f3 into the arches, through acquisition of the 1B arch enhancer, conferred a new capacity for mesenchymal growth and skeletal differentiation that led to the formation of a gill cover. Recent discoveries of a stem gnathostome 21 and a stem chondrichthyan 22 with prominent hyoid arch bony opercula are consistent with a scenario in which the last common ancestor of extant jawed vertebrates possessed a single, hyoid arch-derived gill cover. The four posterior gill covers of cartilaginous fishes would therefore have evolved after the two lineages diverged 23 , perhaps coincident with sequence changes in the 1B enhancer permitting robust Pou3f3 expression in the posterior arches.…”

The early elasmobranch Phoebodus : phylogenetic relationships, ecomorphology and a new time-scale for shark evolution

“…generative tooth sets) along the jaw margins (character 68). Hypohyals, reported in Thrinacodus [11] but unknown in Phoebodus, have a distinctly patchy and possibly homoplastic distribution [22], and it seems likely that tooth sets could have proliferated quite independently to populate the differently shaped jaws of xenacanths and phoebodonts.…”

Diverse stem-chondrichthyan oral structures and evidence for an independently acquired acanthodid dentition

“…However, it is difficult to assess whether non-shedding parasymphyseal whorls are homologous due to the unclear condition in psarolepids (variably interpreted as stem sarcopterygians or stem osteichthyans (48-52)), in which a whorl is inferred (48,52) but is yet to be described. Finally, the interposition of many non-shedding stem-chondrichthyan taxa between shedding chondrichthyans and shedding osteichthyans confirms that a shedding dentition evolved twice, in two different ways, in crown-gnathostomes (7,10) (3,5,8,11,16,32,41,45,53,54).…”