First evidence of a functional spiracle in stem chondrichthyan acanthodians, with the oldest known elastic cartilage. Issue 6 (16th February 2020)

• Record Type: Journal Article
• Authors: Burrow, Carole J.; Newman, Michael J.; den Blaauwen, Jan L.
• Is Part Of: Journal of anatomy. Volume 236:Issue 6(2020)
• Page Start: 1154
• Page End: 1159
• Publication Date: 2020-02-16
• Subjects: branchial skeleton -- mineralised cartilage -- pseudobranch -- spiracular function -- stem Chondrichthyes; Anatomy -- Periodicals; 571.3
• DOI: 10.1111/joa.13170
• Languages: English
• ISSNs: 0021-8782

Abstract

Abstract: Spiracles are a general character of gnathostomes (jawed fishes), being present in antiarch placoderms, commonly regarded as the most basal gnathostome group. The presence of spiracular tubes in acanthodians has been deduced from grooves on the neurocranium of the derived acanthodiform Acanthodes bronni from the Permian of Germany, but until now these tubes were presumed to lack an external opening, rendering them non‐functional. Here we describe the external spiracular elements in specimens of the Middle Devonian acanthodiforms Cheiracanthus murchisoni, Cheiracanthus latus and Mesacanthus pusillus from northern Scotland, and the internal structure of these elements in C. murchisoni, demonstrating that the spiracle in acanthodiforms differed from all known extant and extinct fishes in having paired cartilage‐pseudobranch structures. This arrangement represents a transitional state between the presumed basal gnathostome condition with an unconstricted first gill slit (as yet not identified in any fossil) and the derived condition with a spiracle and a single pseudobranch derived from the posterior hemibranch of the mandibular arch. We identify the main tissue forming the pseudobranch as elastic cartilage, a tissue previously unrecorded in fossils. Abstract : In this study, we found the first evidence of a paired spiracular structure in the first gill arch position on ancient stem sharks, representing the only known example of a transitional state between the fully functional first gill postulated (but not yet identified) to have occurred in stem jawed fishes and the single, reduced pseudobranch present in more derived taxa. We have also identified the oldest record of elastic cartilage in the fossil record.