First Shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia Sheds New Light on the Development of Tessellated Calcified Cartilage

Abstract

BackgroundLiving gnathostomes (jawed vertebrates) comprise two divisions, Chondrichthyes (cartilaginous fishes, including euchondrichthyans with prismatic calcified cartilage, and extinct stem chondrichthyans) and Osteichthyes (bony fishes including tetrapods). Most of the early chondrichthyan (‘shark’) record is based upon isolated teeth, spines, and scales, with the oldest articulated sharks that exhibit major diagnostic characters of the group—prismatic calcified cartilage and pelvic claspers in males—being from the latest Devonian, c. 360 Mya. This paucity of information about early chondrichthyan anatomy is mainly due to their lack of endoskeletal bone and consequent low preservation potential.Methodology/Principal FindingsHere we present new data from the first well-preserved chondrichthyan fossil from the early Late Devonian (ca. 380–384 Mya) Gogo Formation Lägerstatte of Western Australia. The specimen is the first Devonian shark body fossil to be acid-prepared, revealing the endoskeletal elements as three-dimensional undistorted units: Meckel’s cartilages, nasal, ceratohyal, basibranchial and possible epibranchial cartilages, plus left and right scapulocoracoids, as well as teeth and scales. This unique specimen is assigned to Gogoselachus lynnbeazleyae n. gen. n. sp.Conclusions/SignificanceThe Meckel’s cartilages show a jaw articulation surface dominated by an expansive cotylus, and a small mandibular knob, an unusual condition for chondrichthyans. The scapulocoracoid of the new specimen shows evidence of two pectoral fin basal articulation facets, differing from the standard condition for early gnathostomes which have either one or three articulations. The tooth structure is intermediate between the ‘primitive’ ctenacanthiform and symmoriiform condition, and more derived forms with a euselachian-type base. Of special interest is the highly distinctive type of calcified cartilage forming the endoskeleton, comprising multiple layers of nonprismatic subpolygonal tesserae separated by a cellular matrix, interpreted as a transitional step toward the tessellated prismatic calcified cartilage that is recognized as the main diagnostic character of the chondrichthyans.

Biological responses of sharks to ocean acidification

“…The physiological effects of simulated end-of-century elevated CO 2 conditions have only been evaluated in four relatively sedentary, benthic species: the temperate lesserspotted (Scyliorhinus canicula) catshark [38] and Port Jackson (H. portusjacksoni) sharks [39,40] and the tropical bamboo (C. punctatum) [32][33][34] and epaulette (H. ocellatum) sharks [35,36] (table 1). Previous studies investigating physiological processes under elevated CO 2 in sharks have been conducted at very high CO 2 levels (.8-10 mm Hg, approximately 10 000-13 000 matm) (e.g.…”

“…The first reports of their most reliable diagnostic feature-the tesserate mode of cartilage mineralization-are from late Devonian deposits (approx. 380 Mya) [2], though the first scales and spines of Chondrichthyans appeared already in the Lower Silurian [3,4]. Presently, the cartilaginous fishes (comprising approximately 1200 species) are found throughout all of the world's oceans, and many occupy high trophic levels in marine habitats [5,6] where they can exert a fundamental influence (top-down control) on the structure and function of communities [7,8].…”

Ultrastructural and developmental features of the tessellated endoskeleton of elasmobranchs (sharks and rays)

“…Whereas the vast majority (~98%) of vertebrate species have bony endoskeletons, the skeletons of elasmobranch fishes (sharks, rays and relatives) are comprised largely of unmineralized hyaline-like cartilage (Leydig, 1852;Hall, 2005;Currey, 2002;Atkins et al 2014). As unmineralized cartilage is considerably less stiff than bone (Ashby et al 1995), it is remarkable that sharks and rays represent such an evolutionary successful taxon, constituting some of the largest and fastest marine apex predators for more than 400 million years (Maisey, 2013;Long et al 2015). The high performance of elasmobranch cartilage is surely linked to the fact that the majority of the skeleton is essentially armored: the uncalcified cartilaginous core of each piece of the skeleton is covered by a layer of mineralized tiles called tesserae, and then further wrapped in an outer sheath of fibrous connective tissue (perichondrium; Fig.…”

Chondrocranial morphology of Carcharopsis wortheni (Chondrichthyes, Euselachii incertae sedis) based on new material from the Fayetteville Shale (upper Mississippian, middle Chesterian)

“…The systematic distribution of ducts and other features of the vascular complex have not been adequately surveyed in other Palaeozoic chondrichthyan teeth, but large ducts are absent in the scanned Gogoselachus tooth figured by Long et al . (, fig. 5).…”