The LRT confirms a 2015 paper: sharks had a semi-bony past

Long et al. 2015
described a few bits and pieces from a Late Devonian shark, Gogoselachus (Fig 1), with a “highly distinctive type of calcified cartilage forming the endoskeleton.”

I came across this 2015 paper a few days ago, long after sharks nested as taxa derived from cartilaginous sturgeon and semi-bony paddlefish ancestors in the Large Reptile Tree (LRT, 1744+ taxa, subset Fig. 2). So having a basal shark with ‘calcified cartilage’ comes as no surprise to the LRT.

Figure 1. Gogoselache restoration based on the few parts shown here.

Figure 1. Gogoselache restoration based on the few parts shown here from Long et al. 2015.

From the Long et al. ‘Background’
“Living 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).”

This is the traditional view of fish interrelations, the one told in lectures and textbooks. See figure 2 for the more complete LRT view.

“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.”

Figure x. Subset of the LRT focusing on fish.

Figure 2. Subset of the LRT focusing on fish.

From the Long et al. 2015 Methodology/Principal Findings
“Here 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.”

From the Long et al. 2015 Conclusions/Significance
“The Meckel’s cartilages show a jaw articulation surface dominated by an expansive cotylus, and a small mandibular knob, an unusual condition for chondrichthyans.”

In the LRT the Meckel’s cartilages are scored as fused mandible bones based on ancestral states in which the mandible bones are not fused.

“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.”

In the LRT phylogenetic analysis, rather than a few interesting and traditionally unexpected traits, elucidates interrelationships. Having bony traces in a basal shark skeleton could have been predicted from the LRT, but the fish portion of the LRT came later, this time providing confirmation rather than prediction.

The Long et al. 2015 paper
earned a fair bit of publicity five years ago.

“Fossil ancestor shows sharks have a bony past.
This study further supports the idea that sharks must have evolved from bony primitive ancestors and lost their bone early on in the race as they acquired their predatory body shape.”

“No bones about it. Sharks evolved cartilage for a reason.
Most people know that sharks have a distinctive, all-cartilage skeleton, but now a fossil from Western Australia has revealed a surprise ‘missing link’ to an earlier, more bony form of the fish.”

“In testing fossil remains discovered by Professor Long in July 2005 at Gogo in the Kimberley in Western Australia, detailed CT scanning analysis has shown that the three-dimensional remnant skeleton contains a small proportion of bone as well as cartilage.”

“Because sharks and rays have entirely cartilaginous skeletons, Professor Long said it was traditionally thought that they were part of a primitive evolutionary pathway, and that bone in other fish was the more advanced condition. But a series of discoveries in recent years has suggested that sharks are “more evolutionarily derived”, and are likely to be descended from bony ancestors.”

Long JA, Burrow CJ, Ginter M, Maisey JG, Trinajstic KM, et al. 2015. First Shark from the Late Devonian (Frasnian) Gogo Formation, Western Australia Sheds New Light on the Development of Tessellated Calcified Cartilage.” PLoS ONE10(5): e0126066. DOI: 10.1371/journal.pone.0126066


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