Cladoselache and Doliodus enter the LRT

The iconic Devonian shark, Cladoselache
(Fig. 1), enters the large reptile tree (LRT, 1463 taxa) nesting with the extant mako shark, Isurus. That’s to be expected. They are both traditional sharks.

Surprisingly,
the Cladoselache specimen employed (Fig. 2) did not have the iconic bullet-shaped rostrum with a terminal mandible used in every illustration we’ve seen for this taxon. Rather, the rostrum extended some distance over the mandible and it expanded laterally, creating a disc-shape in dorsal view.

The laterally extended nasal on Cladoselache
is a trait shared with the related sturgeon, Pseudoscaphirhychus, further tying sharks and sturgeons closer together.

Cladoselache acanthopterygius (Dean 1894; Late Devonian; 1.8m) is a primitiive shark with a deeply forked tail. Most images show a terminal mouth, but  the example here (Fig. 2) has a typically shark-like underslung mouth and small sharp teeth. A robust spine precedes both dorsal fins. The torso is wider than tall producing a ‘cut-water’ near the tail. The pectoral fins do not have major spines.

Once considered a small shark,
Doliodus problematicus (“problematic deceiver”) also enters the LRT,
but not where Miller, Cloutier and Turner 2003 thought it should. They wrote, “This species has been truly problematic. Previously known only from isolated teeth, it has been identified as an acanthodian and a chondrichthyan [= sharks and rays]. This specimen is the oldest shark showing the tooth families in situ, and preserves one of the oldest chondrichthyan braincases. More notably, it shows the presence of paired pectoral fin-spines, previously unknown in cartilaginous fishes [= sharks and rays].” 

Remember,
this team published their paper(s) before the discovery and publication of the placoderm, Entelognathus, a placoderm that changed our understanding of gnathostome origins and the fish family tree.

Turner and Miller 2004 wrote,
“The most important feature of this fossil is its paired pectoral spines. These suggest that many isolated fossil spines might have belonged to sharks rather than acanthodians as previously believed. Features of the fossil blur the distinction between acanthodians and early chondrichthyans.”

“Textbooks still parrot the conventional thinking that no fossil sharks are found before the late Devonian, but this dogma ignores work from the last three decades. The oldest microfossils definitely attributable to sharks are scales in Silurian strata (440 mya) of Siberian and Arctic Russia. 

“Early Silurian deposits in the Tarim Basin of western China have also yielded fin spines associated with sharklike scales. Are these fossils true sharks? If so, the lineage was apparently toothless for millions of years. Are these fossils true sharks? If so, the lineage was apparently toothless for millions of years. The first indisputable shark teeth do not turn up until about 50 million years after the appearance of these first putative shark scales in the late Ordovician.”

In the LRT,
big-eyed Doliodus (Fig. 4) nests with other big-eyed, spiny-finned acanthodians in the clade of bony fish, not sharks. Some sharks have double-tipped teeth. So does Doliodus by convergence. Sharks are also known from a tooth battery, a conveyer belt lineup of teeth waiting to rotate into place. So does Dolidus, by convergence, although it should be noted that most tetrapods continually replace their teeth. The ‘millimeter-size teeth’ of Doliodus all point toward the tongue (Fig. 4), so the next teeth in the battery rotate to this position, rather than simply ascend or descend into position.

Doliodus problematicus (originally Diplodus, Woodward 1892; Miller et al. 2003; Early Devonian, 409 mya) was considered the transitional taxon between acanthodians, like Brachyacanthus, and today’s sharks, like Isurus. Here Doliodus is not related to sharks, but to small bony fish, like Brachyacanthus, and large predatory bony fish, like Xiphactinus, among tested taxa. Doliodus eyes are enormous on a minimal snout. Fin spines precede the pectoral fins. No squamosal is known. That the teeth point toward the tongue makes one wonder if related and apparently toothless Brachyacanthus had similarly oriented teeth, rendered invisible during crushing and fossilization by their Z-plane orientation.

The cladogram topology in figure 3
is not traditional. Neither is the rest of the LRT. Even so, the LRT continues to provide solutions to long-standing problems and all sister taxa document a gradual accumulation of derived traits. They look like each other, which is how evolution is supported to work.

None of this could be done
without the discoveries of countless paleontologists over the last 200 years. Additionally, none of this could be done without computer software that enables the creation of cladograms from characters and taxa, and software that enables the addition of digital colors to digital images.

References
Dean B 1894a. Contributions to the morphology of Cladoselache (Cladodus). Journal of Morphology 9:87–114.
Dean B 1894b. A new cladodont from the Ohio Waverly, Cladoselache newberryi, n.sp. Transactions of the New York Academy of Science, 13: 115–119.
Miller RF, Cloutier R and Turner S 2003. The oldest articulated chondrichthyan from the Early Devonian period. Nature 435:501–504.
Turner S and Miller RF 2004. New ideas about old sharks. American Scientist 93:244–252.