Earlier I tried to understand
Doliodus latispinosus (Whiteaves, 1881; Maisey et al. 2018; originally Diplodus, problematicus (“problematic deceiver”) Woodward 1889; Fig. 1) and failed. Traditionally Doliodus has been considered part of the acanthodian-chondrichthyan (spiny shark to shark) transition, but the the large reptile tree (LRT 1643+ taxa) nests acanthodians far from chondrichthyans and Doliodus apart from both.
Figure 1. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.
Maisey et al. 2018 reported,
“Based on these data, Doliodus and pucapampellids both fall outside the chondrichthyan crown, but their relative phylogenetic positions on the chondrichthyan stem are unclear.
The phylogenetic position of Doliodus seems less elusive; it possessed an ‘acanthodianlike’ complex of dermal spines, including pectoral fin spines, prepectoral, admedian, and prepelvic spines, and possibly dorsal and pelvic fin spines, in conjunction with numerous ‘chondrichthyan-like’ endoskeletal features and a heterodont ‘sharklike’ dentition. Doliodus can be viewed as a quintessential component of the evolutionary transition between ‘acanthodians’ and ‘conventionally defined chondrichthyans’, leaving little doubt that the chondrichthyan total group includes ‘acanthodians’ (now widely perceived to be a paraphyletic group, populating the basal part of the chondrichthyan stem).”
Figure 1. Skull of Iniopteryx in situ and reconstructed.
Doliodus re-enters the LRT with iniopterygids
(Fig. 2), a strange clade basal to chondrichtheys (ratfish) and elasmobranchs (sharks and kin). It is worthwhile to note that they, too, have a spine-stiffed pectoral fin, supporting an extended base of gracile rays…unlike spiny sharks. The eyes are enormous on a minimal snout. No squamosal is known.
This was probably a nocturnal bottom feeder
with a wide skull, wider than the narrow (as determined by the inter-fin distance) torso. Perhaps this flat taxon was transitional between low pectoral fins of most fish and the high-set pectoral fins that set iniopterygians apart.
Figure 2. The Iniopterygidae include Iniopteryx, Promexyele, Iniopera and Sibyrhynchus. These reconstructions are from Zangerl and Case 1973 and the captions label them “tentative.”
Miller, Cloutier and Turner 2003 also reported on Doliodus.
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].”
Check the cladogram (Fig. 3).
The LRT resolves this issue clearly. This is a novel hypothesis of interrelationships based on taxon inclusion. If anyone published the same interrelationships earlier, let me know so I can promote that citation.
Later, just 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. 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.”
To answer that question:
Yes. Some traditional sharks (Fig. 3, green clade) were toothless for millions of years before toothy sharks appeared.
The other answer to that question is:
check the cladogram (Fig. 3). Some traditional sharks (in the green clade) now appear outside the clade that includes ratfish and most sharks, rays and skates (pink clade) and some traditional sharks (peach clade) now appear basal to the basal dichotomy of bony fish (blue and gold clades).
Figure 4. Subset of the LRT focusing on basal vertebrates (fish). Esox and Ictalurus are highlighted. This cladogram reflects the latest results, which are still not completely resolving internal issues in the teleost clade. Tetrapods arise from the yellow clade at left.
In the LRT,
big-eyed, flat-skulled Doliodus (Fig. 4) nests with other big-eyed, spiny-finned iniopterygians in the clade that leads to bony fish, not sharks. Like the related Xenacanthus, Doliodus has double-tipped teeth.
Sharks are also known from a tooth battery,
a conveyer belt lineup of teeth waiting to rotate into place, as in Dolidus. 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, as in tetrapods.
Maisey JG et al. (6 co-authors) 2018. Doliodus and Pucapampellids: Contrasting perspectives on stem chondrichthyan morphology. Chapter 5 in Evolution and Development of Fishes.
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.
Whiteaves JF. 1881. On some fossil fishes, Crustacea and Mollusca from the Devonian rocks at Campbellton, NB, with descriptions of five new species. Can Nat 10:93–101.
Woodward AS. 1889. Acanthodian fishes from the Devonian of Canada. Ann Mag Nat Hist 4:183–184.