The big-eyed placoderm, Stenosteus, nests with the basal shark, Cobelodus

Sunday, January 21, 2024 17:41

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Two oddballs among their own ranks link up today.
Late Devonian Stenosteus and Middle Carboniferous Cobelodus (Fig 1) share a long list of traits. Placoderms, like Stenosteus, continue to interweave themselves into shark origins.

While this appears to resolve one problem, it creates a few others.

Figure 1. Above: The basal shark, Cobelodus, to scale with the big-eyed placoderm, Stenosteus. Green arrow points to possible homologus structures. A narrow plastron at the base of the not-preserved pectoral fins in the placoderm and jointed rods arising from the base of the pectoral fins not seen in other sharks have similar shapes and locations. The rostrum of both fish are similar.

Cobelodus aculeatus
(Zangerl and Case 1976, originally Styptobasis aculeata Cope 1891; FMNH UF576; Pennsylvanian, Middle to Late Carboniferous; 2m) is a large, shark-like taxon originally considered a holocephalian. Here it nests with Falcatus as ‘the best known Paleozoic elasmobranch’ (ca 1976).

Zangerl and Case note,
“In the differentiation of the neurocranium and the visceral skeleton, Cobelodus presents a morphological condition not previously documented in any chondrichthyan, or, for that matter, any other fish.”

Large eyes suggest a deep sea niche.
The pectoral fins have a 30cm trailing feeler/clasper supported by flexible cartilage.

In Stenosteus (Fig 1, Carr 1996) these same structures form the rim of a long narrow plastron. So, once again, fish skull and pectoral elements appear to splinter and subdivide to become something else.

Figure 2. The postorbital and quadrate are highlighted here for Cobelodus, Stenosteus, Gemuendina and Jagorina.

Earlier
flat and wide Stenosteus was allied with Gemuendina and Jagorina, two ray-like placoderms with a new jaw joint formed by the descending process of the postfrontal.

It’s worth noting that only Hobelodus (Fig 2) has that same descending postfrontal at an early stage of development associated with an appropriate external quadrate (= labial cartilage).

Figure 3. The extraordinary preservation of two Falcatus interacting (MV5385 and MV 5386) in one fossil described by Lund 1985. The ‘extragnathal teeth’ are enlarged in the box. The small size of the lateral fins is noable in this taxon, which also have medial pectoral extensions that likely formed from ancestral plastron elements.

New insights into a related shark,
Falcatus (Fig 2), indicates the break-up of the external quadrate (red = labial cartilage) into a collection of ‘extragnathal (= outside the jaws) teeth’.

Considering their homology,
those ‘extragnathals’ may look like teeth, but they are not teeth – but they serve as teeth to hold on to the male ‘antler’ during mating rituals (Fig 3). Males also have such ‘teeth’. Perhaps they are used for holding on to other male ‘antlers’ in domination contests. Lund 1985 notes that males greatly outnumber females in the Bear Gulch Formation. You’ll note that traditional mandibular teeth are next to indiscernable in this taxon, as in all taxa presented today.

This appears to be a novel hypothesis of interrelationships.
If not please provide a citation so I can promote it here.

References
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72.
Carr RK 1996. Stenosteus angustopectus sp. nov. from the Cleveland shale (Famennian) of Northern Ohio with a review of selenosteid (Placodermi) systematics. Kirtlandia 49:19–43.
Coates MI, Tietjen K, Olsen AM and Finarelli JA 2019. High-performance suction feeding in an early elasmobranch. Sci. Adv. 2019; 5 : eaax2742
Lund R 1985. The morphology of Falcatus falcatus (St John and Worthen), a Mississippian stethacanthid chondrichthyan from the Bear Gulch Limestone of Montana. Journal of Vertebrate Paleontology 5(1):1–19.
St. John OH and Worthen AH 1875. Palaeontology of Illinois. Descriptions of fossil fishes. Geological Survey of Illinois, 6: 245–488.
St. John OH and Worthen AH 1883. Description of fossil fishes: a partial revision of tehe Cochliodonts and Pasmmodonts. Geological Survey of Illiniois 7:55–264.
Zangerl R and Case GR 1976. Cobelodus aculeatus (Cope) an anacanthous shark from Pennsylvanian black shales of North America. Palaeontographica, Abt. A 154:107–157.