Delphyodontos: a young petalodont, not a finless cannibal embryo with an enlarged abdomen

Lund 1980 reported,
“A new species of Lower Carboniferous holocephalan chondrichthyan, Delphyodontos dacriformes, is described from two fetal specimens. The well-developed slashing and piercing dentition, enlargement of the abdominal region, and fecal material indicate the probable evolution of intrauterine oophagy and viviparity in Paleozoic Chondrichthyes.”

Figure 1. Delphyodontos to scale with coeval Obruchevodus, a likely older individual and junior synonym. The aqua and blue areas beneath the skull are large pectoral fins, not abdominal bulges.

After DGS tracing
the ‘enlargement of the abdominal region’ in Delphyodontos (Figs. 1, 2) turns out to be a pair of large pectoral fins, as in petalodont ratfish, like coeval and larger Obrechevodus (Figs. 1, 3). The ‘fecal mass’ is nowhere near the large intestine, anus and anal fin, but appears in other petalodonts (Fig. 3) as a pigmented mass or liver. The ‘slashing and piercing dentition’ refers to the robust teeth typical of Petalodontiformes.

Figure 2. Referred specimen of Delphyodontos, CM 35455 in situ and colorized using DGS methods here. The subtle difference between the fins and the matrix are important here. The so-called ‘fecal mass’ is more likely the liver, as in other petalodonts.

These three new identifications
reduce the possibility that Delphyodontos was a cannibal embryo.

Delphyodontus dacriformes
(Lund 1980; Bear Gulch, Late Carbonferous; second specimen, 2.9cm) was named earlier than Obrechevodus, but considered a finless embryo with a teardrop-shaped body capable of oophagy (eating siblings within the womb). The ‘abdominal enlargement’ is actually a pair of pectoral fins, as in Obruchevodus (Fig. 3) and Belantsea (Fig. 5).

Figure 3.Obruchevodus in situ, counterplate flipped, graphic from Grogan et al. 2014 and colorized here. ‘P’ = pigmented structure as in Belantsea (Fig. 4) and Dephyodontos (Fig. 2). Whatever it is, it keeps showing up in petalodonts.

Obruchevodus griffithi
(Grogan et al. 2014; Bear Gulch, Late Carboniferous) is a petalodont close to Belantsea (below) exposing the internal notochord, skull and other supports more than the external skin.

Figure 4. Delphyodontos holotype.

The similar holotype specimen of Delphyodontos
(Fig. 4) was not tested due to low resolution imagery published in Lund 1980.

FIgure 4. Belantsea is a petalodont also from the Bear Gulch Formation.

Belantsea montana
(Fig. 5, Lund 1989; Early Mississipian, Early Carboniferous; 25 cm in length) is a mid-sized petalodont ratfish with typically giant, fan-like pectoral fins and a small diphycercal tail. The dark spot on the torso is the liver and other internal organs.

Figure 5. The queen triggerfish, Balistes, is convergent with Carbonifermous petalodonts.

Petalodonts of the Carboniferous
were convergent with extant queen triggerfish (Fig. 5) in their morphology and dentition. The procumbent premaxillary dentition of triggerfish evolved to feed on coral.

I did not have firsthand access to this specimen
yet was better able to identify elements overlooked by Lund 1980 by using DGS methods and comparable specimens unavailable in 1980. Those other petalodonts (above) certainly helped.

References
Grogan ED, Lund R and Fath M 2014. A new petalodont chondrichthyan from the bear gulch limestone of montana, USA, with reassessment of Netsepoye hawesi and comments on the morphology of holomorphic petalodonts. Paleontological Journal 48 (9): 1003–1014.
Lund R 1980. Viviparity and Intrauterine Feeding in a New Holocephalan Fish from the Lower Carboniferous of Montana. Science, New Series 209(4457):687–699.
Lund R 1989. New petalodonts (Chondrichthyes) from the Uppper Mississippian Bear Gulch limestone (Namurian E2b) of Montana. Journal of Vertebrate Paleontology 9(3):350–369.

Wiki/Obruchevous – not yet posted
wiki//Delphyodontos
wiki/Belantsea
wiki/Petalodontiformes