What is Pachycormus?

As documented yesterday,
the relationship of the clade Pachycormiformes with other fish clades remains traditionally unclear.

Clearly
in the large reptile tree (LRT, 1620+ taxa, subset Fig. 5) members the Pachycormiformes nest near the base of the Teleostei with Hybodus (Fig. 2). If you are guessing Pachycormus (Fig. 1) and Hybodus have never been tested together before, you may be right. If I am mistaken, let me know and I will promote that citation.

Figure 1. Pachycormus fossil. Pelvic fins vestigial near vestigial anal fin. See figure 3 for closeup of the skull.

Pachycormus macropterus (Agassiz 1833; Wretman, Blom and Kear 2016; Early Jurassic, 1m long) is the original member of the Pachycormiformes. Here it nests with Hybodus (Fig. 2) outside the base of the Teleostei under the last common ancestor method.

(Remember
amphibian-like reptiles, like Silvanerpeton? That’s why classification HAS TO BE clade-based, not trait-based. Otherwise you miss out on all of the wonderful relationships the LRT has revealed.)

Figure 1. Diagram of Hybodus in vivo and skeleton plus teeth.
Figure 2. Diagram of Hybodus in vivo and skeleton plus teeth.

Gill bars, multiple gill slits and a skeleton largely made of cartilage
are retained and the nasal is robust in Pachycormus (Fig. 3). Here DGS helps identify and simplify skeletal elements in a crushed fossil.

Figure 8. Pachycormus macropterus has a new skull reconstruction. Originally I did this without template or guidance. Now osteoglossiformes provide a good blueprint.
Figure 8. Pachycormus macropterus has a new skull reconstruction. Originally I did this without template or guidance. Now osteoglossiformes provide a good blueprint.

This tuna-mimic
(Fig. 3) is considered basal to larger later suspension feeders, like Leedsichthys, the largest known ray fin fish. If so, the LRT makes Leedsichthys a shark, not a ray-fin fish.

Figure 1 (added 01/27/2020 with a current interpretation of skull bones on Hybodus, plus a reconstruction. Note the retention of external gill bars.
Figure 4 (added 01/27/2020 with a current interpretation of skull bones on Hybodus, plus a reconstruction. Note the retention of external gill bars.

Tradtionally taxon exclusion
has kept the shark-like Hybodus out of Pachycormus studies and vice versa. These are the closest tested taxa known to give rise to teleost fishes (sans several now migrated taxa) in the LRT (Fig. 5).

Hybodus basanus (Agassiz 1837; H. reticulatus (Early Jurassic skull); 2m in length, Permian –Late Cretaceous) nests between sharks + chimaeroids and Teleostei in the LRT. Note the spines on the dorsal fins. These are homologous with spines on spiny sharks and basal placoderms like Gregorius. The skull is transitional between sharks and bony fish in the LRT, despite the presence of large gill bars (yellow) lateral to the jaws and a skeleton largely made of cartilage.

Figure x. Newly revised fish subset of the LRT

Another purported pachycormiform,
Protosphyraena, nests closer to Ohmdenia in the LRT, as updated yesterday.

FIgure 1. The BRLS specimen attributed to Pachycormus by Cawley et al. now nesting with Hydrolycus, sans the large fangs.
Figure 6. The BRLS specimen attributed to Pachycormus by Cawley et al. now nesting with Hydrolycus, sans the large fangs.

Cawley et al. 2018 reported on 3D specimens
attributed to Pachycormus. After testing against a wider gamut of taxa, their BRLSI M1332 specimen (Fig. 6) nests with the extant fanged fish, Hydrolycus (Fig. 7).

Figure 1. Hydrolycus, the extant dogtooth characin seems to have unique fangs. But a closely related extinct taxon, Protosphyraena, also has fangs.
Figure 7. Hydrolycus, the extant dogtooth characin seems to have unique fangs. But a closely related extinct taxon, Protosphyraena, also has fangs.

References
Agassiz L 1833, 1837 in Agassiz L. 1833-1843. Recherches sur les Poissons fossiles-I, I, III, Neuchatel, pp 1420.
Cawley JJ, Kriwet J, Kug S and Benton MJ 2019. The stem group teleost Pachycormus Pachycormiformes: Pachycormidae) from the Upper Lias (Lower Jurassic) of Strawberry Bank, UK. PalZ 93(2):285–302.
Wretman L, Blom H and Kear BP 2016. Resolution of the Early Jurassic actinopterygian fish Pachycormus and a dispersal hypothesis for Pachycormiformes. Journal of Vertebrate Paleontology 36(5):e1206022. Online

wiki/Hybodus
wiki/Pachycormus

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