Gregorius rexi: not a ratfish in the LRT

Revised November 07, 10 and 17 2019
with a revision to the LRT that moves Gregorius closer to Hybodus, basal to Placodermi.

Talk about a transitional taxon…
Gregorius rexi (Lund and Grogan 2004; 11cm long; Early Carboniferous; CM 35490) is a small fish from the famous Bear Gulch Formation in Montana. Traditionally it is considered a type of ratfish.

By contrast,
in the large reptile tree (LRT, 1593 taxa; Fig. 3), Gregorius is a late surviving member of an Early Devonian genesis representing the most primitive ray-fin fish splitting from Hybodus. Gregorius is the last common ancestor of all bony fish and placoderms. Ratfish are a bit more primitive.

Figure 1. Gregorius rexi enlarged and to to scale with its cousin in the LRT, Robustichthys. Gregorius still has a dorsal spine and an odd soft of diphycercal tail.

Figure 1. Gregorius rexi enlarged and to to scale with its cousin in the LRT, Coccosteus.  Gregorius still has a dorsal spine and an odd soft of diphycercal tail.

Gregorius is not far from catfish,
still tucked inside the placoderms. Thunnus, the tuna, is the most primitive extant ray-fin fish among taxa derived from a sister to Gregorius.

Figure 5. Subset of the LRT focusing on fish. Pachycormus nests at the base of the revised Telostei (green) clade.

Figure 2. Subset of the LRT focusing on fish. Pachycormus nests at the base of the revised Telostei (green) clade.

In the meantime,
I’ve been learning more about ray fin fish. Some taxa have moved around as mistakes are discovered and corrections are made. The four-eyed fish now nests with the mudskipper, as an example. The general topology of the tree has otherwise stayed much the same as the Bootstrap scores get better. When that portion is complete, we’ll review the changes.


References
Lund R and Grogan E 2004. Five new euchondrocephalan Chondrichthyes from the Bear Gulch Limestone (Serpukhovian, Namurian E2b) of Montana, USA. Recent Advances in the Origin and Early Radiation of Vertebrates 505-531.

https://people.sju.edu/~egrogan/BearGulch/pages_fish_species/Gregorius_rexi.html

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