Orodus: another overlooked taxon at the shark-bony fish transition

Another overlooked human ancestor
enters the large reptile tree (LRT, 1793+ taxa) and with it, new light is shed on the history of how we came to be.

Figure 1. Orodus greggi in situ, FMNH specimen. See figure 2 for reconstruction.

Figure 1. Orodus greggi in situ, FMNH specimen. See figure 2 for reconstruction. This black triangle results after Photoshop removal of the original distortion due to perspective visible in the original photo.

Orodus greggi 
(Agassiz 1838, Late Pennsylvanian to Early Permian 300mya, 2m long) is a later surviving descendant at the shark-bony fish split, descending from Hybodus and basal to tiny Prohalecites.

Note: these are both late survivors of a Middle Silurian radiation based on phylogenetic and chronological bracketing. That gives both taxa plenty of time to evolve individual traits that appear, but do not remove both taxa from their phylogenetic order in the LRT.

The mandible of Orodus is massive
(probably a newly evolved trait). The cranium is narrow. The fins are larger than those illustrated by Zangrel 1981. The FMNH specimen preserves skin and gill slits.

Note: the distance between the pectoral fins and skull shrinks in Prohalecites, one way to make five gill opercula shrink to just one.

The FMNH (Field Museum) specimen of Orodus
would make a wonderful project for a PhD candidate. Not much has been written about it. It might be a good idea to run it through an x-ray machine to see the now covered coronoid process.

Figure 2. Orodus reconstructed using DGS from figure 1 alongside Prohalecites x10 and to scale.

Figure 2. Orodus reconstructed using DGS from figure 1 alongside Prohalecites x10 and to scale.

Earlier we looked at
Prohalecites (Fig. 2) a tiny descendent of Orodus also nesting in the LRT between sharks and bony fish and discussed the increasingly common instances of phylogenetic miniaturization at the genesis of major clades.


References
Agassiz L 1838.
 Recherches Sur Les Poissons Fossiles. Tome III (livr. 11). Imprimérie de Petitpierre, Neuchatel 73-140.

wiki/Prohalecites
wiki/Orodus

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.