Rhizodus

''Rhizodus'' ("root tooth") is an extinct genus of basal, finned tetrapodomorphs (the group of sarcopterygians that contains modern tetrapods and their extinct relatives). It belonged to Rhizodontida, one of the earliest-diverging tetrapodomorph clades. Two valid species have been described, both of which lived during the Early Carboniferous epoch. The type species ''R. hibberti'' is known from the Viséan stage of the United Kingdom, whereas the species ''R. serpukhovensis'' is from the Serpukhovian of Russia. Some fossils referred to the genus ''Rhizodus'' have also been found in North America.

History

A partial lower jaw attributed to a rhizodontiform, PIN no. 2878/581, was discovered in 2000 in the Zaborie quarry of Serpukhovsky District, Russia. The specimen consists of the middle part of the lower jaw, which includes the two coronoid fangs as well as marginal teeth. In 2022, it was formally described as the holotype of the new species ''R. serpukhovensis''. Dating from the Serpukhovian, it was the youngest known specimen of ''Rhizodus'', and the first known from Russia.

No near-complete adult skeleton of ''Rhizodus'' has been found, and the known material is limited to isolated fragments. Given explanations include taphonomic bias due to large body size, but also weak endochondral ossification in rhizodontids, and a lack of strong cranial sutures between skull bones.

Description

The most notable characteristics of ''Rhizodus'', compared to other giant rhizodonts such as ''Barameda'', were the two fangs located near the front of its jaws, followed by other teeth scaling downwards in size. ''Rhizodus'' was a giant apex predator that resided in freshwater lakes, river systems and large swamps, with ''R. hibberti'' measuring long and weighing . It fed on small to medium-sized amphibians, using its teeth to kill prey and rip it into digestible sizes, rather than swallowing prey whole like other, smaller-toothed sarcopterygians.

Fossil skin imprints show that ''Rhizodus'' had large, plate-like scales, similar to those found on modern day arapaima.

In ''Rhizodus'', the pelvic fins consisted of a femur articulating with three radial bones. This is unlike modern sarcopterygians and other known fossils, where the femur articulates with two bones, the tibia and fibula, which is believed to be the basal condition. The femur and the three radial bones in ''Rhizodus'' are thought to originate respectively from the stylopod and zeugopod, although without homology between the individual radials in ''Rhizodus'' and those in other tetrapods. The pectoral fins comprised a humerus articulating with a radius and ulna, making them closer to the tetrapod condition.

Taxonomy

''Rhizodus'' is part of the eponymous clade Rhizodontida, usually considered to be an early-diverging tetrapodomorph branch. However, the discovery of '' Hongyu'' in 2017, showing a mosaic of rhizodont-like and tetrapod-like characters, has questioned this placement. The phylogeny below comes from a study performed by Clement et al. in 2021, which recovered Rhizodontida as basal tetrapodomorphs positioned crownwards of ''Kenichthys'' and ''Tungsenia''.

While the genera ''Rhizodus'', ''Strepsodus'' and '' Archichthys'' are considered well-established, distinguishing characters focus on the symphysial tusks, leaving identification of other material more uncertain due to the fragmentary nature of remains. A 2020 study by Johanson et al. suggested dividing rhizodont humeri into four morphogroups. Morphogroup III specimens, characterized by a large preaxial process extending dorso-ventrally, were identified with ''Rhizodus'', tentatively extending the range of the genus to the Tournaisian.

Diet

The diet of ''Rhizodus'' included medium-sized fish and tetrapods. It has been proposed that ''Rhizodus'' lunged at terrestrial, shorebound prey, just like a modern-day crocodile.

References

Works cited

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{{Taxonbar, from=Q7320749

Rhizodontida
Prehistoric lobe-finned fish genera
Viséan genera
Serpukhovian life
Mississippian sarcopterygians of Europe
Mississippian sarcopterygians of North America
Fossils of Great Britain
Fossils of Russia
Fossil taxa described in 1840
Taxa named by Richard Owen