Rimasuchus

History and naming

The first fossil of Rimasuchus an incomplete skull with associated mandible, was collected by lieutenant colonel Arthur H. Lloyd in the early 20th century in Wadi Moghara, Egypt. The holotype specimen, CGM 15597, was given to the Egyptian Geological Museum and described by Fourtau in 1918 under the name Crocodylus lloydi.

Eventually other skulls further south in Africa ended up being assigned to "Crocodylus" lloydi, with the oldest and southern-most material stemming from Namibia and the youngest fossils from Tanzania and Kenya in East Africa. This would have given the taxon a massive range both geographically and stratigraphically, spanning large parts of the continent from the Miocene up to the Pleistocene.

However revisions of these fossils soon reduced the range of "Crocodylus" lloydi back to its original state. In 2003 the Namibian material was named Crocodylus gariepensis by Martin Pickford, who suggested that it was the true ancestor of the Nile crocodile. Pickford found his argument strengthened when shortly prior to the release of the publication "C." lloydi was found to be a distinct genus, named Rimasuchus. In 2010 Christopher Brochu and colleagues named Crocodylus anthropophagus based on remains from Tanzania and two years later Crocodylus thorbjarnarsoni was named from Kenya. With the Kenyan specimens assigned to their own species, previous size estimates indicating that Rimasuchus grew to massive sizes and even preyed on humans were rendered out of date. Furthermore, with their description Rimasuchus was effectively removed from the fossil record of East Africa and the Pleistocene.

The generic name "Rimasuchus" comes from the Latin words rima, meaning "crack", which is referencing the East African rift valley. However the fact that later studies have shown the absence of Rimasuchus in the region renders the name somewhat ironic. The species name honors Arthur H. Lloyd, who initially discovered the fossils in Egypt.

Description

Rimasuchus has a brevirostrine skull, meaning the snout is short and broad. The premaxillae are wider than long. The suture between the premaxillae and maxillae on the palate is relatively straight.

Phylogeny

Early studies on Rimasuchus generally thought it to be a member of the genus Crocodylus within the Crocodylinae. Given its age it was believed for some time to have been the direct ancestor to the modern Nile crocodile.

A 2018 tip dating study by Lee & Yates simultaneously using morphological, molecular (DNA sequencing), and stratigraphic (fossil age) data established the inter-relationships within Crocodylidae and in 2021, Hekkala et al. were able to use paleogenomics, extracting DNA from the extinct Voay, to better establish the relationships within Crocodylidae, including the subfamilies Crocodylinae and Osteolaeminae. Their results confirmed *Rimasuchus''' status as an osteolaemine. In their phylogenetic tree Rimasuchus was recovered as the closest relative to the two modern dwarf crocodile species and more derived than *Brochuchus''.

Restricting the analysis to work based on morphological data alone also finds it to be an osteolaemine, although the internal relationships of the clade differ slightly. Here, it takes a more basal position, neither especially close to the extant forms nor to the long-snouted Euthecodon.

References

References

1. (1918). "Contribution à l'étude des vertébrés miocènes de l'Egypte". Geological Survey of Egypt.
2. (2004). "Crocodile remains from the Burdigalian (lower Miocene) of Gebel Zelten (Libya)". Geodiversitas.
3. (1986). "Evolution of the crocodiles in East and North Africa". Cahiers de Paléontologie.
4. (2010). "A New Horned Crocodile from the Plio-Pleistocene Hominid Sites at Olduvai Gorge, Tanzania". [[PLoS ONE]].
5. (2012). "A giant crocodile from the Plio-Pleistocene of Kenya, the phylogenetic relationships of Neogene African crocodylines, and the antiquity of ''Crocodylus'' in Africa". Journal of Vertebrate Paleontology.
6. (2003). "Lothagam: The Dawn of Humanity in Eastern Africa". Columbia University Press.
7. (2017). "Pliocene crocodiles from Kanapoi, Turkana Basin, Kenya.". Journal of Human Evolution.
8. (2025-03-17). "Evolutionary trend of the broad-snouted crocodile from the Eocene, Early Miocene and recent ones from Egypt". [[Scientific Reports]].
9. Pickford, M.. (2003). "A new species of crocodile from Early and Middle Miocene deposits of the Lower Orange River Valley, Namibia, and the origins of the Nile crocodile (''Crocodylus niloticus'')". Memoirs of the Geological Survey of Namibia.
10. Michael S. Y. Lee. (27 June 2018). "Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil". [[Proceedings of the Royal Society B]].
11. (2021-04-27). "Paleogenomics illuminates the evolutionary history of the extinct Holocene "horned" crocodile of Madagascar, Voay robustus". Communications Biology.
12. (6 September 2021). "Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem". [[PeerJ]].