Temporal range: 23.03–.781Ma Early Miocene – Early Pleistocene
Rimasuchus is an extinct genus of crocodile from the Neogene period of Africa and the Middle East. Its name comes from the Latin words rima, meaning "crack" (referencing the East African rift valley where it was discovered) and suchus, which means "crocodile". Rimasuchus is a member of the subfamily Crocodylinae, which includes over 20 species, eight of which are extinct. The type and only species, Rimasuchus lloydi, lived alongside other crocodiles such as the Nile crocodile. It preyed on large mammals, including early humans.
Rimasuchus grew to 7 meters or more in length. Unlike most living crocodiles, it is brevirostrine, or broad-snouted. Rimasuchus has characteristically short and broad premaxillae, as well as a deep mandibular symphysis connecting the two sides of the upper jaw. The premaxillae do not project as far back on the upper surface of the skull as those of other crocodiles and are noticeably wider than they are long on the palate (unlike those of Crocodylus niloticus and Crocodylus cataphractus). The external nares are positioned close to the tip of the snout. Like other crocodiles, Rimasuchus has an occlusal groove, or notch, between the premaxilla and maxilla that receives the fourth mandibular tooth. However, in Rimasuchus the groove is noticeably shorter anteroposteriorly than that of C. niloticus. The preorbital region is flat and there is no nasal promontorium, or raised nasal region, as in C. niloticus.
The teeth of Rimasuchus are robust and blunt, unlike those of other crocodiles. They become more bulbous toward the rear of the jaws. The crowns of the teeth are rarely sharp, although crown sharpness tends to lessen with size in crocodylians. The teeth are uniquely bicarinate, meaning that there are ridges on the front and back.
Remains of Rimasuchus show variations in size and proportions, and it is likely that they represent different ontogenic (growth) stages. There has been very little study so far on ontogenic variation in Rimasuchus and living crocodiles. There are many similarities between specimens of R. lloydi and C. niloticus, and it is likely that some specimens of Rimasuchus actually represent particularly large examples of C. niloticus.
Rimasuchus lloydi was first described in 1918 and originally placed in the genus Crocodylus under the name C. lloydi. After cladistic analysis suggested that it did not belong in Crocodylus, however, the new genus was erected in 2003. Among crocodylines, Rimasuchus is most closely related to the living dwarf crocodile Osteolaemus and is placed within the group Osteolaeminae.
Distribution and habitat
Common over much of East Africa, Rimasuchus fossils have been found in close association with fossils of Nile crocodiles. Their fossils have been found in Kenya, Egypt (including the Sinai), Libya, Chad, Ethiopia, Uganda, Sudan, Tanzania and Tunisia. From Africa their range stretched east into Saudi Arabia. While earlier specimens have been recovered from north Africa, how the species evolved and spread is still uncertain.
As large, semiterrestrial predators, Rimasuchus probably competed directly with Nile crocodiles, eventually being out-competed by them as their shared habitat became more arid.
Rimasuchus fossils, along with those of Euthecodon brumpti, are the most common crocodile fossils in the Lake Turkana basin and Koobi fora ridge in northeast Kenya, where the two crocodiles likely lived in the lake and surrounding rivers. Their bones have been found in situ with other vertebrates including several species of fish and birds, mammals such as pigs, elephants, giraffes, cows, rhinoceri, primates, even hominids such as Australopithecus afarensis. Aside from Euthecodon, other predators included the Slender snouted and Nile crocodiles, the extinct gavialoid Eogavialis andrewsi, Dinofelis aronoki and Metailurus (sabre-toothed cats), Bear dogs, hyenas, and early members of the genus Canis.
Rocks from Turkana containing Rimasuchus fossils are usually poorly lithified fluvial or lacustrine sandstones or mudstones that were deposited in lakes and rivers. These deposits come from the Nawata Formation and the Nachukui Formation of the Tortonian age and Pliocene epoch, respectively. Fossils from these rocks usually dates from 1.88 ± 0.02 to 7.44 ± 0.05 million years old.
Predation on early humans
Crocodiles were known predators of early humans. Remains of Rimasuchus, Nile crocodiles, and the extinct Crocodylus anthropophagus have been recovered from sites in association with early human bones. In the Olduvai Gorge of Tanzania, predation has been documented in the form of crocodile tooth marks on the bones of several hominid specimens, namely OH 7, OH 8 and OH 35, all of which belong to Homo habilis. Bone damage in some H. habilis remains include bisected tooth pits and punctures, which are diagnostic of crocodilian predation. The damaged hominin bones have been found at what were considered hominin "living sites" that also included stone artifacts and butchered bones. In addition, the bone accumulations at Beds I and II of the gorge have been specifically attributed to Rimasuchus. A trench containing bones of early bovines, horses, OH 7, and several stone tools has also yielded Rimasuchus teeth.
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