''Crocodylus thorbjarnarsoni'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...

from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

of the

Turkana Basin An '' Acacia'' tree in the Kokiselei river, northern Kenya The greater Turkana Basin in East Africa (mainly northwestern Kenya and southern Ethiopia, smaller parts of eastern Uganda and southeastern South Sudan) determines a large endorheic bas ...

Kenya ) , national_anthem = "Ee Mungu Nguvu Yetu"() , image_map = , map_caption = , image_map2 = , capital = Nairobi , coordinates = , largest_city = Nairobi , ...

. It is closely related to the species ''

Crocodylus anthropophagus ''Crocodylus anthropophagus'' is an extinct species of crocodile from the Pleistocene of Tanzania. It lived 1.84 million years ago. It was a large-sized predator reaching a length of . Etymology ''Crocodylus anthropophagus'' was first named by ...

'', which lived during the same time in

Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...

. ''C. thorbjarnarsoni'' could be the largest known true crocodile, with the largest skull found indicating a possible total length up to . It may have been a predator of early

hominin The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The t ...

s. ''Crocodylus thorbjarnarsoni'' was named by Christopher Brochu and Glenn Storrs in 2012 in honor of John Thorbjarnarson, a conservationist who worked to protect endangered crocodilians.

Description

''Crocodylus thorbjarnarsoni'' is distinguished from other crocodiles by its broad snout. It has small raised rims on the

prefrontal bone The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern and ...

s in front of the eyes, a feature also seen in some

Nile crocodile The Nile crocodile (''Crocodylus niloticus'') is a large crocodilian native to freshwater habitats in Africa, where it is present in 26 countries. It is widely distributed throughout sub-Saharan Africa, occurring mostly in the central, eastern ...

individuals. The

squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral c ...

s form raised rims along the sides of the

skull table The skull roof, or the roofing bones of the skull, are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium. In comparati ...

, similar to the crests in ''C. anthropophagus'' but much smaller. Also like ''C. anthropophagus'', it has nostrils that open slightly forward rather than directly upward. The largest ''C. thorbjarnarsoni'' skull found (KNM-ER 1682) measures from the tip of the snout to the back of the skull table, in comparison, the largest known extant ''

Crocodylus ''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae. Taxonomy The generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant (living) species and 5 extinct species ...

'' skull is that of a saltwater crocodile, measuring . Based on regression analysis for ''Crocodylus'', this corresponds to a total length of but such analysis have been shown to underestimate the size of very large individuals by as much as 20%, which means it could have been as long as .

Paleoecology

''Crocodylus thorbjarnarsoni'' likely preyed on human ancestors like ''

Paranthropus ''Paranthropus'' is a genus of extinct hominin which contains two widely accepted species: ''Paranthropus robustus, P. robustus'' and ''P. boisei''. However, the validity of ''Paranthropus'' is contested, and it is sometimes considered to be sy ...

'' and early members of the genus ''

Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus ''Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relate ...

'', both of which are known from the Turkana Basin. Direct evidence of crocodilian predation is known from bite marks on hominin bones from the

Olduvai Gorge The Olduvai Gorge or Oldupai Gorge in Tanzania is one of the most important paleoanthropology, paleoanthropological localities in the world; the many sites exposed by the gorge have proven invaluable in furthering understanding of early human ev ...

, and these marks were likely made by the closely related crocodile ''C. anthropophagus'' (''anthropophagus'' means "human eater" in

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

). No hominin bones from the Turkana Basin bear crocodilian bite marks, so there is no direct evidence that ''C. thorbjarnarsoni'' preyed on hominins. However, modern Nile crocodiles are known to consume adult humans, and since ''C. thorbjarnarsoni'' was larger than any Nile crocodile, it easily could have eaten smaller-bodied human ancestors. Brochu and Storrs hypothesized that the lack of bite marks could have been due to hominin's awareness of crocodiles and ability to evade them, explaining that "this conflict—eat and drink, but maybe die—was presumably foremost amongst the concerns our predecessors felt when approaching ancient waterways inhabited by ''Crocodylus thorbjarnarsoni''." Another explanation was that ''C. thorbjarnarsoni'' may have eaten hominins whole with little need for biting, since it was much larger.

Specimens

''Crocodylus thorbjarnarsoni'' is known from nine skulls, all of which are housed in the

National Museum of Kenya The National Museums of Kenya (NMK) is a state corporation that manages museums, sites and monuments in Kenya. It carries out heritage research, and has expertise in subjects ranging from palaeontology, archeology, ethnography and biodiversity ...

. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

is a nearly complete

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

and

lower jaw In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

called KNM-ER 1683 and comes from the approximately 2-million-year-old

Koobi Fora Formation Koobi is tilapia salted dried fish in Ghana. The fish has an indigenous flavor in stews and soup in the Ghanaian local delicacies. It is used to prepare Ghanaian foods such as fufu, soup, plantain and stew and other local Ghanaian cuisines. Pr ...

on the eastern shore of

Lake Turkana Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...

. The skulls KNM-ER 1681 and KNM-ER 1682 have also been found from the formation. Three other skulls are known from the Nachukui Formation, west of the holotype's locality. KNM-WT 38977 is from the 2.5- to 3.4-million-year-old Lower Lomekwi Member, KNM-LT 26305 is from the 3.9-million-year-old Kaiyumung Member, and KNM-LT 421 is from the 4.2- to 5.0-million-year-old Apak Member. Three additional skulls called KNM-KP 18338, KNM-KP 30604, and KNM-KP 30619 are known from the southern Turkana Basin in the Kanapoi Formation, dating between 4.07 and 4.12 million years. KNM-ER 1682, KNM-LT 421, KNM-LT 26305, and KNM-KP 30619 were previously assigned to ''

Rimasuchus lloydi ''Rimasuchus'' is an extinct genus of crocodile from the Miocene of Egypt and possibly Libya. Only one species - ''Rimasuchus lloydi'' - is currently known. It was previously thought to be a species of '' Crocodylus'', but is now thought to be ...

'', and their reassignment to ''C. thorbjarnarsoni'' reduces the range of ''R. lloydi'' to Northern Africa.

Phylogeny

The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

below is from a 2021 study based on the finding of a new ''

'' partial

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

in the

of northern

References

{{Taxonbar, from=Q1647824 Crocodylidae Fossil taxa described in 2012 Neogene reptiles of Africa Pliocene crocodylomorphs Pleistocene crocodylomorphs