Sarcosuchus

Credit: Rhunevild on Imgur.

*Sarcosuchus* is an extinct genus of crocodile that lived in Africa and South America during the Early Cretaceous.

## History

Several expeditions from the Sahara from 1946-1959 led by Albert-Félix de Lapparent found several large crocodyliform fossils from the Continental Intercalaire Formation, some found in Foggara Ben Draou near Aoulef (informally "Aoulef Crocodile"), with others from the Ain el Guettar Formation, Gara Kamboute, southern Tunisia. They found a fragmented skull, teeth, scutes and vertebrae. In 1957, the region now considered the Elrhaz Formation (north Niger) hosted several large isolated teeth, found by H. Faure. France De Broin identified these as from a long-snouted crocodile/  In 1964, the research team f the French CEA found a near-complete skull in Gadoufaoua in north Niger. The skull was shipped to Paris and became the holotype for *Sarcosuchus imperator* in 1966. The name comes from the Greek σάρξ (sarx), "flesh" and σοῦχος (souchus), "crocodile".

In 1977, a new *Sarcosuchus* was erected; S. hartti, from 19th century Late Hauterivian pebby conglomerate and green Ilhas Group shales, Recôncavo Basin, northeast Brazil. Charles Hrrt (1867) discovered two isolated teeth, sending them to Othniel Charles Marsh - who then erected *"Crocodylus" hartti*. In 1907, these remains (with some new material) were assigned *"Goniopholis" hartti* The fragmented mandible, osteoderm and two teeth assigned *S. hartti* are now in the British Museum of Natural History and assigned the genus *Sarcosuchus*.

Paul Sereno found material from the Aoufous Formation, Morocco in 1995, 1997 and the follow-up 2000 trip. Partial skeletons, several skulls and 20 tons of other remains were found from the Elrhaz Formation (Late Aptian-Early Albian). It took a year to prepare this material. More fossils were found in the Ifezouane Formation, Morocco in 2010. Some teeth from the Nalut area, northwest Libya (Hauterivian-Berremian?) might belong to S. imperator. Sarcosuchus indet. material includes articulated osteoderms, isolated teeth, a left scapula, mandibular fragment, dorsal vertebrae, illium and the proximal fragment of the femur were discovered from the Oum Ed Dhiab Member in 2018.

## Description

*Sarcosuchus* was a relative of crocodiles, but much larger. Fully grown *Sarcosuchus* may have reached 9-9.5 meters (29.5-31.2 feet) long and had a weight of 3.5-4.3 metric tons (3.9-4.7 short tons). It's eyes were somewhat telescoped, and it's snout is ~75% of the skull's total length. 35 teeth lined each side of the upper jaw, and 31 with the lower. Additionally, the lower jaw is much shorter and creates an overbite. When the jaws closed, there was a gap caused by the overbite. Young individuals snouts resemble the gharial, and adult individuals are broader. *Sarcosuchus* has an extensive bulla, which has been compared to the ghara in gharials Unlike the ghara which is only found in males, the bulla is present in all individuals. It's purpose is unknown, and was not sexually dimorphic. The osteoderms of *Sarcosuchus* are similar to goniopholodids such as *Shunosuchus* and *Goniopholis*. These form a surface that is interrupted from the posterior segment of the neck down to the middle of the tail, similar to basal crocodyliforms and *Araripesuchus*. This is different from crocodiles, who show discontinuity of the neck and body osteoderms.

A common size estimation tactic in crocodilians is using the length of the skull (midline from the tip to the skull's table), since living crocodilians have a strong correlation between skull length of subadults and adults regardless of their sex. Sereno et al. (2001) used this to combat the lack of postcranial material.

The two regression equations were used on S. imperator, created based on measurements from 17 captive Indian gharials and 28 wild Australian saltwater crocodiles. Both datasets were supplemented by measurements of 1.5 meter (4.92 feet)-long individuals in literature. The type S. imperator, also the largest, is 1.6 meters (5.25 feet) long; 1.5 meters (4.92 feet) at the midline. This individual was likely 11.65 meters (38.2 feet) long. The snout-vent length was found by using linear equations of the saltwater crocodile, finding it was 5.7 meters (18.7 feet).

Thus, they fund a weight of 8 metric tons (8.8 short tons). These show *Sarcosuchus* could grow larger than previously thought, but is greater than *Rhamphosuchus*, *Deinosuchus* and *Purussaurus* (based on estimates at the time). However, femur extrapolation from a subadult was much smaller than Sereno found based on modern forms. O'Brien et al. (2019) find the largest S. imperator as 9.5 meters (31 feet) long and 4.3 metric tons (4.7 short tons) in weight based on longirostrine crocodilian skulls. This was very close to the femur estimate (9.1 meters (29.9 feet)).

## Classification

*Sarcosuchus* is commonly referred to Pholidosauridae (a crocodyliforme outside Crocodylia). This clade is most related to *Terminonaris*. Most pholidosaurids have long, slender and are known from a mosaic of environemnts (some fresh, others saltwater). *Sarcosuchus* stands out among this group for being a generalist carnivore, which many pholidosaurs were generalist piscivores. A 2019 study found *Sarcosuchus* has a more derived position in Tethysuchia, phylogenetically closer to Dyrosauridae. Fortier et al. (2011) recovered;

## Paleobiology

### Ontogeny

Sereno examined thin layers from trunk osteoderms from a probable subadult (80% size) and found ~40 arrested growth lines. Sereno found *S. imperator* took 50-60 years to reach maturity. Given modern crocodilians rarely reach this stage, *S. imperator* likely underwent a period of rapid juvenile growth. A similar strategy was employed by the massive *Deinosuchus* based on similar criteria.

### Diet

Based on the snout of adult *S. imperator* compared to the gharial and narrow-snouted crocodiles, alongside the lack of interlocking smooth, sturdy-crowned teeth when the jaws closed, Sereno et al. proposed *S. imperator* was a generalist like the Nile crocodile. This would have included prey such as the abundant dinosaurs present in the region. However, a 2014 biomechanics study suggests *Sarcosuchus* was unable to perform the death roll. This implies *S. imperator* would have had to use different techniques than modern crocodilians to dismember their prey.

## Paleoecology

*S. imperator* remains were found in part of the Ténéré Desert (named Gadoufaoua), specifically in the Tegama Group, Elrhaz Formation (Late Aptian-Early Albian, ~112 million years ago). The stratigraphy and aquatic fauna in the region proposes it was an inland fluvial environment. It was entirely freshwater, humid and tropical. *S. imperator* swam with *Lepidotes* and *Mawsonia*. Dinosaurs include *Lurdusaurus* (the most common dinosaur in the area), *Ouranosaurus*, *Nigersaurus*, *Suchomimus*, *Eocarcharia*, *Kryptops* and an unnamed sauropod.

*S. hartti* lived in the Recôncavo Basin of Brazil, Ilhas Formation, Bahia series. This region was a shallow lacustrine ecosystem dating to the Late Aptian, which is similar to *S. imperator*. Aquatic fauna was similar, including *Lepidotes* and two *Mawsonia* species. Dinosaurs are extremely fragmented; an indeterminate theropod and iguanodontid.

## References

* https://paleobiodb.org/classic/basicTaxonInfo?taxon\_no=214793
* https://en.wikipedia.org/wiki/Sarcosuchus