Aegisuchus
Aegisuchus is an extinct monospecific genus of giant, flat-headed crocodyliform within the family Aegyptosuchidae. It was found in the Kem Kem Formation of southeast Morocco, which dates back to the Cenomanian age of the Late Cretaceous epoch.[1] The type species Aegisuchus witmeri was named in 2012 by paleontologists Casey Holliday and Nicholas Gardner, who nicknamed it "Shieldcroc" for the shield-like shape of its skull.[2] A. witmeri is known from a single partial skull including the braincase and skull roof.[1]
Aegisuchus is known only from a partial braincase and skull roof cataloged as ROM 54530. It is diagnosed by several autapomorphies, or unique features. At the center of the skull table is a raised and rough-surfaced boss on the parietal bone that is shaped like a circle. On either side of this boss are holes called dorsotemporal fenestrae, and the surrounding bone is relatively smooth. The quadrate bone in the temporal region of the skull has a rectangular projection called the adductor tubercle, which served as an attachment for muscles that closed the jaw. At the front of the skull table, projections on the laterosphenoid bones called capitate processes face out to the side. This feature is also seen in the skulls of living gharials, but evolved independently in each group. Also on the front surface are two holes of the dorsotemporal fenestrae, which pass through the skull and open at the skull table. On the front surface, a ridge of bone or torus makes up the lateral edge of each hole. The back of the skull is wide, with large projections on the exoccipital bones that would have anchored large epaxial muscles in the top part of the neck.[1]
At 40 cubic centimetres, the braincase of Aegisuchus is much larger in volume than that of any other crocodyliform. Based on the ratio of braincase to skull length in other crocodilians, the total skull length of Aegisuchus is estimated to have been 2.08 to 2.86 metres (6.8 to 9.4 ft) in length. A similar ratio between braincase and body length puts Aegisuchus at 15 to 21 metres (49 to 69 ft) long when based on the proportions of long-snouted gharials, or 16 to 22 metres (52 to 72 ft) long when based on the proportions of short-snouted crocodiles.[1] However, these proportions have been met with a lot of scrutiny, and it is more likely that Aegisuchus reached less than 15 metres (49 ft) in length.[1]
The circular boss of roughened bone on the skull table is one of the most unusual features of Aegisuchus. As in most crocodilians, the rough-surfaced region was probably covered in a thick skin that tightly adhered to the skull. Surrounding the boss, the smooth-surfaced region bears several deep parallel channels for blood vessels, suggesting that thicker, more complex skin tissue covered this region. Vascularization is not seen in any other crocodilian, and may have been unique to Aegisuchus. Given that the blood vessel channels run into the braincase, the vascularized tissue may have served a thermoregulatory role by heating blood going to the brain and eyes. The central boss may have been used in mating displays, appearing as an eyespot. Modern crocodilians raise their heads out of the water as a social signal in mating displays; as a close relative of crocodilians, Aegisuchus likely had similar mating rituals.[1]
