Carnotaurus does not have osteoderms

Carnotaurus skin

Skin impressions from the caudal region of Carnotaurus, from Czerkas & Czerkas (1997). The large, circular indentations are impressions of the shield scales.

Carnotaurus sastrei is fairly well-known as having extensive skin impressions preserved along with its skeleton. The most distinctive feature of these impressions is the large “bumps” which are scattered across the skin. These bumps are referred to as osteoderms commonly in online discussions and even in the scientific literature on a least one occasion (Lindoso et al. 2013). However, they are not actually osteoderms but instead a type of scale with no bony component. This misconception likely arose from the superficial resemblance as well as the vague original description of the impressions. Bonaparte (1985) made no mention of any skin, while Bonaparte et al. (1990) only dedicated two sentences to the scales.

“The surface of the skin is made of rather low, conical protuberances of about 4 to 5 cm diameter, each with a modest keel and separated from one another by about 8 to 10 cm. The surface between the protuberances is rough, with rather rounded, low, and small granules about 5 mm in diameter that are separated from one another by narrow furrows.”

Czerkas & Czerkas (1997) provided the most detailed description of Carnotaurus integument, importantly noting that the bumps do not have bony cores. Unfortunately, this paper has remained obscure compared to the preceding ones and is rarely cited.

“Despite the comparatively soft definition, the typical dinosaurian scale pattern composed of individual turbercules in rosette patterns can be readily distinguished. The size of the individual scales range [sic] from about five to ten to twelve millimeters in diameter. Overall, there is a basic uniformity consisting of the smaller tubercules over the entire animal. Carnotaurus also has distinctive, low conical studs about eight to ten centimeters apart from each other, aligned in irregular rows along the sides of the animal. These structures are about four to five centimeters in diameter. Smaller studs were scattered towards the lower portions, with increasingly larger studs on the upper parts of the animal. The height of the larger conical studs is uncertain but may easily have been between three to five centimeters, possibly more. These may be unique to theropods, or perhaps the saurichia [sic] as somewhat similar conical structures have also been found on sauropods. The abundant conical studs that distinguishes [sic] the skin of Carnotaurus from other dinosaurs were probably made of a hypertrophied cluster of compacted scales, similar to what is seen in the dermal spines along the dorsal midline of hadrosaurs. Together, they take on the form of a single conical stud which often appears to have a slight median ridge or keel. There is no indication of a bony core, or ossification taking place within the conical studs. The absence of scutes or bony nodules is significant because one might expect that such prominent dermal structures would have would have some ossification for support, reflecting the shape of dermal ornamentation. However, as with hadrosaurs and sauropods, it is once again demonstrated, now among theropods, that the skin of dinosaurs can be unpredictably ornamental and complex.”

If not osteoderms, then what exactly are the bumps? “Protuberances” and “studs”, as they were previously called, are not very specific or helpful terms. In his revision of dinosaur scale terminology, Bell (2012) outlined different types that match the scales of Carnotaurus.

“Herein, the terms ‘basement’ or ‘basement-scales’ are used to describe the scales that form the major part of the integumentary surface. Collections of similar-sized basement-scales in a mosaic-like arrangement were described as ‘cluster areas’ by Osborn and this terminology is followed here. Basement-scales form the background pattern onto which larger and sporadically arranged scales are often imposed. These larger scales are frequently (but not invariably) of different morphology to the basement-scales and are referred to as ‘feature-scales’. […] Shield scales are circular or ovoid and interspersed among the surrounding scales as feature-scales. Consequently, they are notably larger than the surrounding basement-scales, ranging from 7 mm to several centimetres in diameter and have been referred to as ‘‘limpet-like’’ by some authors. Shields are typically flat or domed, and their surfaces may be smooth or corrugated.”

Using Bell’s definitions, the bumps on Carnotaurus are feature-scales since they are large (4-5 cm diameter) and surrounded by much smaller scales (0.5-1.2 cm diameter). More specifically, they are shield scales because they are circular and subconical in shape with keels (3-5 cm height). The surrounding scales are basement-scales that form cluster areas. Feature-scales have been found in one other theropod, Albertosaurus sarcophagus, which are also shield scales although far smaller (0.7 cm diameter; Bell et al. 2017). Ceratosaurus nasicornis is one of two theropods with preserved osteoderms, which have been found articulated along the back of the holotype skeleton as well as isolated (Gilmore, 1920; Madsen & Welles, 2000). The other is Tyrannosaurus rex, which has an osteoderm on top of each postorbital that Carr (2020) termed the epipostorbital.

Addendum (8/13/2021)

A complete redescription of the Carnotaurus skin impressions has just been published (Hendrickx & Bell, 2021). This paper describes the integument in much greater detail than previous papers and includes high-resolution, color photos of the material.

References

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