Morphology and distribution of scales, dermal ossifications, and other non‐feather integumentary structures in non‐avialan theropod dinosaurs

Abstract

Modern birds are typified by the presence of feathers, complex evolutionary innovations that were already widespread in the group of theropod dinosaurs (Maniraptoriformes) that include crown Aves. Squamous or scaly reptilian‐like skin is, however, considered the plesiomorphic condition for theropods and dinosaurs more broadly. Here, we review the morphology and distribution of non‐feathered integumentary structures in non‐avialan theropods, covering squamous skin and naked skin as well as dermal ossifications. The integumentary record of non‐averostran theropods is limited to tracks, which ubiquitously show a covering of tiny reticulate scales on the plantar surface of the pes. This is consistent also with younger averostran body fossils, which confirm an arthral arrangement of the digital pads. Among averostrans, squamous skin is confirmed in Ceratosauria (Carnotaurus), Allosauroidea (Allosaurus, Concavenator, Lourinhanosaurus), Compsognathidae (Juravenator), and Tyrannosauroidea (Santanaraptor, Albertosaurus, Daspletosaurus, Gorgosaurus, Tarbosaurus, Tyrannosaurus), whereas dermal ossifications consisting of sagittate and mosaic osteoderms are restricted to Ceratosaurus. Naked, non‐scale bearing skin is found in the contentious tetanuran Sciurumimus, ornithomimosaurians (Ornithomimus) and possibly tyrannosauroids (Santanaraptor), and also on the patagia of scansoriopterygids (Ambopteryx, Yi). Scales are surprisingly conservative among non‐avialan theropods compared to some dinosaurian groups (e.g. hadrosaurids); however, the limited preservation of tegument on most specimens hinders further interrogation. Scale patterns vary among and/or within body regions in Carnotaurus, Concavenator and Juravenator, and include polarised, snake‐like ventral scales on the tail of the latter two genera. Unusual but more uniformly distributed patterning also occurs in Tyrannosaurus, whereas feature scales are present only in Albertosaurus and Carnotaurus. Few theropods currently show compelling evidence for the co‐occurrence of scales and feathers (e.g. Juravenator, Sinornithosaurus), although reticulate scales were probably retained on the mani and pedes of many theropods with a heavy plumage. Feathers and filamentous structures appear to have replaced widespread scaly integuments in maniraptorans. Theropod skin, and that of dinosaurs more broadly, remains a virtually untapped area of study and the appropriation of commonly used techniques in other palaeontological fields to the study of skin holds great promise for future insights into the biology, taphonomy and relationships of these extinct animals.

The exquisitely preserved integument of Psittacosaurus and the scaly skin of ceratopsian dinosaurs

“…More crownward birds evolved a mesarthral condition, in which the interpad crease corresponds to the interphalangeal joint; however, both conditions are present in extant birds and can vary among individuals 54 . In non-avian theropods, an arthral arrangement is present in allosauroids ( Concavenator 55 ), tyrannosauroids ( Santanaraptor 6 ), and maniraptorans ( Sinornithosaurus [GMC 91, STM 5-172]; Anchiornis [STM-0-7] 6 , 29 ). To our knowledge, Psittacosaurus is the first ornithischian to preserve direct evidence of the arthral pad configuration.…”

“…T he first report of scaly skin in a non-avian dinosaur (hereafter, dinosaur) was that of a sauropod by Mantell in 1852, but which was incorrectly identified as a giant crocodylian at the time [1][2][3][4] . The discovery of "typical" reptilian scales among dinosaurs for the remainder of the nineteenth and much of the twentieth century has been regarded with some degree of ambivalence 5,6 , although the discovery of feathered specimens from Liaoning Province of China in the 1990s (e.g. refs.…”

Oldest preserved umbilical scar reveals dinosaurs had ‘belly buttons’

“…To our knowledge, an umbilical scar has not been previously reported in any fossil amniote [ 27 ], although the ‘umbilical cord’ was described in a Devonian-aged viviparous placoderm fish [ 28 ]. Embryonic skin has been reported in only two non-avian dinosaurs [ 29 – 31 ], neither of which show indications of an embryonic umbilicus, and few other specimens preserve naked integument in this region of the abdomen that can be compared with SMF R 4970. Two unique specimens that do—the hadrosaurid Edmontosaurus annectens ‘mummy’ AMNH FARB 5060 and the juvenile ankylosaurian Liaoningosaurus paradoxus IVPP V12560—show no indication of any structure that could be interpreted as an umbilicus, even though the latter was less than a year old at the time of death [ 32 ].…”

The Exquisitely Preserved Dinosaur Skin of Psittacosaurus and the Scaly Skin of Ceratopsian Dinosaurs

“…Scale terminology largely follows that outlined by Bell (2012) and Hendrickx et al (2022). The term 'inclusion' (or scale inclusion) follows its use in crocodylian literature to refer to small, variably-shaped scales that ll irregular gaps between adjacent larger, more uniformly sized/shaped scales that form the main basement.…”

“…the rst REPORT of scaly skin in a non-avian dinosaur (hereafter, dinosaur) was that of a sauropod by Mantell in 1852, but which was incorrectly identi ed as a giant crocodylian at the time (Mantell 1852, Czerkas 1997, Upchurch et al 2015. The discovery of 'typical' reptilian scales among dinosaurs for the remainder of the 19th and much of the 20th Century has been regarded with some degree of ambivalence (Czerkas 1994, Hendrickx et al 2022, although the discovery of feathered specimens from Liaoning Province of China in the 1990s (e.g., Chiappe andWitmer 2002, Chiappe 2007) has since spurred an intense interest in the integument of dinosaurs. However, outside Hadrosauridae, which includes several 'mummi ed' specimens covered with skin (see review by Bell 2014), the scaly integument of dinosaurs is still surprisingly poorly known.…”