Oldest preserved umbilical scar reveals dinosaurs had ‘belly buttons’

Abstract

Background In egg-laying amniotes, the developing embryo is tethered to a number of the extraembryonic membranes including the yolk sac and allantois that deliver oxygen and nutrients and remove metabolic waste products throughout embryonic development. Prior to, or soon after hatching, these membranes detach from the animal leaving a temporary or permanent umbilical scar (umbilicus) equivalent to the navel or ‘belly button’ in some placental mammals, including humans. Although ubiquitous in modern mammals and reptiles (including birds), at least early in their ontogeny, the umbilicus has not been identified in any pre-Cenozoic amniote. Results We report the oldest preserved umbilicus in a fossil amniote from a ~130-million-year-old early-branching ceratopsian dinosaur, Psittacosaurus. Under laser-stimulated fluorescence (LSF), the umbilicus is revealed as an elongate midline structure delimited by a row of paired scales on the abdomen. The relatively late ontogenetic stage (close to sexual maturity) estimated for the individual indicates that the umbilicus was probably retained throughout life. Conclusions Unlike most extant reptiles and birds that lose this scar within days to weeks after hatching, the umbilicus of Psittacosaurus persisted at least until sexual maturity, similar to some lizards and crocodylians with which it shares the closest morphological resemblance. This discovery is the oldest record of an amniote umbilicus and the first in a non-avian dinosaur. However, given the variability of this structure in extant reptilian analogues, a persistent umbilical scar may not have been present in all non-avian dinosaurs.

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

“…It is not necessary to repeat those descriptions here. SMF R 4970 is remarkable for the extent of soft tissue preservation, which has revealed unprecedented integumentary structures including the dorsal row of bristles on the tail, the cloaca, umbilical scar, as well as evidence of countershading 16,19,[23][24][25] . Additional integumentary features either not previously recognized or expounded upon include: (1) the keratinous jugal "horn" 23 ; (2) enlarged scales of the ischial callosity 19,25 ; (3) the row(s) of feature scales on the mid-distal tail, and; (4) the arthral arrangement of the digital pads.…”

“…Bell et al 24 recently assessed the age of SMF R 4970 based on femoral length. To summarize those findings, the right femur of SMF R 4970 is ~140 mm long, which is similar to the femoral lengths of the specimens of P. houi (a senior synonym of P .…”

“…Age of the individual. Bell et al 24 recently assessed the age of SMF R 4970 based on femoral length. To summarize those findings, the right femur of SMF R 4970 is ~140 mm long, which is similar to the femoral lengths of the specimens of P. houi (a senior synonym of P. lujiatunensis; see Mayr et al 23 ) IVPP V12617 (138 mm) and V18344 (145 mm), LPM R00128 (135 mm) and R00138 (144 mm) and PKUP V1053 (149 mm) and V1056 (135 mm), which belong to ~6-7 year old subadults (see Table 1 and Fig.…”

“…It is also the sole marginocephalian to include skin from the region of the head, limbs and tail and the only dinosaur in which the keratinous cranial horn is preserved 23 (see “skin morphology in ceratopsian dinosaurs” below). More importantly, the integument of Psittacosaurus SMF R 4970 preserves evidence of color patterns and countershading and is the only dinosaur to preserve an umbilical scar 24 or the cloaca 19 , 25 . The latter was recently revealed to be unique among tetrapods in having a V-shaped convergence of the two darkly-pigmented lateral lips, in addition to a bulbous dorsal lobe 25 .…”

“…In recent years, laser-stimulated fluorescence (LSF) has become a powerful tool in paleontology for highlighting and/or revealing additional soft tissue details in fossils that are otherwise unseen under white light conditions 28 , 29 . The application of this technology to Psittacosaurus SMF R 4970 has quite literally illuminated new aspects of the tail bristles 23 and color patterns 19 and permitted the identification of the cloaca 25 and umbilical scar 24 . In addition, Vinther et al 19 (their supplementary information) provided some preliminary observations of the scale architecture and taphonomy of SMF R 4970 using this technique.…”