Newly detected data from Haestasaurus and review of sauropod skin morphology suggests Early Jurassic origin of skin papillae

Abstract

Discovered in 1852, the scaly skin belonging to Haestasaurus becklesii was the first to be described in any non-avian dinosaur. Accordingly, it has played a crucial role in the reconstruction of sauropod integument and dinosaurs more broadly. Here, we reassess this historic specimen using Laser-Stimulated Fluorescence (LSF), revealing extensive, previously unknown regions of skin that augment prior interpretations of its integumentary morphology and taphonomy. Under white light, polygonal-subrounded, convex scales are visible on one side of the block ('side A'), but LSF reveals extensive smaller and more flattened scales, which are diagenetically fragmented, on the reverse block surface ('side B'). Contrary to the prior interpretation that the visible scales are the epidermal undersides, the presence of convex, intrascale papilliform textures on side A suggests that the external skin surface is exposed. We define intrascale papillae and provide a review of sauropod skin morphology, which clarifies that intrascale papillae are unique to and widespread across stem Neosauropoda, and likely have an evolutionary origin in the Early Jurassic. Intrascale papillae may ultimately have been integral to the evolution of gigantism in this charismatic clade.

Fig. 1. ‘Side A’ of NHMUK R1868, showing relatively large subrounded or polygonal scales (pentagonal–heptagonal) with generally positive relief.

Photographs under a white light and b, c laser-stimulated fluorescence. d Close-up of small intrascale papillae covering the skin surface. e Interpretive outline drawing, excluding intrascale papillae. Scale bar for a, b and e are equivalent. All scale bars as indicated.

Fig. 2. ‘Side B’ of NHMUK R1868, showing relatively small, flattened, subrounded or polygonal scales (quadrangular–octagonal).

Photographs under a white light and b, c laser-stimulated fluorescence. d Interpretive outline drawing. Scale bar for all parts as indicated.

Fig. 3. Photograph of a portion of ‘side B’ of NHMUK R1868 under laser-stimulated fluorescence, showing diagenetic fragmentation of epidermal scales.

Location within the entire block is indicated in Fig. 2a. Arrows identify several scales in various stages of fragmentation, while the red circle surrounds three separated fragments, which are interpreted to form a single scale that has broken apart. Scale bar as indicated.

Fig. 4. Exemplary sauropod skin specimens showing similar intrascale papilliform surface textures to those of NHMUK R1868.

a Skin from the scapular region of the non-titanosauriform macronarian Tehuelchesaurus benitezii (MPEF-PV 1125/1), from the Cañadón Calcáreo Formation (Upper Jurassic) of Argentina (photo: E. Ruigomez). b Section of integument (CMC VP8075) from the diplodocid Diplodocus sp., from the Mother’s Day Quarry within the Morrison Formation (Upper Jurassic) of Montana, USA (photo: M. Rubin). c Single scale of an indeterminate sauropod, most likely Apatosaurus sp. (part of MWC 5537), from the Mygatt-Moore Quarry within the Morrison Formation (Upper Jurassic) of Colorado, USA (photo: J. McHugh). Arrows in b and c indicate exemplary papillae that are worn and exposed in cross section. All scale bars equal 1 cm.

Fig. 5. Time-calibrated phylogeny showing the known distribution of intrascale papillae within Eusauropoda.

Tree topology and time-calibrations for non-macronarian nodes are simplified from Mannion et al.: Fig. 44, with two exceptions: Camarasaurus is presented as a non-titanosauriform macronarian (after Mannion et al.: Fig. 40), and Barosaurus replaces Supersaurus as the sister taxon of Diplodocus (after Xu et al.: Fig. 3). Tree topology and time-calibrations for macronarian nodes are simplified from Mannion et al.: Fig. 40, with removal of the clade containing Janenschia and Haestasaurus. Green bolded genera indicate taxa for which preserved skin is confidently known, while black bold for Apatosaurus reflects possible skin occurrence. Known papilliform scale textures are indicated for each bolded taxon. Arrows indicate continuation of a particular lineage into the Late Cretaceous (not shown). Note: the presence of papilliform scale surfaces does not necessarily exclude the presence of smooth scale surfaces in other body regions.