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| Borealopelta markmitchelli Brown, Henderson, Vinther, et al., 2017 |
Abstract
Although the evolution and function of “exaggerated” bony projections in ornithischian dinosaurs has been subject to significant debate recently, our understanding of the structure and morphology of their epidermal keratinized coverings is greatly limited. The holotype of Borealopelta, a new nodosaurid ankylosaur, preserves osteoderms and extensive epidermal structures (dark organic residues), in anatomic position across the entire precaudal length. Contrasting previous specimens, organic epiosteodermal scales, often in the form of horn-like (keratinous) sheaths, cap and exaggerate nearly all osteoderms, allowing for morphometric and allometric analyses of both the bony osteoderms and their horny sheaths. A total of 172 osteoderms were quantified, with osteoderm spine length and height being positively allometric with respect to basal length and width. Despite tight correlations between the different measures amongst all other osteoderms, the large parascapular spines represent consistent outliers. Thickness and relative contribution of the keratinized epiosteodermal scales/sheaths varies greatly by region, ranging from 2% to 6% for posterior thoracics, to ∼25% (1.3×) for the parascapular spines—similar to horn sheaths in some bovid analogues. Relative to the bony cores, the horny portions of the spines are strongly positively allometric (slope = 2.3, CI = 1.8–2.8). Strong allometric scaling, species-specific morphology, and significant keratinous extension of the cervicoscapular spines is consistent with elaboration under socio-sexual selection. This marks the first allometric analysis of ornithischian soft tissues.
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| Figure 1: Dorsal view of TMP 2011.033.0001, showing both photocomposite and schematic line drawing. (A) Photocomposite dorsal view of TMP 2011.033.0001. (B) Schematic line drawing of (A) showing osteoderm regions by color. (C) Inset showing constituent blocks of TMP 2011.033.0001, and their relative position within a body outline in dorsal view. Photocomposite (A), created using separate, orthogonal images of blocks A–C, D, E, F–I, and J and combined digitally to reduce parallax. Blocks F, G, H, and I represent reflected counterpart. |
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| Figure 2: Single dorsal photograph of TMP 2011.033.0001. Sacral region represents original part—reflected counterpart shown in Fig. 4. Scale equals 1 m. |
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| Figure 3: Interpretive scientific illustration of TMP 2011.033.0001 in dorsal view. Sacral region represents original part—reflected counterpart shown in Fig. 4. Scale equals 1 m. |
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| Figure 4: Composite dorsal view of TMP 2011.033.0001. Photocomposite created using separate images of blocks A–C, D, E, F–I, and J (see Fig. 1) and combined digitally to both reduce parallax and remove gaps. Blocks F, G, H and I represent reflected counterpart of sacral part in Fig. 2. Photographs of individual blocks were digitally modified (brightness, contrast, etc.) to removed different lighting conditions, and to illustrate an average composite of the entire specimen. Scale equals 1 m. |
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| Figure 15: Comparisons of the size of the bony core and keratinous sheath of the parascapular spine of Borealopelta to modern bovid and squamate analogues. (A) Absolute size of the bone core (horncore or osteoderm) (yellow) and the overlying keratinous/horn sheath (grey) for the parascapular spine of TMP 2011.033.0001 (top) as well as averages for several bovid and squamate taxa (lower). (B) Schematic representations of the relative bony and keratinous components of select spines/horns (adjusted to same size). Data for Oreamnos americanus (n = 6, 20) and Oreamnos harringtoni (n = 10, 13) from Mead & Lawler (1995), Bos (n = 18) from Grigson (1975), Antilocapra (n = 3) and Bison (n = 18) from Borkovic (2013), Ovis nivicola (n = 2), Ovis dalli (n = 2), Ovis ammon (n = 2), Ovis canadensis nelsoni (n = 5), Ovis canadensis canadensis (n = 8), Capra ibex sibirica (n = 4) and Capra ibex ibex (n = 5) from Bubenik (1990), Trioceros (n = 1) from TMP 1990.007.0350, Phrynosoma solare (n = 1) from LACM 123351, and P. asio (n = 1) from WLH 1093. |
Conclusion
The combined results showing that the osteoderm spines, and their keratinous coverings, are positively allometric (regionally); and that the anterior portion of the osteoderm series is both highly variable and has species specific morphology, provided new insights into the function and evolution of these structures. Similar results have been obtained from analysis of the exaggerated structures of most other ornithischian clades: Hadrosauridae (Dodson, 1975; Evans, 2010; McGarrity, Campione & Evans, 2013), Ceratopsia (Currie et al., 2016; Dodson, 1976; Hone, Wood & Knell, 2016; Lehman, 1990), Pachycephalosauria (Horner & Goodwin, 2009; Schott et al., 2011). These results in other ornithischian clades have been used to support the hypothesis that these exaggerated structures may have functioned, and evolved, in the context of socio-sexual selection (Hone, Wood & Knell, 2016; Hopson, 1975; Sampson, 1997). Similar hypotheses have been proposed for thyreophoran spines and plates (Carpenter, 1998; Hopson, 1977; Main et al., 2005; Padian & Horner, 2011), but until now had lacked commensurate morphometric backing. This argument is strengthened further when the parascapular spine is considered. Not only does this element show a different pattern of scaling than the rest of the series, but the absolute sizes of the keratin sheath and bony core are similar to the horns of extant bovids, and the relative sizes similar to the horns of some extant squamates, both of which are thought to function in socio-sexual display (Bustard, 1958; Farlow & Dodson, 1975; Geist, 1966). Combined with recent evidence suggesting this spine may, in life, have been pigmented differently than the rest of the osteoderms (Brown et al., 2017), this suggests this spine may have function as a visual socio-sexual display signal with conspecifics.
Caleb M. Brown. 2017. An Exceptionally Preserved Armored Dinosaur reveals the Morphology and Allometry of Osteoderms and their Horny Epidermal Coverings. PeerJ. 5:e4066. DOI: 10.7717/peerj.4066
New research analyses body armour of Borealopelta RoyalTyrrellMuseum.wordpress.com/2017/11/29/new-research-analyses-body-armour-of-borealopelta/ via @RoyalTyrrell
