Morphology and osteohistology of a large‐bodied caenagnathid (Theropoda, Oviraptorosauria) from the Hell Creek Formation (Montana): implications for size‐based classifications and growth reconstruction in theropods

Abstract

Oviraptorosaurs, like many coelurosaurians, are frequently diagnosed using incomplete or fragmentary skeletal remains, with factors such as body size often used to assign material to a particular taxon or as a basis for describing new species. Here we describe a partial skeleton, ROM VP 65884, from the Hell Creek Formation (Montana, USA), and through morphological and phylogenetic comparisons identify it as belonging to Caenagnathidae, and probably referable to Anzu wyliei. We perform multi-element osteohistological sampling of the tibia, fibula, manual phalanx, rib and gastralium of ROM VP 65884 to determine the ontogenetic status of the individual, and to perform a detailed assessment of the intra-skeletal variation present in a series of histological proxies for growth and development. Skeletochronological analysis suggests ROM VP 65884 was still actively growing at time of death. Lacunar area was variable, but consistent with values reported for other dinosaurs and vertebrates. Osteocyte lacunar density was found to be relatively similar to the few previous measurements known from coelurosaurian dinosaurs, and preserved no ontogenetic pattern. Osteocyte lacunar density (OLD) was much higher in sampled rib and gastralia elements when compared to limbs, and the high variability in OLD overall lends further support against the hypothesis of this feature acting as a proxy for mass-specific growth rate. Our results stress the importance of using osteohistological analyses to constrain variation and permit ontogenetic assessment, and suggest caution be taken when using size as a determinant for referral of disassociated elements to specific or new coelurosaur taxa.

Caenagnathids of the Dinosaur Park Formation (Campanian) of Alberta, Canada: anatomy, osteohistology, taxonomy, and evolution

“…In both specimens, the endosteal layer is composed of several layers and secondary osteons are well developed, showing multiple internal lamellae, each of which is evidence of prolonged secondary remodeling (Bromage et al 2009). No ontogenetic series of caenagnathids is yet known, but some insights into their general growth patterns can be made from the new material sampled here and previous descriptions of caenagnathid osteohistology Cullen et al 2020;. Several lines of evidence suggest that growth early in life was relatively rapid, as in other coelurosaurs (Varricchio 1993;Skutschas et al 2016;Woodward et al 2020).…”

“…Although comparing disparate bones to assess maturity is not ideal, several factors suggest that this is unlikely to introduce significant error into the current study. Intraskeletal histovariability in theropods (Lee and O'Connor 2013), including caenagnathids (Cullen et al 2020), can lead to significant differences in quantitative variables like osteocyte lacunar density, line of arrested growth (LAG) spacing, and LAG counts. However, assessments of relative maturity remain consistent between these bones, especially where an external fundamental system (EFS; Padian and Lamm 2013) is present (Lee and O'Connor 2013).…”

“…Funston et al (2015) suggested that Caenagnathus collinsi was intermediate in body size between Chirostenotes pergracilis and Anzu wyliei, but the large pubes described here (UALVP 56638) are the same size as those of CM 78000 (Anzu wyliei), showing that Caenagnathus collinsi was much larger than currently estimated. However, Cullen et al (2020) showed that large material probably representing Anzu wyliei was still actively growing, and thus Anzu wyliei may also have been larger than current-ly recognized. Funston et al (2015) referred some large isolated elements to Caenagnathus collinsi on the basis of larger size and some morphological differences, but in light of new evidence, some of these referrals are doubtful.…”

“…UALVP 56638 is similar in size and length to the pubes of the type of Anzu wyliei (CM 78000), which suggests Caenagnathus collinsi was a large animal. Cullen et al (2020) recently described some large elements probably referable to Anzu wyliei (ROM VP 65884), and their osteohistological analyses of the ribs, gastralia, tibia, fibula, and manual phalanx show that this individual was still actively growing, despite its large size. Thus, known specimens of Anzu wyliei may not represent their maximum body size, and it may have been significantly larger at adulthood than Caenagnathus collinsi.…”

“…The two youngest specimens are likely TMP 1985.043.0070 and UALVP 57349 (Funston and Currie 2018), a pair of fused dentaries and a tibia, respectively. In each, there is a high proportion of woven-fibered bone, dense vasculature, and dense osteocyte lacunae (although osteocyte lacunar density is unlikely to reflect mass-specific growth rates; see Cullen et al 2020). UALVP 57349 preserves a possible CGM in the mid-cortex, which, rather than manifesting as a distinct LAG, is represented by an annulus of parallel-fibered bone .…”