Extreme Ontogenetic Changes in a Ceratosaurian Theropod

Abstract

Ontogenetic variation is documented within many dinosaur species, but extreme ontogenetic changes are rare among dinosaurs, particularly among theropods. Here, we analyze 19 specimens of the Jurassic ceratosaurian theropod Limusaurus inextricabilis, representing six ontogenetic stages based on body size and histological data. Among 78 ontogenetic changes we identify in these specimens, the most unexpected one is the change from fully toothed jaws in the hatchling and juvenile individuals to a completely toothless beaked jaw in the more mature individuals, representing the first fossil record of ontogenetic edentulism among the jawed vertebrates. Jaw morphological data, including those derived from Mi-CT and SR-μCT scanning of Limusaurus specimens, reveal dental alveolar vestiges and indicate that ontogenetic tooth loss in Limusaurus is a gradual, complex process. Our discovery has significant implications for understanding the evolution of the beak, an important feeding structure present in several tetrapod clades, including modern birds. This radical morphological change suggests a dietary shift, probably from omnivory for juvenile Limusaurus to herbivory for adult Limusaurus, which is also supported by additional evidence from gastroliths and stable isotopes. Incorporating new ontogenetic information from Limusaurus into phylogenetic analyses demonstrates surprisingly little effect on its placement when data from different stages are used exclusively, in contrast to previous analyses of tyrannosaurids, but produces subtle differences extending beyond the placement of Limusaurus.

Dental anatomy of the apex predator Sinraptor dongi (Theropoda: Allosauroidea) from the Late Jurassic of China

“…The distribution of apomorphic dental character states was visualized on a fully topologically constrained tree (sensu Hendrickx et al 2020) using WinClada 1.00.08 (Nixon 2002) on the basis of a Nexus file, with the data matrix and the phylogenetic tree, both created with Mesquite 3.2 (Maddison and Maddison 2017). The tree topology is congruent with the results recovered by Müller et al (2018), fifth phylogenetic analysis (which uses the data matrix of Langer et al 2017) for non-neotheropod saurischians, Ezcurra (2017) for non-averostran neotheropods, Rauhut and Carrano (2016) and Wang et al (2017) for Ceratosauria, Carrano et al (2012) and Rauhut et al (2012 for noncoelurosaurian tetanurans, Brusatte and Carr (2016) for Tyranno-sauroidea, and Cau et al (2017) on the basis of the data set of Brusatte et al (2014) for neocoelurosaurs (i.e., Compsognathidae + Maniraptoriformes; sensu Hendrickx et al 2019).…”

“…The fauna is dominated by herbivorous dinosaurs belonging to five clades ( Fig. 1A): Sauropodomorpha with Mamenchisaurus sinocanadorum Russell and Zheng,1993; Tienshanosaurus chitaiensis Young, 1937; and Bellusaurus sui Dong, 1990(and see Dong 1992; Moore et al 2018); Theropoda with Limusaurus inextricabilis Xu et al, 2009 (and see Wang et al 2016Wang et al , 2017; Ornithischia clade indet. With Gongbusaurus wucaiwanensis Dong, 1989; Stegosauria with Jiangjunosaurus junggarensis Jia et al, 2007; and Marginocephalia with Yinlong downsi Xu et al, 2006b (and see Han et al 2016Han et al , 2018; and Hualianceratops wucaiwanensis Han et al, 2015. In addition to the large-bodied predator Sinraptor dongi (i.e., approximately 7 m in length; Tanke and Currie 1998), carnivorous theropods are represented by five small-to medium-sized dinosaurs (Fig.…”

Developmental patterns and variation among early theropods

“…; Wang et al. ). Characters such as the presence/absence of the trochanteric shelf, which is highly variable throughout ontogeny and among individuals (Griffin & Nesbitt, ,b), may have to be excluded from or highly altered in analyses to provide phylogenetically informative patterns.…”

“…Therefore, understanding what phylogenetic characters are variable in ontogeny, and how the scoring of these characters influences the phylogenetic placement of taxa that may only be known from immature individuals, is important for properly reconstructing evolution (Butler & Zhao, 2009;Evans et al 2011;Fowler et al 2011;Tsuihiji et al 2011;Campione et al 2013;Tsai & Fordyce, 2014). This problem has been especially noted in early dinosaurs and their relatives, which possess many phylogenetically important characters that are variable during development (Tykoski, 2005;Griffin & Nesbitt, 2016a,b;Barta et al 2018;Wang et al 2017). Characters such as the presence/absence of the trochanteric shelf, which is highly variable throughout ontogeny and among individuals (Griffin & Nesbitt, 2016a,b), may have to be excluded from or highly altered in analyses to provide phylogenetically informative patterns.…”

A Small-Bodied Troodontid (Dinosauria, Theropoda) from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China

“…The most unusual feature of LH PV39 is the presence of the extremely dorsoventrally compressed sacral centra. Spool-like sacral centra is typical for most theropods including coelophysids (Tykoski 2005), neoceratosaurian (Wang et al . 2017), basal tetanuran (Hu 1993), tyrannosaurids (Brochu 2003), ornithomimus (Kobayashi & Lü 2003; Makovicky et al .…”