A Phylogenetic Approach to Ontogeny and Heterochrony in the Fossil Record: Cranial Evolution and Development in Anguimorphan Lizards (Reptilia: Squamata)

Abstract

The incorporation of ontogeny into the interpretation of the vertebrate fossil record promises major advances in palaeontology, systematics, and macroevolution. Here, a key additional component, the incorporation of phylogenetic bracketing into ontogenetic considerations, is demonstrated using cranial anatomy in anguimorphan lizards, a diverse modern clade with an extensive fossil record. The obstacles of fragmentary disarticulated fossil material and low representation in museum collections are overcome by using detailed analysis of individual elements and binning into broad ontogenetic stages, respectively. Results indicate the prevalence of classical macroevolutionary phenomena, notably heterochrony and homoplasy (convergence), throughout anguimorphan evolution. Furthermore, two problematic fossil anguimorph taxa are examined, both of which are unusually small for their clades, suggesting either immaturity or dwarfism. Using extant phylogenetic brackets of ontogenetic trajectories to distinguish between these hypotheses, it is shown that the holotype of one of these taxa is indeed a juvenile (also calling into question its taxonomy) and that the other is a dwarf. It is expected that a phylogenetic approach to ontogeny will yield similar insights across a broad range of fossil and extant organisms.

The Braincase Anatomy of the Late Cretaceous DinosaurAlioramus(Theropoda: Tyrannosauroidea)

“…5). These observations are consistent with the hypothesis that tyrannosaurid frontals widen with postnatal growth (Carr, 1999;Currie, 2003;Carr and Williamson, 2004)-an ontogenetic trajectory present in many, if not most, amniote clades, at least for the postorbial region of the frontal (e.g., Abdala et al, 2001;Bever, 2008;Bhullar, 2012). Basal tyrannosauroids, such as Dilong paradoxus (Xu et al, 2004), also have long frontals, suggesting that widening of the rostral cranial vault may be both an ontogenetic and phylogenetic trend within Tyrannosauroidea.…”

“…The presence of a stout process in IGM 100/1844 is consistent with its subadult status, but it may also reflect the overall low form of the entire skull that is apomorphic in Alioramus. A relative lengthening of the process during the postnatal ontogeny of tyrannosaurs is in contrast to the more generally recognized trajectory in reptiles, in which is the process becomes stouter with age (e.g., Bever et al, 2005;Bhullar, 2012). The rostral corner of the process continues as a ridge that becomes confluent with the lateral bounding wall of the subsellar recess.…”

“…Together, these crests separate the lateral wall of the braincase from the supratemporal fossa dorsally. In large, presumably adult, Daspletosaurus (CMN 8506) and Tyrannosaurus (AMNH FARB 5117) the combined crest is prominent with a sharp lateral edge; the indistinct condition of the ridge in IGM 100/1844 likely is a reflection of its immaturity, as this condition is also seen in immature Daspletosaurus (TMP 1994.143.0001) and is known to characterize immature specimens of other reptile taxa (e.g., Bever et al, 2005;Bhullar, 2012).…”

Postnatal Development of the Skull of Dinilysia patagonica (Squamata‐Stem Serpentes)

“…). Additionally, the supratemporal processes become larger in the posterolateral direction, in similar fashion to those of lizards (Maisano, ; Bhullar, ), Anilius scytale , and figured juvenile and adult specimens of Cylindrophis ruffus (Cundall and Irish, : Fig. 2.35).…”

“…The paroccipital process, a characteristic feature of the opisthotic bone of reptiles, usually consists of an elongated, posterolaterally directed structure with a distal expanded tip capped by cartilage. During ontogeny, this process elongates and its cartilaginous extremity undergoes endochondral calcification and fuses with the distal tip in adults (Maisano, ,b; Torres‐Carvajal, ; Tarazona et al, ; Bhullar, ). A paroccipital process is present in adult specimens of Dinilysia (Estes et al, ; Zaher and Scanferla, ), and our observations of the ontogenetic sequence demonstrate that the genesis of this feature is the result of progressive elongation during postnatal development (Fig.…”