Effects of phylogenetic uncertainty on fossil identification illustrated by a new and enigmatic Eocene iguanian

Abstract

Fossil identifications made in a phylogenetic framework are beholden to specific tree hypotheses. Without phylogenetic consensus, the systematic provenance of any given fossil can be volatile. Paleobiogeographic and divergence time hypotheses are contingent on the accurate systematic placement of fossils. Thus, fossil diagnoses should consider multiple topologies when phylogenetic resolution or clear apomorphies are lacking. However, such analyses are infrequently performed. Pleurodonta (Squamata: Iguania) is an ancient and frequently-studied lizard clade for which phylogenetic resolution is notoriously elusive. I describe a skull fossil of a new pleurodontan lizard taxon from the Eocene deposits of the Willwood Formation, Wyoming, and use the new taxon as a case-study to explore the effects of phylogenetic uncertainty on fossil identification. The relationships of the new taxon differ considerably among analyses, and resulting interpretations are correspondingly disparate. These results illustrate generalizable and severe issues with fossil interpretations made without consideration of alternative phylogenetic hypotheses.

The making of calibration sausage exemplified by recalibrating the transcriptomic timetree of jawed vertebrates

“…Above (Materials and methods: Calibrations: Node 132 – Iguanidae) I argue for using the age of Kopidosaurus , about 53 million years, as the minimum age of Iguanidae. Kopidosaurus was named and described from a largely complete skull by Scarpetta (2020) in a publication where the words “oldest” and “older” do not occur at all, and “first” and “ancient” only occur in other contexts – even though Scarpetta (2019) had just published on calibration dates for molecular divergence date analyses. The reason may be that he did not think Kopidosaurus was the oldest iguanid; one of the two matrices he used for phylogenetic analyses contained the 56-Ma-old Suzanniwana , and his analyses found it as an iguanid (Scarpetta, 2020: supp.…”

“…Moreover, he was most likely aware that the publication that named and described Suzanniwana (Smith, 2009) also named and described Anolbanolis from the same site and age and argued that both of them – known from large numbers of isolated skull bones – were iguanids. Yet, Anolbanolis has never, to the best of my knowledge, been included in any phylogenetic analysis; and Conrad (2015), not mentioning Anolbanolis and not cited by Scarpetta (2020), had found the phylogenetic position of Suzanniwana difficult to resolve in the analysis of a dataset that included a much larger sample of early pan-iguanians.…”

“…Burbrink et al (2020) found extremely short internal branch lengths for the basal radiation of Iguanidae; similarly, Scarpetta (2020) found the phylogeny of Iguanidae difficult to resolve, which likewise suggests a fast radiation (but might also be a consequence of the sparse taxon sampling in both matrices). Paleoecologically, the recovery phase immediately after the Cretaceous-Paleogene boundary suggests itself as the time of such a radiation.…”

“…Conrad (2015), nowhere mentioning Anolbanolis , stated that Suzanniwana was one of the two “taxa with the most volatile positions within this analysis”, but only published the Adams consensus of that analysis, which shows Suzanniwana as part of a polytomy that also encompasses Corytophanidae and a clade containing all other extant iguanids – whether Suzanniwana remains inside Iguanidae in all of the 98 most parsimonious trees or is placed as the sister-group of Iguanidae in some could only be determined by repeating the analysis. Scarpetta (2020: supp. inf.)…”

“…The oldest certain iguanid, then, is the oldest one known from articulated remains: the fairly highly nested Kopidosaurus , even though it is not clear where it is nested exactly (Scarpetta, 2020). Being slightly older than a 52.59 ± 0.12 Ma old tuff that overlies it (Scarpetta, 2020), and being followed by Afairiguana (which forms an exclusive clade with the extant Polychrus and Dactyloidae in the analysis of Conrad, 2015), the highly nested corytophanid Babibasiliscus and the less highly nested corytophanid Geiseltaliellus (Conrad, 2015) within the next five million years, it establishes a rather tight minimum age of 53 Ma for this calibration point, very close to the abovementioned 56 Ma.…”