Abstract
Neptunidraco ammoniticus is a thalattosuchian crocodylomorph from the Rosso Ammonitico Veronese Formation (RAVF, Middle Jurassic) of northern Italy. Erected from one partial specimen, Neptunidraco is pivotal in reconstructing thalattosuchian evolution, being it the oldest known member of Metriorhynchidae. Two additional RAVF thalattosuchians have been referred to Neptunidraco. A revised diagnosis of N. ammoniticus is provided here. Using a well-sampled phylogenetic data set of Crocodylomorpha, the affinities of all three RAVF thalattosuchian specimens are investigated simultaneously for the first time using parsimony tree-search strategies and Bayesian inference using the Fossilized Birth-Death with Sampled Ancestor (FBDSA) model. The results of the alternative analyses are not consistent in the placement of the RAVF specimens. The holotype of N. ammoniticus is consequently referred to Metriorhynchidae incertae sedis. The first referred specimen is recovered in various alternative placements among Metriorhynchoidea. The third and most fragmentary specimen is recovered as a crocodylomorph of uncertain affinities in the parsimony analysis and in the undated Bayesian analysis, and a metriorhynchoid sister taxon of the second RAVF specimen in the tip-dated Bayesian analysis. Only a subset of the results in the parsimony-based analyses supports the referral of the latter two specimens to Neptunidraco. The unusually high rate of morphological divergence for the Neptunidraco branch, inferred in previous iterations of the Bayesian inference analyses but not recovered in the novel analysis, was likely an artifact of the a priori constraint of all RAVF thalattosuchians into a single taxonomic unit, and of the arbitrarily fixed tip-age priors for the terminal taxa. These results confirm the utility of specimen-level morphological analysis and of combined tree-search strategies for inferring the affinities and the inclusiveness of fragmentary but significant fossil taxa, and reinforce the importance of incorporating stratigraphic uncertainty as prior in tip-dated Bayesian inference analyses.
Highlights
The thalattosuchian fossil record from the Rosso Ammonitico Veronese Formation (RAVF, Middle-Upper Jurassic) of the Veneto Region is represented by a very limited number of specimens collected between the 18th and the 20th Centuries (Cau, 2014; Cau & Fanti, 2011; Cau & Fanti, 2015)
The two specimens from the RAVF show some of the mandibular features listed in the differential diagnosis of N. ammoniticus, these are widespread among Metriorhynchoidea, and are not sufficient for unambiguously referring FMCR SP3839 and Museo Paleontologico Universitario di Padova (MGP) 6552 to that species
With the exclusion of the subset of topologies placing FMCR SP3839 among neosuchians, discussed here, there is no evidence of a second clade of crocodylomorphs in the RAVF, because all known archosaurian material so far recovered from this formation since the 18th Century has consistently been referred to Thalattosuchia (Cau, 2014; Cau & Fanti, 2015)
Summary
The thalattosuchian fossil record from the Rosso Ammonitico Veronese Formation (RAVF, Middle-Upper Jurassic) of the Veneto Region (northern Italy) is represented by a very limited number of specimens collected between the 18th and the 20th Centuries (Cau, 2014; Cau & Fanti, 2011; Cau & Fanti, 2015). The only valid taxon is Neptunidraco ammoniticus Cau & Fanti (2011), based on a specimen which includes a partial skull and mandible (both missing the rostral end) semi-articulated to the cervical series, and is dated to the latest Bajocian-earliest Bathonian (Cau & Fanti, 2011). The specimen, housed in the Museo Civico of Rovereto (Trento Province, northern Italy) under accession number FMCR SP3839 (Figs. 1G and 1H), shares metriorhynchid and geosaurine synapomorphies and is indistinguishable in the overlapping elements from the other two Rosso Ammonitico Veronese Formation specimens (Cau, 2014): its referral to Neptunidraco was based on both stratigraphic and morphological arguments, the affinities of this specimen have never been tested using a quantitative analysis
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