A new phylogeny of ichthyosaurs (Reptilia: Diapsida)

Abstract

The largest phylogenetic analysis of ichthyosaurs to date is presented, with 114 ingroup taxa coded at species level. Completeness of the taxa included varied from >98% to <2%; ten taxa were removed a priori using Concatabominations, due to incompleteness and taxonomic uncertainty. The data were analysed using three widely used optimisation criteria: maximum parsimony, maximum likelihood, and Bayesian inference; while similar, each criterion produced different topologies, support, and levels of resolution. Maximum parsimony found a well-resolved consensus tree with minor improvement from a posteriori pruning of unstable taxa; however, general support remains low. Tree resolution was reduced more by taxa that lacked codings from phylogenetically important regions of the tree, rather than by those that simply lacked many codings. Resolution present in the most likely tree is poorly supported; sister relationships cannot be confirmed, although similarities are found to the most parsimonious tree. Bayesian inference found poorly resolved consensus trees. While more resolved, an equal-distribution rate prior is significantly worse than the null gammadistribution rate prior for morphological data, but suggests rate heterogeneity across ichthyosaur phylogeny. Tree comparisons under each analytical criterion failed to select a single best tree, however, the Bayesian inference tree with gamma-distribution rate prior is selected as the best tree based on recent analyses showing improved accuracy using this criterion. Unequivocally resolved clades include Ichthyopterygia, Ichthyosauria, Shastasauria, Euichthyosauria, Parvipelvia, and Neoichthyosauria, but with variation in their taxonomic components. Mixosauridae and Ophthalmosauridae are similarly recovered, but their definitions are modified to stem-based definitions to prevent substantial variation of included taxa. Several genera are not monophyletic: Brachypterygius,

Does exceptional preservation distort our view of disparity in the fossil record?

“…Our chosen cladistic matrix is modified from Moon [27] and comprises 112 ichthyosauriforms scored for 287 discrete characters, where characters 100, 204, 205, and 218 are ordered. Phalarodon major was removed as it is a nomen dubium [28] and the character states for Suevoleviathan integer and S. disinteger were combined as the latter is a junior synonym of the former [29].…”

Ontogeny of the braincase inStenopterygius(Reptilia, Ichthyosauria) from the Lower Jurassic of Germany

“…2F, G) (note that there is no way of knowing whether these are from the same embryos as the basioccipitals discussed above). One has a distinct paired internal carotid foramen, not unlike that of more basal ichthyosaurs (e.g., Maisch and Matzke, 2000;Moon, 2017), whereas the other retains traces of a paired morphology, along the lines of the early juvenile stage, in which a single foramen is partially divided by a dorsal ridge, forming two channels. Of the embryonic basisphenoids, one is not more substantially ossified than the other, but the basisphenoid with the paired morphology is slightly larger (widths: 7 and 9 mm, respectively).…”

“…Braincase morphology has also been used as a source of characters in phylogenetic analyses of ichthyosaurian ingroup relationships. Features such as the shape of the quadrate, the relative size of the extracondylar area of the basioccipital, the shape of the medial head of the stapes, the shape of the parabasisphenoid, and the relative contribution of the supraoccipital to the dorsal and lateral edges of the foramen magnum, among others, are thought to vary across the clade (see Moon, 2017). Detailed morphological studies have been carried out on reconstructed braincases of Jurassic ichthyosaurs such as Ichthyosaurus (McGowan, 1973b), Ophthalmosaurus icenicus (Moon and Kirton, 2016), Hauffiopteryx ?typicus (Marek et al, 2015), and more recently Protoichthyosaurus prostaxalis (Lomax et al, 2019).…”

Peer Review #2 of "A new specimen of Palvennia hoybergeti: implications for cranial and pectoral girdle anatomy in ophthalmosaurid ichthyosaurs (v0.1)"

“…The two additional specimens described above, PMO 230.097 and PMO 224.250, are confidently referred to Ophthalmosauridae on the basis of possessing a humerus (Figs. 15, 19) with a platelike dorsal ridge (Moon, 2017) and lacking a tuberosity on the anterodistal extremity of the humerus as well as lacking notching on forefin elements (Moon & Kirton, 2016). PMO 230.097 represents a relatively small ophthalmosaurid that shares some humeral characters with the SML ophthalmosaurid Keilhauia nui (Delsett et al, 2017), including a relatively small deltopectoral PeerJ reviewing PDF | (2018:05:28469:1:1:NEW 28 Aug 2018)…”