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Osteology and phylogeny of Robustichthys luopingensis, the largest holostean fish in the Middle Triassic.

ABSTRACT: The extinct ray-finned fish taxon Robustichthys luopingensis from Luoping, eastern Yunnan, China represents the largest holostean known in the Middle Triassic. Despite its potential significance for investigating the holostean phylogeny and reconstructing the Triassic marine ecosystems, Robustichthys has so far not been described in detail and its phylogenetic position within the Holostei was controversy. This study provides a redescription and revision of Robustichthys based upon a comparative study of eight type specimens and nine new specimens. Newly recognized information includes a toothed parasphenoid, a pair of premaxillae not pierced by the olfactory nerve, a splint-like quadratojugal, a hatchet-shaped hyomandibula, an hourglass-shaped symplectic, anterior and posterior ceratohyals, a complete series of branchiostegal rays, and sclerotic bones. A revised reconstruction of Robustichthys is presented. Results of a cladistic analysis confirmed Robustichthys as an ionoscopiform within the Halecomorphi; the previous placements of Robustichthys as a basal ginglymodian and Ionoscopidae as a basal amiiform clade are not supported. The sister group relationship between Sinamiinae (Sinamia and Ikechaoamia) and Amiinae (Amia and Cyclurus) within the Amiidae is newly recognized. This revised topology provides new insights into the evolution and historical paleoecology of halecomorph fishes.

SUBMITTER: Xu GH

PROVIDER: S-EPMC6596411 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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Osteology and phylogeny of Robustichthys luopingensis, the largest holostean fish in the Middle Triassic.

Xu Guang-Hui GH

PeerJ 20190624

The extinct ray-finned fish taxon Robustichthys luopingensis from Luoping, eastern Yunnan, China represents the largest holostean known in the Middle Triassic. Despite its potential significance for investigating the holostean phylogeny and reconstructing the Triassic marine ecosystems, Robustichthys has so far not been described in detail and its phylogenetic position within the Holostei was controversy. This study provides a redescription and revision of Robustichthys base ...[more]

PMID: 31275762

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Project description:BackgroundAlligator lizards (Gerrhonotinae) are a well-known group of extant North American lizard. Although many fossils were previously referred to Gerrhonotinae, most of those fossils are isolated and fragmentary cranial elements that could not be placed in a precise phylogenetic context, and only a handful of known fossils are articulated skulls. The fossil record has provided limited information on the biogeography and phylogeny of Gerrhonotinae.ResultsWe redescribe a nearly complete articulated fossil skull from the Pliocene sediments of the Anza-Borrego Desert in southern California, and refer the specimen to the alligator lizard genus Elgaria. The fossil is a representative of a newly described species, Elgaria peludoverde. We created a morphological matrix to assess the phylogeny of alligator lizards and facilitate identifications of fossil gerrhonotines. The matrix contains a considerably expanded taxonomic sample relative to previous morphological studies of gerrhonotines, and we sampled two specimens for many species to partially account for intraspecific variation. Specimen-based phylogenetic analyses of our dataset using Bayesian inference and parsimony inferred that Elgaria peludoverde is part of crown Elgaria. The new species is potentially related to the extant species Elgaria kingii and Elgaria paucicarinata, but that relationship was not strongly supported, probably because of extensive variation among Elgaria. We explored several alternative biogeographic scenarios implied by the geographic and temporal occurrence of the new species and its potential phylogenetic placements.ConclusionsElgaria peludoverde is the first described extinct species of Elgaria and provides new information on the biogeographic history and diversification of Elgaria. Our research expands the understanding of phylogenetic relationships and biogeography of alligator lizards and strengthens the foundation of future investigations. The osteological data and phylogenetic matrix that we provided will be critical for future efforts to place fossil gerrhonotines. Despite limited intraspecific sampled sizes, we encountered substantial variation among gerrhonotines, demonstrating the value of exploring patterns of variation for morphological phylogenetics and for the phylogenetic placement of fossils. Future osteological investigations on the species we examined and on species we did not examine will continue to augment our knowledge of patterns of variation in alligator lizards and aid in phylogenetics and fossil placement.

Dataset Information

Osteology and phylogeny of Robustichthys luopingensis, the largest holostean fish in the Middle Triassic.

Publications

Osteology and phylogeny of <i>Robustichthys luopingensis</i>, the largest holostean fish in the Middle Triassic.

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