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First filter feeding in the Early Triassic: cranial morphological convergence between Hupehsuchus and baleen whales - PubMed

  • ️Sun Jan 01 2023

First filter feeding in the Early Triassic: cranial morphological convergence between Hupehsuchus and baleen whales

Zi-Chen Fang et al. BMC Ecol Evol. 2023.

Abstract

Modern baleen whales are unique as large-sized filter feeders, but their roles were replicated much earlier by diverse marine reptiles of the Mesozoic. Here, we investigate convergence in skull morphology between modern baleen whales and one of the earliest marine reptiles, the basal ichthyosauromorph Hupehsuchus nanchangensis, from the Early Triassic, a time of rapid recovery of life following profound mass extinction. Two new specimens reveal the skull morphology especially in dorsal view. The snout of Hupehsuchus is highly convergent with modern baleen whales, as shown in a morphometric analysis including 130 modern aquatic amniotes. Convergences in the snout include the unfused upper jaw, specialized intermediate space in the divided premaxilla and grooves around the labial margin. Hupehsuchus had enlarged its buccal cavity to enable efficient filter feeding and probably used soft tissues like baleen to expel the water from the oral cavity. Coordinated with the rigid trunk and pachyostotic ribs suggests low speeds of aquatic locomotion, Hupehsuchus probably employed continuous ram filter feeding as in extant bowhead and right whales. The Early Triassic palaeoenvironment of a restrictive lagoon with low productivity drove Hupehsuchus to feed on zooplankton, which facilitated ecosystem recovery in the Nanzhang-Yuan'an Fauna at the beginning of the Mesozoic.

Keywords: Ichthyosauromorph; Marine reptiles; Mesozoic; Mysticeti; Nanzhang-Yuan’an Fauna.

© 2023. The Author(s).

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1

New specimens of Hupehsuchus nanchangensis (A) Photograph of 2020-NYF-84-4. (B) Photograph of WGSC V26007. Scale bar = 5 cm

Fig. 2
Fig. 2

Comparison of the skull roof of Hupehsuchus nanchangensis and the modern baleen whale. (A) Reconstruction of skull roof of Hupehsuchus nanchangensis, based on the new specimens, 2020-NYF-84-4 and WGSC V26007. (B) Skull roof of an adult minke whale from [28] - used following the attainment of appropriate copyright permission. The skull roofs are signed with 9-landmark configuration and descriptions seen in Table 1. The grey region is the paired nasals

Fig. 3
Fig. 3

The morphological details in skulls of Hupehsuchus nanchangensis A-B. The cranial photograph and interpretative drawing of 2020-NYF-84-4 in dorsal view. Dashed lines indicate the long grooves that run along the labial margin. C-D. The cranial photograph and interpretative drawing of WGSC V26007 in dorsal view. Dashed line represents broken in specimen. E. The palatal view of the skull of referred specimen WGSC V26000, the black lines outline the groove-like depressions, and the grey regions highlight the bulges around the palatal margin Abbreviations:bo: basioccipital; bp: basisphenoid; cb: ceratobranchial; eo: exoccipital; f: frontal; hy: hyoid; j: jugal; l: lacrimal; m: maxilla; mnd: mandible; n: nasal; op: opisthotic; p: parietal; pm: premaxilla; pob: postorbital; ptf: postfrontal; prf: prefrontal; q: quadrate; qj: quadratojugal; sp: splenial; sq: squamosal; so: supraoccipital; st: supratemporal; utf: upper temporal fenestra. Scale bars = 2 cm.

Fig. 4
Fig. 4

Morphospace of the skull roof in marine amniotes based on landmark analysis. (A) Distribution of key taxonomic groups. (B) Distribution of the ecomorphological guilds of predators based on prey size. The analysis includes nine landmark points, and the two-dimensional morphospace plots are based on the first two principal components, with Hupehsuchus placed in context of data from modern marine mammals

Fig. 5
Fig. 5

Phylogenetic tree of major groups of marine tetrapods with selected terrestrial sister taxa modified from [81], following the attainment of appropriate copyright permission. The groups and species are marked with their feeding guilds. The blue branches indicate marine-adapted tetrapods and brown branches represent their terrestrial sister taxa. The classifications of feeding guilds are from [34] for mammals, [78] and [81] for reptiles

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