The earliest equatorial record of frogs from the Late Triassic of Arizona

Abstract

Crown-group frogs (Anura) originated over 200 Ma according to molecular phylogenetic analyses, though only a few fossils from high latitudes chronicle the first approximately 60 Myr of frog evolution and distribution. We report fossils that represent both the first Late Triassic and the earliest equatorial record of Salientia, the group that includes stem and crown-frogs. These small fossils consist of complete and partial ilia with anteriorly directed, elongate and distally hollow iliac blades. These features of these ilia, including the lack of a prominent dorsal protuberance and a shaft that is much longer than the acetabular region, suggest a closer affinity to crown-group Anura than to Early Triassic stem anurans Triadobatrachus from Madagascar and Czatkobatrachus from Poland, both high-latitude records. The new fossils demonstrate that crown anurans may have been present in the Late Triassic equatorial region of Pangea. Furthermore, the presence of Early Jurassic anurans in the same stratigraphic sequence ( Prosalirus bitis from the Kayenta Formation) suggests that anurans survived the climatic aridification of this region in the early Mesozoic. These fossils highlight the importance of the targeted collection of microfossils and provide further evidence for the presence of crown-group representatives of terrestrial vertebrates prior to the end-Triassic extinction.

Keywords: Chinle Formation; Norian; evolution; frog; ilium; microfossils.

Figure 1.

Time-calibrated stratigraphic and biogeographic distribution of Triassic Period and Jurassic Period anuran specimens. (a) Time-calibrated stratigraphic column for the Chinle Formation (Upper Triassic) and overlying Glen Canyon Group of the southwestern USA. Frog icons mark the stratigraphic position at which each specimen was collected; orange indicates a Triassic locality, whereas blue indicates the locality of the Jurassic Prosalirus bitis. (b) Palaeogeographic distribution of early Mesozoic salientian fossils. Orange icons indicate Triassic forms (Czatkobatrachus, Triadobatrachus, and the Chinle forms), whereas blue indicates Jurassic forms (Prosalirus, Vieraella). The Chinle exemplar, the right ilium DMNH 2018-05-0002, is shown in lateral view in photograph (c) and in lateral (d) and medial (e) CT surface images. Abbreviations: ace: acetabulum; ap: anterior process; dp: dorsal process. Small arrow in (d) indicates anterior direction. Scale bar equals 1.0 mm. (Online version in colour.)

Figure 2.

Comparisons of the primary Late Triassic specimen from the Chinle Formation (DMNH-2018-05-2002) to the ilia of stem anurans, including Triadobatrachus (MNHN-F-MAE126), Czatkobatrachus (ZPAL Ab IV/114; surface file shared by T. Sulej), Prosalirus (MCZ-VP-9324A) and extant anurans from the earliest diverging crown lineages, including Ascaphus truei (UF-Herp-80664), Leiopelma hamiltoni (CAS-Herp-53931), Alytes obstetricans (CAS-SU-21691) and Barbourula busguangensis (KU 309278). All figured in right lateral view with anterior to the right. Abbreviations: ace: acetabulum. Scale bar equals 1.0 mm. (Online version in colour.)