Renewed emphasis on microvertebrate fossils recovers the oldest frogs in North America

Article by Dr. Adam D. Marsh, Lead Paleontologist,Petrified Forest National Park

The Late Triassic Chinle Formation (~228–208 Ma) and age-equivalent strata in the southwestern United States (Arizona, Utah, New Mexico, Colorado, Wyoming, and Texas) is well known for the relatively high diversity of vertebrate macrofossils representing medium-to-large-bodied groups such as the Phytosauria, Aetosauria, Metoposauridae, Rauisuchidae, and Dicynodontia (Long and Murry, 1995; Heckert and Lucas, 2002). These fossils have been somewhat easy to discover, excavate, prepare, and curate for more than 100 years (Parker, 2006) owing to their large size and the bentonitic, easily-eroded siltstones laid down by rivers, lakes, streams, and soils in the Late Triassic. However, relatively little research has been done on the microvertebrates of the Chinle Formation (but see Heckert, 2004; Kligman et al., 2017, 2018), leading to a wide gap in knowledge of the early evolution of modern small-bodied groups such as frogs.

Figure 1. Volumetric rendering of a frog ilium from the Chinle Formation of Arizona placed within a silhouette of an early frog.

Silhouette by Nobu Tamura via phylopic.org used under the Creative Commons Attribution 3.0 Unported license (https://creativecommons.org/licenses/by/3.0/).

Until recently, the only amphibians known from the Chinle Formation belonged to the Metoposauridae, a long-extinct group of large-bodied, flat-headed temnospondyls that were ambush predators in Late Triassic lakes and rivers. Metoposaurids had incredibly robust skulls, pectoral girdles, and vertebral intercentra and are among the most common fossils found in the Chinle Formation of Arizona. The earliest members of Lissamphibia, a monophyletic group that includes the modern amphibian lineage (frogs, salamanders, and caecilians; Marjanović and Laurin, 2007), were small-bodied and far less abundant; two stem-frogs are known from the Early Triassic high latitudes of what is now Poland and Madagascar (Piveteau, 1936; Rage and Roček, 1989; Evans and Borsuk−Białynicka, 1998, 2009; Ascarrunz et al., 2016) and a putative stem-caecilian has recently been reported from the Late Triassic of Colorado (Pardo et al., 2017), and the next youngest members of those groups are known from the Early Jurassic Kayenta Formation on the Navajo Nation (Jenkins and Walsh, 1993; Shubin and Jenkins, 1995). For frogs, this results in a gap in the fossil record of approximately 60 million years. Fortunately, new fossils from Petrified Forest National Park (PEFO) and sites from a neighboring ranch and St. Johns, Arizona are starting to fill that gap.

Figure 2. Side views of three fossil frog ilia from the Chinle Formation from Petrified Forest National Park and neighboring lands. See text for abbreviations. Scale bars equal 1 mm.

Last summer, a collaborative team from Virginia Tech (VT) and PEFO was conducting fieldwork on a ranch adjacent to the park with permission from the landowner and repository agreement with the Perot Museum of Nature and Science (DMNH) in Dallas, Texas. The crew was quarrying at a known microfossil locality (~213 Ma; Ramezani et al., 2011) by excavating small blocks of rock (<1 kg), splitting them, searching for microfossils with a hand lens, and wrapping intriguing fossiliferous samples in tissue paper. After several days, the VT crew departed with several anatomically interesting samples needing closer inspection, including a frog ilium (Figure 1). The shape of frog ilia is characteristic of that group and is unlike that of metoposaurids or other small vertebrates. While that specimen was being CT scanned for detailed study, the research team discovered more frog ilia from slightly older strata (221-219 Ma; Atchley et al., 2013; Ramezani et al., 2014) via screen washing sediment at PEFO and at the collections from the Placerias Quarry housed at the Museum of Northern Arizona (MNA) in Flagstaff (Figure 2). Together, these new frog fossils belong to the group Salientia (modern frogs and their closest evolutionary ancestors) and represent both the earliest frogs in North America and the earliest frogs from the equatorial region of Pangea (Stocker et al., 2019). The new fossil frogs are more similar to the younger Early Jurassic fossil frog Prosalirus bitis than older Early Triassic frogs, suggesting that frogs were not extirpated from the region during the drying event and hypothesized mass extinction at the end of the Triassic Period (Figure 3; Figure 4; Atchley et al., 2013; Blackburn et al., 2013; Nordt et al., 2015).

Figure 3. Stratigraphic and geographic data of Late Triassic and Early Jurassic frogs, including an age-calibrated stratigraphic section from northern Arizona and paleogeographic reconstruction of Pangea in the Late Triassic/Early Jurassic. References for radiometric dates are provided in the text.

New fossil discoveries are not only important for understanding the early evolution of modern animal groups like lissamphibians and providing examples of the interaction between changing climates and ecosystems, they can emphasize the utility of federal-private partnerships in the National Park Service. Collaborations like these can bolster strengthen park science programs, collect empirical data for management decisions, and create interpretive content that has the ability to spread far beyond park and museum visitors. Working with outside researchers and other museum institutions, NPS units like Petrified Forest National Park provide the opportunity to explore new scientific spaces and reach audiences that are interesting in learning more about the fossil resources on their federal lands.

Figure 4. A Late Triassic frog clings to the snout of a phytosaur.

Art by Andrey Atuchin, used with permission.

The research article featured here can be read and downloaded for free. CT scan data are available. The project was funded by David B. Jones, the National Science Foundation, the Petrified Forest Museum Association, the Friends of Petrified Forest National Park, and the Virginia Tech Department of Geosciences

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