CONICET | Buscador de Institutos y Recursos Humanos

The Upper Jurassic Morrison Formation (~151 Ma) in western North America is well known for its dinosaur fauna. However, the formation also preserves a significant and diverse biota of plants, non-dinosaurian vertebrates (e.g., fishes, amphibians, turtles, crocodylomorphs, pterosaurs, and mammals, invertebrates (e.g., spinicaudatans, ostracods, crayfishes, gastropods, bivalves, insects) and insect traces, most of which have been only preliminarily studied. In the last years, new field trips to Jurassic Salad Bar, a plant-producing locality in southeastern Utah, have produced new arthropod body fossils, such as additional crayfishes, spinicaudatans, and insects, and we summarize and briefly describe these new specimens reposited in the Utah Field House of Natural History State Park Museum, Vernal, Utah, USA (FHPR) here. The Jurassic Salad Bar, was discovered in 2016 near Bears Ears National Monument, west of the town of Blanding in San Juan County, southeastern Utah, USA. The site contains an abundant flora consisting of ferns, a horsetail, a seed fern, four types of ginkgophytes, a cycadophyte, and up to six different types of conifers (Kirkland et al., 2022; Foster et al., this volume). In addition, the locality preserves spinicaudatan carapaces, rare amioid fishes, and amphibians (Foster et al., 2022). The Jurassic Salad Bar site (hereafter JSB) consists of a several-decimeter thick layer of soft, finely-laminated shale containing most of the plant and occasional small bone material. The site probably represents a freshwater pond or a small lake deposit. The fossiliferous interval occurs near the middle of an ~80 m-thick section of the Brushy Basin Member of the Morrison Formation in southeastern Utah.Specimens include impressions/compressions of insects and crustaceans (e.g., crayfishes, spinicaudatans). A thin, brown organic film somewhat similar to the color and preservational appearance of the plant material is observed on JSB fossil arthropods (Lara et al., 2020). The JSB fossil insect fauna is limited but promising, having not been recognized from the site until 2019. The first recognized and formally described insect specimen (Fig. 1a, b) from JSB was a nepomorph hemipteran, Morrisonnepa jurassica with broad similarities to modern giant water bugs (Lara et al., 2020). Morrisonnepa jurassica (FHPR 17299) is a relatively large insect, with preserved length of 25.4 mm. The insect consists of an incomplete specimen in dorsal view, with mostly disarticulated elements including a forewing (hemelytra), clavus, possible head, and scutellum, five (articulated) abdominal segments, and a pair of short and tubular structures (similar to respiratory processes observed in true water bugs [Lara et al., 2020]). The most recent findings from the JSB include a new crayfish, spinicaudatans, a beetle, and other unidentified arthropods. The small crayfish (FHPR 11301 [Figure 1c]) appears to be a different form from those previously reported and described by Hasiotis and Kirkland (1997) and is not associated with burrows. The specimen (total length ⁓30 mm) is preserved in ventral view and consists of cephalothorax, abdomen, and telson. The morphological structures partly preserved include: carapace, rostrum, ?maxillipeds, cephalothorax sternum, five pairs of thoracic appendages (pereiopods: the first three pairs are chelate), and uropods. Interestingly, the spinicaudatans include at least ten specimens (e.g., FHPR 10861) preserved as shells but not yet identified (Fig. 1d). Spinicaudatans are also common at the Montezuma Creek locality east of Bluff, Utah. There are at least four possible caddisfly cases (?Taktonargus, e.g., FHPR 10844) from the Jurassic Salad Bar and all appear to have been lined exteriorly with unidentified fecal pellets (Fig. 1e).A beetle (Fig. 1f) is represented by an isolated and elongated elytra (FHPR 18689), 8.0 mm long by 5.5 mm, showing ~12 longitudinal rows of small striae, which show up best as positive-relief natural casts on the counterpart. The striae run parallel to each other and the margins. The elytra is punctate, with striae composed of rows of small indentations. Finally, FHPR 11308 is ~14.3 mm long, and preserves structures similar to eyes on stalks, antennae, and a cheliped (Fig. 1g). This specimen probably likely close to decapod crustaceans (freshwater ?hermit crabs). Futures studies will give us a more accurate taxonomic position. FHPR 18672 also appears to be a fragmentary arthropod fossil with some exoskeletal structure preserved as impressions in the matrix (Fig. 1h).These fossils represent the first arthropod records at the Jurassic Salad Bar locality, Morrison Formation (Upper Jurassic), San Juan County (Utah, USA). The presence of multiple species of insect body fossils is so far unique within the Morrison Formation. Likewise, these findings add new anatomical information to different arthropod groups and allow us to better understand how the arthropod assemblages were integrated during the Late Jurassic in this part of the Laurasian continent.

enviar mensaje