Asterotrygon

The holotype, FMNH PF 15166; deposited in the Field Museum of Natural History.

*Asterotrygon* is an extinct genus of eagle myliobatiforme batoid ray that lived in Wyoming during the Early Eocene.

## History

Before description, *Heliobatis* was the only ray known from the Green River Formation. In 1980, AMNH P 11557, a female stingray with two aborted fetuses, was discovered. Another specimen, FMNH PF 15166, this time a female and still-developing embryonic fetus, was discovered. The type was erected in 2004 with the latter coined as the holotype and other specimens referred. While *Asterotrygon* is known from a few dozen specimens, contemporaries such as *Heliobatis* are known by the hundreds. It was named the "star stingray" after the star-like denticle base; the specific name is after Thomas Maloney, donator of the paratype (AMNH P 11557) to the museum. As an early stingray, although not the first, it is rare to see such great preservation when most taxa only preserve teeth and dermal denticles.

## Description

Its bauplan is similar to typical rays through the flat, rounded disk structure with a head, pectoral fins and a narrow stinging tail. The smallest juveniles (SMMP 83.25) are 7.97 centimeters (3.14 inches) long whereas the largest adults (FMNH 15166) are 65 centimeters (26 inches) long. The disk is rounder than its contemporaries, with the upper surface covered in denticles, each forming a small hook. Unlike *Heliobatis* and modern rays, a small dorsal fin sits in from of the stingers. While most bear a cartilaginous rod that stretches from the stinger to the end of the tail, this genus retains distinct vertebrae in the entire length of the tail, which itself is slightly thicker at the base than other stingrays. It lacks thoracic ribs, possessing thoracolumbar synarcual cartilage instead, like all stingrays. Small fin-like folds are found at the tip of the tail. The puboischiadic girdle is plesiomorphous in being narrow and arch-shaped. Neurocranial traits connect it with modern stingrays, such as the hyomandibulae being fully distinct from the lower jaw and the hyomandibulae being fused by the hyomandibular-Meckelian ligament (as in cartilaginous fish). Despite losing this, it retains some calcified cartilage where the ligament was once located. The postorbital processes are broad, flat and resemble a shelf.

## Classification

It was a basal Myliobatoidei, with only *Hexatrygon* being more basal. Despite being contemporaries, it and *Heliobatis* were not closely related, and modern stingrays are also not closely related (the closest being Urolophidae and *Plesiobatis*). One study found it was outside other stingray lineages, and so decided to coin Asterotrygonidae for convenience. It is likely a descendant from populations that originated from the west Pacific or Indo-Pacific, where *Heliobatis* was probably a product of American taxa. Because they are not close relatives, both sides probably colonized the Green River Formation in two, separate but closely-timed waves; and since both are not found outside of the formation, it is thought that they probably evolved to be specialized in the lakes.

## Paleobiology

Several morphotypes are observed in specimens, which may suggest that additional unnamed species exist. The specimens found indicate that Fossil Lake was a breeding ground, including a mating pair, a female with an embryo inside and an adult with newborns alongside it. The specimen with the embryo suggests that *Asterotrygon* gave live birth and had viviparity. One study found it was outside other stingray lineages, and so decided to coin Asterotrygonidae for convenience. Studies published before naming referred to it as an undescribed ray or the fat-tailed ray. It is distinguished from *Heliobatis* through dense denticle covering (some hook-like), covering the tail. The tail was likely not as flexible as the latter. It was estimated to be ~40x rarer than *Heliobatis*.

## Paleoecology

The Green River fauna swam in subtropical lakes within a mountainous region, which existed for about 15 million years. Of these are three major bodies: Fossil Lake, Lake Gosiute and Luke Uinta. Most stingray fossils are known from the second, although it was the shortest-lived. The only other locality to preserve complete stingrays is the Monte Bolca Formation of Italy, which has a greater diversity of species. Fossil lake was semitropical, with the modern Gulf Coast and southern Atlantic regions as analogous. It is found in two localities: F-1 or "18-inch-layer" that has limestone deposited over several hundred years, thought to be a deep section of the lake with an anoxic bottom; and F-2, which reaches up to 4 meters (13 feet) thick and was deposited over (possibly) several thousand years, representing a section of the lake that was closer to the north- and southeastern shores, where water was richer in dissolved oxygen and organic material content.

## References

https://en.wikipedia.org/wiki/Asterotrygon