Phareodus testis and Diplomystus dentatus | 100% Natural | Green River Formation | Wyoming

In the ancient waters of Fossil Lake, a formidable predator roamed. The Phareodus testis, a smaller but equally fascinating cousin of Phareodus encaustus. Both species boasted striking features, including pointy teeth and an impressive pectoral fin.

The Bony-Tongue Fish Family

Phareodus encaustus and Phareodus testis belong to the order Osteoglossiformes and family Osteoglossidae. Today, this family comprises 10 modern species that thrive in tropical freshwater habitats across South America, Africa, Asia, and Australia. Interestingly, these fish are mouth-brooders, where parents carefully nurture eggs and hatchlings in their mouths.

Fossil Discoveries

Fossil records reveal that adult Phareodus specimens often went solo, whereas juveniles frequently gathered in large groups. This suggests that Phareodus fish schooled together during their youth before becoming solitary predators as adults. The presence of smaller fish in their jaws and stomachs confirms their predatory nature. Adaptations like rearward-oriented fins and large, sharp teeth enabled Phareodus to chase down prey with ease.

Size Comparison

While Phareodus encaustus reached an impressive 30 inches in length, its smaller counterpart, Phareodus testis, measured around 20 inches. Despite their size difference, both species played vital roles in their ancient ecosystem.

Diplomystus is an extinct genus of freshwater clupeomorph fish distantly related to modern-day extant herrings, alewives, and sardines. The genus was first named and described by Edward Drinker Cope in 1877.

Ray-finned Fish - Diplomystus dentatus

Order Ellimmichthyiformes, Family Paraclupeidae

The last known species of the Ellimmichthyiformes order went extinct sometime in the middle Eocene. D. dentatus is a primitive relative of the modern day herring. The genus Diplomystus is also known from fossil deposits in China.

D. dentatus is the 2nd most common fossil fish found from Fossil Lake. Specimens ranging from embryonic size (about 0.7 inches) to full-grown adults (about 26 inches) are common. Smaller-sized specimens are more commonly found in mid-lake than near-shore deposits. This suggests that D. dentatus spawned in open water.

This species' upturned mouth indicates it fed at the surface of Fossil Lake. D. dentatus specimens are commonly found with other fish stuck in their mouths, including other D. dentatus specimens.