Note: This article was originally published on August 31, 2019. It has since been substantially updated and re-published on September 19, 2024.

The Morrison Formation of the western United States is one of the most famous deposits of late Jurassic strata anywhere in the world. It is here that dinosaur fossils from famous species like Allosaurus, Stegosaurus, Apatosaurus, and others were discovered and continue to be uncovered by paleontologists to this day. While the Morrison Formation is world-renowned for its superb dinosaur fossils, this landscape was home to many other species that dwelt here 150 million years ago. In addition to dinosaurs, fossils of pterosaurs, crocodiles, lizards, turtles, amphibians, fish, and invertebrates have also been uncovered. As you can see from the above list, there are many aquatic or semi-aquatic animals that are mentioned. This may sound bizarre because, as anyone who has even a vague knowledge of the Morrison Formation knows, this landscape was arid and dry for much of the year during the late Jurassic; click here for a video that talks about this. However, since fossilization is most likely to occur in areas that are prone to flooding, it would make sense that many of the fossils that we find come from creatures that made their homes in and around the water.

One of those creatures was Morrolepis schaefferi, one of the fish species that lived in western North America during the Late Jurassic. Morrolepis, meaning “Morrison scale”, belonged to the family Coccolepidae which sits within the group Palaeonisciformes. The species M. schaefferi is named in honor of the paleo-ichthyologist Bobb Schaeffer, who extensively studied North American prehistoric fish. Morrolepis seems to be most closely related to Sunolepis, a coccolepid fish from China dating to the Jurassic/Cretaceous transition, but Morrolepis’ pelvic fins were smaller and positioned further forward on the underside, a larger and more forward-positioned anal fin, a more forward-positioned dorsal fin (Kirkland 1998, pages 514-515).

Morrolepis schaefferi is known from three specimens found within the middle layers of the Brushy Basin Member of the Morrison Formation. Two were found in western Colorado, while the third was found on the lands of the Navajo Indian Reservation within the southeastern corner of Utah (Kirkland 1998, page 516). The Brushy Basin Member is the uppermost member of the Morrison Formation and dates from the middle of the Kimmeridgian Stage to the middle of the Tithonian Stage of the late Jurassic Period, circa 152-147 MYA (Hasiotis 2004). The three specimens would therefore date to around 150 MYA.

Morrolepis measured eight inches long, far larger than its contemporary, the minnow-sized Hulettia, but at the same time it was far smaller than its other major contemporary, the three-foot-long lungfish Ceratodus. Lungfish were the most common fish found in the Morrison Formation. Being able to breathe when you’re out of water is very helpful if you live in a landscape that has a long dry season and is prone to droughts. According to Jim Kirkland’s 1998 description of Morrolepis, the fish was covered in very thin cycloid scales except for a row of thickened ganoid scales along the lateral line (Kirkland 1998, page 514).

We know quite a bit about Morrolepis’ anatomy based upon the fossils that have been uncovered, but how would it have lived? We know from geology that Morrolepis’ remains were found inland. Therefore, it was not a marine species, but was instead a freshwater species. The Morrison Formation was, as said before, a largely dry area, but there were a few places where there were permanent sources of fresh water. The landscape was mostly flat, and rivers that flow through flat terrain usually flow very slowly because the incline of the land is barely noticeable. Moreover, flat-land rivers tend to be very wide but very shallow, unless they happen to be cutting through a gorge or ravine. So, it appears that Morrolepis was at home in standing or slow-moving water, such as ponds, lakes, and slow-moving rivers. Water bodies that are standing or slow-moving usually have a lot of aquatic vegetation. This is because seeds and spores of aquatic plants have a better chance of taking root and growing because the current won’t sweep them away, like in faster-moving streams and rivers. Therefore, in water bodies such as this, there is a sufficient amount of aquatic plants and algae. In some circumstances, the water might appear to be green due to the heavy concentration of algae (visit Kissena Park in Queens, New York if you don’t believe me; the lake there looks like pea soup). So, what we have so far is a slow-moving river that is wide but shallow, and probably has a fair amount of aquatic vegetation in it – ambush country.

Predatory fish that live in this type of environment are almost exclusively ambush predators, waiting under cover for prey to pass by too close, and then suddenly lunging forward and gobbling them up. Like many of palaeeoniscoid fish, Morrolepis had large eyes set close to the front of the head, ideal for spotting its prey. Also, if the water was indeed so thick with algae that it appeared to be dyed green, visibility would be very low. Large eyes would compensate for the murky water. A large mouth lined with noticeably long spiky teeth would seem to be a good go-to method for swallowing down small prey in water that had low visibility. With a gaping maw like that, even if your aim was not 100% accurate, you still stood a fair chance of catching your victim anyway. Due to the environment that it lived in, it wouldn’t be far-fetched to imagine that Morrolepis was patterned in blotches or wavy stripes, and presumably would have been colored in various shades of tan, brown, and green to camouflage it in the murky muddy water and match the surrounding submerged vegetation.

Morrolepis schaefferi. © Jason R. Abdale (August 26, 2024)

Kirkland, James I. (1998). “Morrison Fishes”. Modern Geology, volume 22 (January 1998). Pages 503-533.
https://www.researchgate.net/publication/292223569_Morrison_fishes.

Hasiotis, Stephen T. (2004). “Reconnaissance of Upper Jurassic Morrison Formation ichnofossils, Rocky Mountain Region, USA: Paleoenvironmental, stratigraphic, and paleoclimatic significance of terrestrial and freshwater ichnocoenoses”. Sedimentary Geology, volume 167, issues 3-4 (May 15, 2004). Pages 177-268.
https://www.sciencedirect.com/science/article/abs/pii/S0037073804000090.

Categories: Paleontology, Uncategorized

Tags: America, art, fish, Jurassic Period, Late Jurassic, Mesozoic, Morrison Formation, Morrolepis, North America, palaeoniscoid, paleo-art, paleontology, Prehistoric