A RE-EXAMINATION OF THE FOSSIL EVIDENCE FOR THE PRESENCE OF FISH SPECIES WITHIN THE LATE CRETACEOUS COLERAINE FORMATION OF MINNESOTA’S MESABI IRON RANGE

Fishes of global seas in the Late Cretaceous Period display a rich and diverse selection of species, with arguably even more diversity than we see today. Fishes have been evolving for more than 420 million years, and extremely high Late Cretaceous sea levels offered vast new habitats for expansion and diversification among all types of fishes (e.g., those with bony skeletons as well as the cartilaginous fishes, such as sharks and rays).

Fossils of bony fishes are recovered within deposits of the Late Cretaceous Coleraine Formation of Minnesota’s iron ranges. These remains are comprised mostly of isolated vertebrae and teeth. Occasionally, fish scales can be found within the matrix, though they are typically fragmented. Fossil scales from Lepisosteiformes (i.e. gars) remain the only currently identified scale material.

The vertebrae recovered range in a variety of sizes from 1mm to 6cm in diameter. The length of each vertebrae is typically shorter than its circular diameter. They are concave on both the front and back surfaces, and exhibit openings or hollowed supports along the exterior surface. The largest collected specimen is a caudal vertebra of an Icthyodectes sp., a Cretaceous fish from the order Icthyodectiformes that could grow in length up to 13 feet (4m) in length. This specimen is unique in that its length (7cm) is greater than its diameter (5cm). Ichthyodectiforms are within Osteoglossomorpha, a Superorder that includes modern day Elopiformes (e.g., tarpons and allies) and Hiodontiformes (e.g., mooneyes).

Fish teeth preserved in the Coleraine Formation are usually slender and exhibit a slight curve or recurve in their length. They are difficult to distinguish from some of the marine reptile specimens recovered from the same localities. An example of a crushing palette tooth, presumably a pycnodontid fish, has been recovered as well. This rounded and stubby tooth is currently undergoing examination.

While few individual species, like Icthyodectes or gar, can be selectively described from the Coleraine Formation fossil record, more as yet unknown or unidentified species are likely contained in the collection. The range of vertebrae alone should indicate a wider variety of species that includes many smaller-sized feeder fishes similar to herring.