Semionotus

Semionotus was a genus of prehistoric fish which lived throughout the world during much of the Mesozoic Era. It was named by the famous Swiss ichthyologist and paleontologist Louis Agassiz in 1843 (type species: Semionotus bergeri). Semionotus first appeared during the early Triassic Period about 247 million years ago, and went extinct during the early Cretaceous Period about 125 million years ago. That’s a genus lifespan of almost one hundred and twenty million years, far longer than the typical one-to-ten million year lifespan of most genera.

Semionotus had a worldwide distribution, although most fossils have been found within the Northern Hemisphere. This shows that the genus had already spread throughout the prehistoric super-continent of Pangea before it began to break up at the beginning of the Jurassic Period about 200 MYA.

There are currently 30 recognized species of Semionotus, although this genus has often been treated as a “waste basket” taxon by paleontologists and it’s possible that some of these animals have been mis-identified. Almost all Semionotus fossils were found within freshwater deposits, and the few specimens which weren’t were found within rocks that were laid down in coastal, estuary, or river mouth environments. This indicates that Semionotus, like some fish today, was able to tolerate small percentages of salt and was therefore able to live in water which was slightly brackish.

Despite a superficial resemblance to a carp, Semionotus was actually more closely related to gars, as is evident from its rows of thickened rhomboid scales. These rows of thick overlapping interlocking chunks of enamel served much the same purpose as chain-mail armor. In fact, some paleontologists have drawn a link between the wear patterns on some meat-eating dinosaur teeth to these fish, saying that the act of biting into these fish with their durable scales had actually damaged the carnivore’s teeth in the process.

Skeleton of Semionotus elegans, from the early Jurassic Period of New Jersey, USA. Image from Olsen, Paul Eric; McCune, Amy R. (1991). “Morphology of the Semionotus elegans species group from the Early Jurassic part of the Newark Supergroup of eastern North America with comments on the Family Semionotidae (Neopterygii)”. Journal of Vertebrate Paleontology, volume 11, issue 3 (September 30, 1991). Page 272.

Most species of Semionotus grew to be about a foot long, although a few could be quite hefty in size. For example, S. kanabensis, which lived in the Chinle Formation and Kayenta Formation of the American Southwest during the late Triassic and early Jurassic Periods, grew to be 4 feet long and likely weighed upwards of 80 pounds, which is very impressive by today’s standards of freshwater angling. We know that Semionotus lived in schools, sometimes numbering in dozens of individuals, because paleontologists have found slabs of rock with multiple skeletons of this fish preserved together. In order for groups of a dozen or more animals measuring as much as 4 feet long to live together, there must have been some very large waterways within these environments. There also had to have been abundant sources of food in order to sustain groups of such large-sized animals.

Form tends to follow function – if Semionotus looks like a carp, chances are it lived like a carp. One way to determine how it lived its life is to determine what its diet was. The website Fossilworks describes Semionotus as a carnivore, specifically a “durophagus” carnivore, meaning that it preferred to eat things with shells such as snails, freshwater clams, and crustaceans. However, its mouth is not designed for cracking through hard shells, unlike its relative Lepidotes, which can be best described as a swimming nutcracker similar in many respects to a modern-day Pacu. Semionotus possessed a small mouth which couldn’t open very wide, its teeth were tiny, and its skull does not appear to be robustly built. Rather than feeding on hard-shelled organisms, it likely fed on worms and other small invertebrates. Modern-day carp are omnivores and will eat pretty much anything, which is part of the reason why they are able to thrive in so many environments and is also how they are often able to grow to a massive size. Another reason for their success is that they are able to survive in water that is of poor quality, either high in pollution or low in oxygen, within waters that would kill most other fish species. One wonders if Semionotus had barbels around its mouth, like some carp species do today, in order to feel its way through muddy murky water and pick up the tell-tale presences of worms, insect larvae, and small freshwater crustaceans.

Skull of Semionotus elegans. Image from Olsen, Paul Eric; McCune, Amy R. (1991). “Morphology of the Semionotus elegans species group from the Early Jurassic part of the Newark Supergroup of eastern North America with comments on the Family Semionotidae (Neopterygii)”. Journal of Vertebrate Paleontology, volume 11, issue 3 (September 30, 1991). Page 274.

Below are two illustrations of what Semionotus would have looked like in life. The illustration is based upon the skeletal remains of the species Semionotus elegans, which was found within New Jersey, USA in rocks dated to the early Jurassic Period. The barbels around its mouth artistic conjecture. I chose to give the animal a drab muddy tan coloration seen in many freshwater species. The first image was made with No.2 and No.3 pencils. The second image was colorized with Crayola and Prismacolor colored pencils.

Semionotus. © Jason R. Abdale (February 21, 2022).

As always, keep your pencils sharp, everyone.

Sources

Olsen, Paul Eric; McCune, Amy R. (1991). “Morphology of the Semionotus elegans species group from the Early Jurassic part of the Newark Supergroup of eastern North America with comments on the Family Semionotidae (Neopterygii)”. Journal of Vertebrate Paleontology, volume 11, issue 3 (September 30, 1991). Pages 269-292.
https://www.ldeo.columbia.edu/~polsen/nbcp/olsen_mccune_91_sm.pdf.

Fossilworks. “Semionotus Agassiz 1843 (ray-finned fish)”. http://www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=35162. Accessed on February 18, 2022.

Fossilworks. “Semionotus elegans Newberry 1888”. http://www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=391328. Accessed on February 18, 2022.

Fossilworks. “Semionotus kanabensis Schaeffer and Dunkle 1950 (ray-finned fish)”. http://www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=124144. Accessed on February 18, 2022.

Utah Geological Survey. “The case for fishing dinosaurs at the St. George Dinosaur Discovery Site at Johnson Farm”, by Andrew R. C. Milner and James I. Kirkland (September 2007). https://geology.utah.gov/map-pub/survey-notes/fishing-dinosaurs-at-johnson-farm/. Accessed on February 18, 2022.



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