Leucichthyops vagans was a species of prehistoric salmonid fish which lived in what’s now western North America during the middle of the Cretaceous Period approximately 100 million years ago.
The species was named in 1919 by Theodore D. A. Cockerell, and was based solely upon fossilized fish scales. These scales were uncovered within the Mowry Shale of Wyoming, the Benton Shale of Colorado, and the Graneros Shale near Sturgis, South Dakota. All of these sediment layers were deposited in saltwater, specifically within the Mowry Sea. This was a large body of saltwater which covered the center of North America during the middle Cretaceous Period until it merged with the Gulf of Mexico to form the Western Interior Seaway. These rock layers date to approximately 100 MYA (Cockerell 1919, pages 180-181).
These fossilized fish scales were identified as belonging to the fish family Salmonidae, which includes salmon, trout, chars, ciscos, graylings, and whitefish. In appearance, Cockerell notes in his report that the fossil scales bear the closest resemblance to those belonging to the species Leucichthys nigripinnis, the Blackfin Cisco, which has since become extinct as of 1996, and to the grayling Thymallus (Cockerell 1919, pages 180-181; “Fisheries Fact Sheet”. Page 84). It was due to the similarity in shape to the scales of Leucichthys that Cockerell named this prehistoric species Leucichthyops. The name Leucichthyops means “pale/light-colored fish face” in ancient Greek, but a better translation of the name in this context would simply be “looks like Leucichthys“. Like many salmonid fishes today, Leucichthyops was probably “anadromous” in its biology, meaning that they spend most of their lives in saltwater, but the adults migrate into freshwater rivers to spawn. Evidence presented for this was the fact that the growth rings within the scales are very distinct from each other, which, according to Cockerell, indicated that the fish was migratory. This migratory behavior is likely the reason for its species name vagans, which is Latin for “wandering”, and from which the word “vagrant” descends. In terms of family affinity, Cockerell suggested that Leucichthyops was a basal member of the family Salmonidae, and therefore likely possessed anatomical features belonging to several salmonid groups (Cockerell 1919, page 181).
Since this study was published in 1919, revisions have been made to our understanding of salmonid evolution and phylogeny. Salmon and graylings, formerly thought to belong to two separate families, are now understood to be subgroups within a single family, Salmonidae. Currently, the family Salmonidae is divided into three sub-families (Crête-Lafrenière et al 2012: e46662; Shedko et al 2013):
- Coregoninae (whitefish)
- Thymallinae (graylings)
- Salmoninae (salmon, trout, char, huchens, and taimens)
In his 1919 report, Cockerell suggests, since Leucichthyops possesses scales which are similar in appearance to both salmon and graylings, that Leucichthyops is probably basal to both groups. If this is true, then this would mean that this prehistoric fish would be a common ancestor to both sub-families (Cockerell 1919, page 181). But how are these two sub-families positioned evolutionarily to each other? According to Crête-Lafrenière et al (2012), the sub-family Thymallinae is the most primitive group within Salmonidae (Crête-Lafrenière et al 2012: e46662). A study conducted the following year proved inconclusive, but the majority of results indicate that the earlier assessment made in 2012 was correct (Shedko et al 2013, pages 628-631). If these two studies are correct, then this would mean that the prehistoric genus Leucichthyops is a basal member of the family Salmonidae and doesn’t fall into any specific sub-family, which is exactly what Theodore Cockerell surmised back in 1919.
According to Crête-Lafrenière et al (2012), the family Salmonidae first appeared around 59 MYA during the Paleocene Epoch of the Paleogene Period. (Crête-Lafrenière et al 2012: e46662). However, according to Shedko et al (2013), the ancestral lineages of the salmoniforms (salmon and their relatives) and the esociforms (pikes and their relatives) split from each other approximately 106 million years ago (Shedko et al 2013, page 623). This is close to the dating of Leucichthyops, which is believed to be a primitive salmon ancestor, which dates to approximately 100 MYA. The relationship of the groups Salmoniformes and Esociformes was also confirmed in another study in 2017 (Betancur-R et al 2017).
Determining size is difficult due to only fossil scales being recovered. Most scales measure 8-9 mm in diameter (Cockerell 1919, pages 180-181). Unfortunately, I wasn’t able to find any measurements for the scales of graylings, which are the most primitive extant members of Salmonidae. Most grayling species measure 2 feet long, and I am therefore assuming that Leucichthyops reached a similar size.
Below is a hypothetical reconstruction of what Leucichthyops vagans may have looked like in life. I decided to illustrate this species similar to graylings due to these fish being the least-derived members of Salmonidae. However, I have also slightly altered the features due to Salmonidae’s close affiliation to the sub-order Esociformes. This includes somewhat altering the shape of the caudal fin so that it’s more pike-like in appearance, rounding the ends of the pectoral and anal fins, and positioning the dorsal fin further posteriorly compared to modern graylings in an effort to evoke the positioning of pike dorsal fins. This illustration was made with a No.2 pencil and colored pencils on printer paper, followed by some touching-up on the computer.
Leucichthyops vagans. © Jason R. Abdale (November 22, 2023).
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Bibliography
Betancur-R, Ricardo; Wiley, Edward O.; Arratia, Gloria; Acero, Arturo; Bailly, Nicolas; Miya, Masaki; Lecointre, Guillaume; Ortí, Guillermo (2017). “Phylogenetic classification of bony fishes”. BMC Evolutionary Biology, volume 17, article number 162 (July 6, 2017).
https://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-017-0958-3.
Cockerell, T. D. A. (1919). “Some American Cretaceous fish scales, with notes on the classification and distribution of Cretaceous fishes”. U. S. Geological Survey. Shorter Contributions to General Geology, professional paper 120. Pages 165-206.
https://www.google.com/books/edition/Professional_Paper/43fdNoV0ybIC?hl=en&gbpv=0.
Crête-Lafrenière, Alexis; Weir, Laura K.; Bernatchez, Louis (2012). “Framing the Salmonidae Family Phylogenetic Portrait: A More Complete Picture from Increased Taxon Sampling”. PLOS ONE, volume 7, issue 10: e46662 (October 5, 2012).
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0046662.
Shedko, S. V.; Miroshnichenko, I. L.; Nemkova, G. A. (2013). “Phylogeny of salmonids (salmoniformes: Salmonidae) and its molecular dating: Analysis of mtDNA data”. Russian Journal of Genetics, volume 49, issue 6 (June 16, 2013). Pages 623-637.
https://www.researchgate.net/publication/260342129_Phylogeny_of_Salmonids_Salmoniformes_Salmonidae_and_its_Molecular_Dating_Analysis_of_mtDNA_Data.
“Fisheries Fact Sheet”. Department of Fisheries and Forestry. Ottawa, Canada. Page 84. https://waves-vagues.dfo-mpo.gc.ca/library-bibliotheque/360067pt1.pdf.
Smithsonian Institute. “Leucichthyops vagans Cockerell”. https://www.si.edu/object/nmnhpaleobiology_3394004.
Categories: Paleontology, Uncategorized
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