Halecodon denticulatus was a species of prehistoric saltwater fish, which is estimated to have measured 20 inches long, and which lived around 105-98 million years ago within the Mowry Sea which once covered the center of North America during the middle of the Cretaceous Period.
The species was named by the paleontologist Theodore D. A. Cockerell in 1919. The one-and-only specimen of this animal was found by one A. R. Schultz near Spring Valley, Uinta County, Wyoming within the rocks of the Aspen Shale (Cockerell 1919, pages 171, 183).
The Aspen Shale was named by the geologist Arthur Clifford Veatch in 1907 after the now-abandoned Aspen railroad station which formerly laid upon the Union Pacific rail-line within the southwestern corner of Wyoming. The formation was described as being composed of dark grey shale, splintery in its texture and fracturing. The Aspen Shale overlies the Bear River Formation, and it underlies the Frontier Formation. The Aspen Shale is almost exactly concurrent with the Mowry Shale, but comparison of various ammonite species found within both formations show that the Aspen Formation was deposited slightly earlier. The ammonite species Neogastroplites cornutus occurs within the lower levels of the Mowry Shale, but it’s also found within the middle layers of the Aspen Shale. Likewise, the species Neogastroplites americanus is found within the middle layers of the Mowry Shale, but it’s found in the upper parts of the Aspen Shale. It’s believed that the lower parts of the Aspen Shale are concurrent with the upper layers of the Thermopolis Shale, which underlies the Mowry Shale within north-central Wyoming (Reeside Jr. and Cobban 1960, pages 7, 11).
Theodore Cockerell didn’t state where exactly the fossil of Halecodon had been found stratigraphically, so we don’t know if the fossil came from the lower, middle, or upper parts of the Aspen Shale, and that makes exact dating problematic. As stated earlier, the lower parts of the Aspen Shale were concurrent with the upper parts of the Thermopolis Shale. The Thermopolis Shale is divided into four sub-units referred to by geologists as “members”, which are, arranged from lowest/oldest to highest/newest: the Rusty Beds Member, the Lower Thermopolis Member, the Muddy Sandstone Member, and the Upper Thermopolis Member (Thom Jr. et al 1935, pages 1-200; Eicher 1958, pages 79-83; Mirsky 1962, pages 1,653-1,680). The Upper Thermopolis Member dates to the late Albian Stage, 105-100 MYA (Druckenmiller 2002, page 29). The upper parts of the Aspen Shale are concurrent with the middle part of the Mowry Shale. The Mowry Shale dates to the late Albian and early Cenomanian Stages of the middle Cretaceous, dating approximately 100-97 MYA (Bremer 2016, pages 1, 6; Finn 2021, page 4). Therefore, the approximate date of 105 MYA might be applied to the basal layer of the Aspen Shale, and the approximate date of 98 MYA may be ascribed to the uppermost layer of the Aspen Shale.
Halecodon denticulatus is known from scant remains consisting of a palatine and ectopterygoid, which both form part of the upper jaw (collection ID code: USNM 8708). Based upon their shape, Theodore Cockerell stated that Halecodon was a member of the family Enchodontidae, and therefore was related to Enchodus, the so-called “saber-toothed herring”, even though it actually wasn’t related to herrings at all. Despite being related to the vampire-esque Enchodus, Halecodon didn’t have the ferociously elongated teeth seen in its more well-known cousin. The shape of the palatine is similar to that seen in the prehistoric fish genus Halec, which Halecodon is named after, but is more slender and bears smaller teeth. In fact, the name Halecodon literally means “Halec tooth”, but it can be better translated as “teeth similar in shape to the teeth of Halec”. The shape of Halecodon’s ectopterygoid is much more basally elongated compared to that seen in Halec, and possesses many small teeth. Altogether, Halecodon’s palatine and ectopterygoid measure 54 mm (2.12 inches) long. The teeth at the end of the palatine bone measure juts 1.7 mm long and are sharply pointed, while the teeth on the ectopterygoid measure only 1 mm long (Cockerell 1919, page 183).
Palatine and ectopterygoid of Halecodon denticulatus, from the Aspen Shale of southwestern Wyoming (collection ID code: USNM 8708). Image from Cockerell (1919), plate XXXVI, figure 7.
https://pubs.usgs.gov/pp/0120i/report.pdf.
How can we determine the total length of this animal if we only have two small bones in the upper jaw to go on? Well, if Halecodon truly is a close relative of Enchodus, then perhaps we can make some size extrapolations. Enchodus is a genus composed of several species, but for this exercise, we’ll look at the species Enchodus zimapanensis, which comes from Zimapan, Mexico and dates to either the Albian or Cenomanian Stages of the middle Cretaceous Period. I chose this species because there are some very excellent illustrations of the fossils within the article that named and described it. The holotype specimen UAHMP 679 measures a total length of 36 cm (14.17 inches) (Fielitz and Gonzalez-Rodriguez 2010, page 1,345). Measurements for the individual bones in the skull are not provided within the article. However, using the illustrations and the accompanying 1 cm scale bars, the length of the palatine and ectopterygoid bones appears to measure 3.75 cm. Using this ratio, the total length for Halecodon denticulatus would have been 51.84 cm (20.4 inches). However, I should point out that it’s possible that Halecodon may have had different physical proportions, and if that’s the case, then these calculations and size estimates are utterly meaningless.
In addition to the unusually large teeth which Enchodus is known for, members of the family Enchodontidae are distinctive for a row of sturgeon-like scutes running down the middle of their backs (Silva and Gallo 2011, pages 483, 498).
So if we were to reconstruct what Halecodon would have looked like, picture a miniature barracuda or wahoo measuring 20 inches long with a line of armor plates along the top of its back like a sturgeon or some species of armored catfish, and that’s how it’s shown in the illustration seen below.
Halecodon denticulatus. © Jason R. Abdale (February 12, 2024).
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Bibliography
Bremer, Jordan M. (2016). “Stratigraphy and Sedimentology of the Cretaceous Mowry Shale in the Northern Bighorn Basin of Wyoming: Implications for Unconventional Resource Exploration and Development”. Master of science thesis, University of Nebraska (July 2016). Pages 1-55.
https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1083&context=geoscidiss.
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 165-203.
https://pubs.usgs.gov/pp/0120i/report.pdf.
Druckenmiller, Pat S. (2002). “Osteology of a New Plesiosaur from the Lower Cretaceous (Albian) Thermopolis Shale of Montana”. Journal of Vertebrate Paleontology, volume 22, issue 1 (March 14, 2002). Pages 29-42.
Eicher, D. L. (1958). “The Thermopolis shale in eastern Wyoming”. Wyoming Geological Association Field Conference Guidebook, no. 13 (1958). Pages 79-83.
Fielitz, Christopher; Gonzalez-Rodriguez, Katia A. (2010). “A New Species of Enchodus (Aulopiformes: Enchodontidae) from the Cretaceous (Albian to Cenomanian) of Zimapán, Hidalgo, Mexico”. Journal of Vertebrate Paleontology, volume 30, issue 5 (September 2010). Pages 1,343-1,351.
https://www.researchgate.net/publication/233476912_A_New_Species_of_Enchodus_Aulopiformes_Enchodontidae_from_the_Cretaceous_Albian_to_Cenomanian_of_Zimapan_Hidalgo_Mexico.
Finn, Thomas M. (2021). “Stratigraphic Cross Sections of the Mowry Shale and Associated Strata in the Wind River Basin, Wyoming”. U.S. Geological Survey Pamphlet to accompany Scientific Investigations Map 3476. Pages 1-14.
https://pubs.usgs.gov/sim/3476/sim3476_pamphlet.pdf.
Mirsky, Arthur (1962). “Stratigraphy of non-marine Upper Jurassic and Lower Cretaceous rocks, southern Big Horn Mountains, Wyoming”. American Association of Petroleum Geologists Bulletin, volume 46, issue 9 (1962). Pages 1,653-1,680.
Reeside, Jr., John B.; Cobban, William A. (1960). “Studies of the Mowry Shale (Cretaceous) and Contemporary Formations in the United States and Canada”. Geological Survey Professional Paper 355. Washington, D.C.: United States Government Printing Office, 1960. Pages 1-126.
https://pubs.usgs.gov/pp/0355/report.pdf.
Silva, Hilda M. A.; Gallo, Valéria (2011). “Taxonomic review and phylogenetic analysis of Enchodontoidei (Teleostei: Aulopiformes)”. Anais da Academia Brasileira de Ciências (Annals of the Brazilian Academy of Sciences), volume 83, issue 2 (2011). Pages 483-511.
https://www.scielo.br/j/aabc/a/vPvQZ4v493QvhCLL9wh9zDb/?format=pdf&lang=en.
Thom Jr., W. T.; Hall, G. M.; Wegemann, C. H.; Moulton, G. F. (1935). “Geology of Big Horn County and the Crow Indian Reservation, Montana, with special reference to the water, coal, oil, and gas resources”. U.S. Geological Survey Bulletin 856 (1935). Pages 1-200.
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