Introduction
Allow me to introduce you to Toxolophosaurus, “the bow-crested lizard”, which lived in Montana during the early to middle of the Cretaceous Period approximately 135-115 million years ago. Despite its name, Toxolophosaurus wasn’t either a dinosaur or a lizard. Instead it was rhynchocephalian reptile, commonly known as a “tuatara”. Tuataras outwardly resemble lizards, but they actually belong to a completely separate group of reptiles, and a much more primitive one at that. In fact, the modern-day tuatara (Sphenodon punctatus) of New Zealand is the only surviving rhynchocephalian today.
Discovery and Naming
Fossils of this animal were first found in the late 1950s in southwestern Montana by one George Cloud. The specimen was uncovered within the rocks of the Kootenai Formation, and consisted of a single lower jaw (both the left and rights sides joined together) with the articular ends missing encased within red siltstone. The fossil was first presented to the Montana geologist Oliver Duncan “O.D.” Blake (“Memorial to Oliver Duncan Blake, 1910-1986”), who afterwards forwarded it to Everett C. Olson, a paleontologist at UCLA. In 1960, Olson named this specimen Toxolophosaurus cloudi. The genus name Toxolophosaurus, meaning “bow-crested lizard”, was given in reference to the arch-like structure of the creature’s teeth. Olson described the teeth as possessing a “sharp, transverse crest, curved anteriorly on lingual margin” (Olson 1960, page 551). The species name cloudi was named in honor of Mr. George Cloud who found the specimen (Olson 1960, page 551). The holotype specimen is currently housed within the Field Museum of Natural History in Chicago, Illinois (collection ID code: FMNH UR 619) (Throckmorton et al 1981, page 586).
Classification and Anatomy
When it was discovered, it was originally thought to belong to a primitive group of reptiles called the trilophosaurids, which existed during the Triassic Period (Olson 1960, pages 551-555). Later in 1981, it was determined that it was actually a prehistoric tuatara (Throckmorton et al 1981, pages 586-597). Toxolophosaurus belonged to the tribe Eilenodontini (along with the genera Ankylosphenodon, Eilenodon, and Priosphenodon), which was part of the sub-family Eilenodontinae. In fact, Toxolophosaurus was classified as the sister-genus to Eilenodon (Herrera-Flores et al, 2018). Members of Eilenodontini are distinguished from other sphenodonts by “propalinal jaw action, deep mandible, retroarticular process reduced, marginal teeth expanded mediolaterally… Eilenodon and Toxolophosaurus were deep-jawed forms with very wide grinding teeth, indicating a herbivorous diet” (“Sphenodontia: Sphenodontinae”).
Based upon the shape of its teeth, Toxolophosaurus was a plant-eater. This makes sense because the other species within the sphenodont tribe Eilenodontini were also herbivorous. Moreover, the particular shape of the teeth as well as the wear patterns upon the tooth surface show similarities to mammalian teeth which are specially adapted for feeding on tough scrubby plants. The front of the lower jaw was “edentulous”, meaning that there weren’t any teeth or tooth sockets present there (Olson 1960, page 551; Throckmorton et al, 1981, pages 586, 595). This is similar to what is seen in some sphenosuchian crocodylomorphs such as Hallopus and Macelognathus. The lack of teeth upon the lower jaw’s “symphysis” (the front of the lower jaw where the left half and right half are joined together) is sometimes touted as evidence for a diet focused on eating eggs. However, given the fact that Toxolophosaurus‘ teeth appear to have been specially adapted for a herbivorous diet, and given that we have numerous examples of plant-eating dinosaurs which lacked teeth in the front of their jaws, I think that the idea that Toxolophosaurus also ate eggs doesn’t have much evidence to back it up.
The only confirmed specimen which we have of Toxolophosaurus had a jaw length of 60 mm (Apesteguia and Carballido 2014, page 314). By extrapolating this measurement in relationship with the known measurements of skull length and overall body length of other sphenodonts, this yields an overall body size of 22.65 inches (57.53 cm) for this individual, which is within the size range of a modern-day tuatara. Its close relative Eilenodon from the Late Jurassic Morrison Formation had a skull length of 109 mm (4.29 inches) (Apesteguia and Carballido 2014, page 314). This would make that animal 40-42 inches long. Because Toxolophosaurus is known from only one confirmed specimen, it’s unknown if it also grew to a similar size. However, since Toxolophosaurus and Eilenodon are sister taxa, it’s very likely that Toxolophosaurus also reached 3 to 3.5 feet long.
Dating
Olson’s 1960 report stated that the fossil was found 223.75 feet (68.2 meters) above the base of the Kootenai Formation within Silver Bow County, Montana. The geologist Oliver Blake examined the locality and measured the strata, and he stated that the entirety of the Kootenai strata at this locality measured 1,210 feet thick (Olson 1960, pages 551, 554). This is in contrast to the U.S. Geological Survey, which says that the Kootenai Formation measures 335 meters (1,099 feet) thick within Silver Bow County, Montana (“Kootenai through Dinwoody Formation: Kootenai, Morrison, Thaynes, Woodside, and Dinwoody Formations”). However, since Blake personally went to the site and measured the thickness of each individual layer at half-inch increments, I’m more inclined to trust his data over that from the USGS. The rock layer which the fossil was found in was described as “Siltstone, calcareous, splintery, calcareous, pale reddish-brown, gray, and orange pink (Vertebrate fossil found 21″ below the top of this bed). 58.5 [feet thick]” (Olson 1960, page 554). Geographically, the fossil’s locality was sited at “T. 4 S., R. 8 W., W. Cent. NE SE Sec. 23”. (Olson 1960, page 551).
The Kootenai Formation is often described as dating to the Aptian and Albian Stages of the middle Cretaceous Period, 120-110 MYA (“Geologic Unit: Kootenai”; Horner and Hanson 2020, page 17), based upon several studies which were carried out during the 1940s and 1950s. However, more recent studies of the Kootenai Formation conducted during the 2000s and 2010s using U-Pb dating revealed that the Kootenai Formation extends much further back than previously supposed, possibly to 140 MYA. The most recent comprehensive study (Finzel and Rosenblume 2020) of the formation’s strata within southwestern Montana, which was where the specimen of Toxolophosaurus was found, gave the ages of 135-110 for the entire formation (Rosenblume et al, 2021).
The Kootenai Formation is divided into smaller geological units called “members”. These are, from lowest/oldest to newest/highest: the Cut Bank Sandstone Member, the Sunburst Sandstone Member, and the Moulton Member (also known as the Greybull Member) (Cohee and Wright 1976, page A23; “Kootenai through Dinwoody Formation: Kootenai, Morrison, Thaynes, Woodside, and Dinwoody Formations”). Given the low placement of the locality’s stratum within the Kootenai’s overall thickness (less than one-fifth of the way up from the formation’s base), it was likely found within the lowest member, the Cut Bank Sandstone Member. According to Rosenblume et al (2021), “In southwestern Montana, the Kootenai Formation consists of four informal members: the Lower clastic, Lower calcareous, Upper clastic, and Upper calcareous or gastropod limestone…This measured section provides stratigraphic constraints and U-Pb detrital zircon maximum depositional ages (MDAs) for all four informal members of the formation; estimated ages are as follows: ~135 Ma for the Lower clastic member, ~115 Ma for the Lower calcareous member, ~112 Ma for the Upper clastic member, and ~110 for the Upper calcareous member (gastropod limestone). These MDAs are consistent with ages determined by previous paleontological work; however, the age of the lowermost interval of the Kootenai Formation remains subject to considerable uncertainty because it generally lacks Cretaceous zircon grains” (Rosenblume et al, 2021). If these dates correlate to the named members described earlier, then this would mean that the Cut Bank Sandstone Member of the Kootenai Formation dates to 135-115 MYA, and therefore the sole specimen which we have of Toxolophosaurus dates sometime to this time span. Unfortunately, it’s impossible at the moment to narrow this date down further.
Possible Specimens from Utah?
Numerous jaw fragments which are similar to those of Toxolophosaurus (but have not yet been definitively determined to belong to that same genus) have also been found within eastern Utah in the rocks of the upper part of the Yellow Cat Member of the Cedar Mountain Formation, 135-132 MYA (Kirkland et al 1997, page 76; Kirkland and Madsen 2007, page 8). The abundance of fossil fragments found here indicates that this animal was relatively common within Utah during that time (“Tate Geological Museum’s Spring Lecture Series 2021: Cretaceous Dinosaurs – part 3”). According to Dr. Jim Kirkland and his colleagues, “A large eilenodontid sphenodont, similar to Toxolophosaurus, is represented by isolated jaws, and a partial skeleton. Given the abundance of this taxon, it probably was an important small terrestrial herbivore during the deposition of the upper Yellow Cat Member” (Kirkland et al 2016, page 133). In a 2021 online lecture, Kirkland stated that this partial skeleton consisted of the front-half of the body including both forelimbs and the shoulder girdle (“Tate Geological Museum’s Spring Lecture Series 2021: Cretaceous Dinosaurs – part 3”). Additionally, another specimen consisting of a complete skull was discovered within the Ruby Ranch Member of the Cedar Mountain Formation, which dates to approximately 118-105 MYA. For the time being, all of these specimens are classified as “cf. Toxolophosaurus” which means “similar in shape to Toxolophosaurus” (Kirkland et al 2016, pages 137, 151). A rigorous compare-and-contrast study will need to be performed between the Toxolophosaurus holotype and all of the Utah specimens to determine if they all belong to the same taxon.
Artwork
Below is my reconstruction of Toxolophosaurus. While I was able to find illustrations and photographs of the holotype lower jaw found in Montana, I wasn’t able to find any images of the fossils which were found in Utah. The head of this reconstruction is based upon the heads of the related genera Eilenodon and Priosphenodon, which had a somewhat boxier skull compared to the modern-day tuatara of New Zealand. The drawing was made with No.2 pencil, No.3 pencil, and colored pencils on printer paper. The eye was painted in Microsoft Paint.
Toxolophosaurus cloudi. © Jason R. Abdale (May 25, 2023).
Bibliography
Articles
Apesteguia, Sebastian; Carballido, Jose L. (2014). “A new eilenodontine (Lepidosauria, Sphenodontidae) from the Lower Cretaceous of central Patagonia”. Journal of Vertebrate Paleontology, volume 34, issue 2 (March 2014). Pages 303-317.
https://www.academia.edu/29689944/A_new_eilenodontine_Lepidosauria_Sphenodontidae_from_the_Lower_Cretaceous_of_central_Patagonia.
Cohee, George V.; Wright, Wilna B. (1976). “Changes in Stratigraphic Nomenclature by the U.S. Geological Survey, 1975”. Geological Survey Bulletin 1422-A. Washington: United States Government Printing Office 1976. Pages A1-A84.
https://pubs.usgs.gov/bul/1422a/report.pdf.
Herrera-Flores Jorge A.; Stubbs Thomas L.; Elsler, Armin; Benton, Michael J. (2018). “Taxonomic reassessment of Clevosaurus latidens Fraser, 1993 (Lepidosauria, Rhynchocephalia) and rhynchocephalian phylogeny based on parsimony and Bayesian inference”. Journal of Paleontology, volume 92, issue 4 (July 2018). Pages 734-742.
https://www.cambridge.org/core/journals/journal-of-paleontology/article/taxonomic-reassessment-of-clevosaurus-latidens-fraser-1993-lepidosauria-rhynchocephalia-and-rhynchocephalian-phylogeny-based-on-parsimony-and-bayesian-inference/AEED7FE732081A56AD8B02CB57451716.
Horner, John R.; Hanson, Dale A. (2020). “Vertebrate Paleontology of Montana”. MBMG Special Publication 122: Geology of Montana, vol. 2: Special Topics. Page 1-46.
https://mbmg.mtech.edu/pdf/geologyvolume/HornerVertebratePaleoFinal.pdf.
Kirkland, James I.; Britt, Brooks; Burge, Donald L.; Carpenter, Kenneth; Cifelli, Richard; DeCourten, Frank; Eaton, Jeffery; Hasiotis, Stephen; Lawton, Tim (1997). “Lower to Middle Cretaceous Dinosaur Faunas of the Central Colorado Plateau: A Key to Understanding 35 Million Years of Tectonics, Evolution, and Biogeography”. Brigham Young University Geology Studies, volume 42, issue 2. Pages 69-103.
https://www.researchgate.net/publication/259021967_Lower_to_middle_Cretaceous_dinosaur_faunas_of_the_central_Colorado_Plateau_a_key_to_understanding_35_million_years_of_tectonics_sedimentology_evolution_and_biogeography.
Kirkland, James I.; Madsen, Scott K. (2007). “The Lower Cretaceous Cedar Mountain Formation, eastern Utah: the view up an always interesting learning curve”. Fieldtrip Guidebook, Geological Society of America, Rocky Mountain Section. 2007 Geological Society of America Rocky Mountain Section Annual Meeting, St. George, Utah (May 4-6, 2007). Pages 1-108.
https://www.researchgate.net/publication/40662964_The_Lower_Cretaceous_Cedar_Mountain_Formation_eastern_Utah_the_view_up_an_always_interesting_learning_curve_papers_from_a_symposium_of_the_Geological_Society_of_America_at_the_annual_meeting_in_St_Geo.
Kirkland, James I.; Suarez, Marina; Suarez, Celina; Hunt-Foster, ReBecca (2016). “The Lower Cretaceous in East-Central Utah—The Cedar Mountain Formation and its Bounding Strata”. Geology of the Intermountain West, volume 3 (October 2016). Pages 101-228.
https://giw.utahgeology.org/giw/index.php/GIW/article/view/11.
Olson Everett C. (1960). “A trilophosaurid reptile from the Kootenai Formation (Lower Cretaceous)”. Journal of Paleontology, volume 34, issue 3 (May 1, 1960). Pages 551-555.
https://archive.org/details/sim_journal-of-paleontology_1960-05_34_3_0/page/550/mode/2up?view=theater.
Rosenblume, Justin; Finzel, Emily; Pearson, David M. (2021). “Early Cretaceous Provenance, Sediment Dispersal, and Foreland Basin Development in Southwestern Montana, North American Cordillera”. Tectonics, volume 40, issue 4 (April 2021).
https://www.researchgate.net/publication/350155032_Early_Cretaceous_Provenance_Sediment_Dispersal_and_Foreland_Basin_Development_in_Southwestern_Montana_North_American_Cordillera.
Throckmorton, Gaylord S.; Hopson, James A.; Parks, Peter (1981). “A re-description of Toxolophosaurus cloudi Olson, a Lower Cretaceous herbivorous sphenodontid reptile”. Journal of Paleontology, volume 55, issue 3 (May 1981). Pages 586-597.
Websites
Geological Society of America. “Memorial to Oliver Duncan Blake, 1910-1986”. https://rock.geosociety.org/net/documents/gsa/memorials/v24/Blake-OD.pdf. Accessed on May 24, 2023.
Palaeos. “Sphenodontia: Sphenodontinae”. http://palaeos.com/vertebrates/sphenodontia/sphenodontinae.html. Accessed on May 25, 2023.
United States Geological Survey. “Kootenai through Dinwoody Formation: Kootenai, Morrison, Thaynes, Woodside, and Dinwoody Formations”. https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=MTKJTRkd%3B0. Accessed on May 24, 2023.
United States Geological Survey National Geologic Map Database. “Geologic Unit: Kootenai”. https://ngmdb.usgs.gov/Geolex/Units/Kootenai_8885.html. Accessed on May 24, 2023.
Videos
YouTube. Tate Geological Museum. “Tate Geological Museum’s Spring Lecture Series 2021: Cretaceous Dinosaurs – part 3 – The Cedar Mountain Formation of Utah: North America’s Most Complete Early Cretaceous Record” (May 11, 2021). https://www.youtube.com/watch?v=Thhb6Jy-Acw. Accessed on March 14, 2022.
Categories: Paleontology, Uncategorized
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