Selmasaurus

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Selmasaurus
Temporal range: Late Cretaceous, Santonian 86.3–83.6  Ma
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Selmasaurus Clean.png
Selmasaurus johnsoni mounted skull in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Superfamily: Mosasauroidea
Family: Mosasauridae
Subfamily: Plioplatecarpinae
Genus: Selmasaurus
Wright & Shannon, 1988
Species
  • S. russelliWright & Shannon, 1988 (type)
  • S. johnsoniPolcyn & Everhart, 2008

Selmasaurus is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Plioplatecarpinae subfamily alongside genera like Angolasaurus and Platecarpus . Two species are known, S. russelli and S. johnsoni; both are exclusively known from Santonian deposits in the United States.

Contents

Selmasaurus is unique among the mosasaurs in that its skull is unusually akinetic, meaning that it is incapable of widening to swallow larger prey. Most mosasaurs have skulls which possess "coupled kinesis" (mesokinesis and streptostyly), that is, parts of the jaw can open widely to accommodate large prey.

History of discovery

Skull roof of the S. russelli holotype specimen GSATC 221, showing frontal, parietal, postorbitofrontals, and left jugal. Dorsal view. GSATC 221, holotype specimen of Selmasaurus russelli, crk.jpg
Skull roof of the S. russelli holotype specimen GSATC 221, showing frontal, parietal, postorbitofrontals, and left jugal. Dorsal view.

First recognized by geologist Samuel Wayne Shannon in his 1975 Master's thesis, "Selected Alabama Mosasaurs", the taxon remained a nomen nudum until it was officially described in 1988 in an article coauthored by Wright. [1] The type specimen, formerly reposited at the Geological Survey of Alabama and cataloged as GSATC 221, was transferred in 2005 to the Alabama Museum of Natural History (Tuscaloosa). The holotype of this genus consists of a very well preserved but incomplete and disarticulated skull, the left atlantal neural arch, atlas centrum, and a single neural arch from a cervical vertebra. Preserved skull elements include the frontal, parietal, left ectopterygoid, left jugal, supratemporals, basioccipital and basisphenoid, and quadrates. The species was named in honor of paleontologist Dale A. Russell, for his extensive work on mosasaurs. The holotype and only known specimen of S. russelli was collected from an unknown location in western Alabama, and for decades, uncertainty surrounded the precise stratigraphic horizon from which the specimen had been recovered. Then in 1998, Caitlín R. Kiernan extracted chalky matrix from the basilar canal of the basiocciptal and identified calcareous nanoplankton that indicated GSATC 221 had originated from basal Campanian beds within the lower unnamed member of the Mooreville Chalk Formation (Selma Group). In her study of Alabama mosasaur biostratigraphy, Kiernan placed S. russelli within the Clidastes Acme Zone, though it was the rarest element in the fauna, accounting for only 0.3% of the biozone's assemblage (one specimen). [2]

A remarkably well preserved and nearly complete Selmasaurus skull and partial postcranial skeleton was discovered by Steve Johnson and family in 1996, from the Santonian or Campanian marine horizon in the Niobrara Formation of Niobrara Chalk, western Kansas. Recovered in 1997 and donated to the Sternberg Museum of Natural History in Hays, Kansas, in 2001, the remains were determined to be a new species of Selmasaurus in 2008 after over a decade of study by Polcyn and Everhart. [3] Named S. johnsoni after its discoverer, the skull is one of the most complete mosasaur skulls recovered and thus provides new anatomical information for Selmasaurus, a better understanding of plioplatecarpine ingroup relationships, extends the geographic and temporal range of the genus, and documents further diversity within Plioplatecarpinae. The holotype and the only known specimen is housed at the Sternberg Museum of Natural History under catalog number FHSM-VP-13910.

Description

Life restoration Selmasaurus2.png
Life restoration

Selmasaurus was a small predatory mosasaur at approximately 3–5 meters in length. It possesses a relatively low number of teeth for a mosasaur, the lowest of any known species at the time of its discovery. [4] Originally classified as a plioplatecarpine mosasaur, it differs from all other plioplatecarpine mosasaurs in several respects, listed below as stated by Polcyn and Everhart (2008): [3]

Suprastapedial process descends to about half the quadrate height, and is in contact with but not fused to the peg-like infrastapedial process; elongate medially constricted, subrectangular parietal table, broadening anteriorally and constricting posteriorly forming narrow parasagittal ridges that diverge at their terminus; posteroventral median process of parietal meets supraoccipital in narrow elongate contact; anteromedial process of supratemporal clasped by medial excavations of the parietal rami for nearly entire length of rami; canals for the basilar arteries enter the basioccipital as two small foramina separated by a median septum, diverge within the basioccipital and cross the basisphenoid suture in a lateral and deeply ventral position.

Classification

Wright and Shannon classified Selmasaurus as a member of the mosasaur subfamily Plioplatecarpinae, which also includes the genera Platecarpus, Plioplatecarpus, and Ectenosaurus, largely on the "basis of the mode of circulation through the basicarnium." The genus may be most closely related to Ectenosaurus, though it possesses a much shorter, stouter skull. Additional specimens would greatly expand our understanding of Selmasaurus russelli.

The cladogram below follows the most resolved topology from a 2011 phylogenetic analysis of the Plioplatecarpinae by paleontologists Takuya Konishi and Michael W. Caldwell. [5]

Clidastes propython

Kourisodon puntledgensis

Russellosaurina

Yaguarasaurus columbianus

Russellosaurus coheni

Tethysaurus nopcsai

Tylosaurus kansasensis

Tylosaurus proriger

Plioplatecarpinae

Ectenosaurus clidastoides

Angolasaurus bocagei

Selmasaurus johnsoni

Selmasaurus russelli

Plesioplatecarpus planifrons

Platecarpus tympaniticus

Latoplatecarpus willistoni

Latoplatecarpus nichollsae

" Platecarpus somenensis "

Plioplatecarpus primaevus

Plioplatecarpus houzeaui

Plioplatecarpus marshi

Related Research Articles

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<span class="mw-page-title-main">Mosasaurinae</span> Subfamily of reptiles

The Mosasaurinae are a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates. Members of the subfamily are informally and collectively known as "mosasaurines" and their fossils have been recovered from every continent except for South America.

<span class="mw-page-title-main">Tylosaurinae</span> Extinct subfamily of lizards

The Tylosaurinae are a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates. Members of the subfamily are informally and collectively known as "tylosaurines" and have been recovered from every continent except for South America. The subfamily includes the genera Tylosaurus, Taniwhasaurus, and Kaikaifilu, although some scientists argue that only Tylosaurus and Taniwhasaurus should be included.

Plioplatecarpinae is a subfamily of mosasaurs, a diverse group of Late Cretaceous marine squamates. Members of the subfamily are informally and collectively known as "plioplatecarpines" and have been recovered from all continents, though the occurrences in Australia remain questionable. The subfamily includes the genera Latoplatecarpus, Platecarpus, Plioplatecarpus and Plesioplatecarpus.

<span class="mw-page-title-main">Halisaurinae</span> Extinct subfamily of lizards

The Halisaurinae are a subfamily of mosasaurs, a group of Late Cretaceous marine lizards. They were small to medium-sized, ranging from just under 3 meters in Eonatator sternbergi to as much as 8 or 9 meters in Pluridens serpentis. They tended to have relatively slender jaws and small, numerous teeth, suggesting a diet of small fish and other prey. Although the skeleton is primitive compared to other Mosasauridae in many respects, halisaurines had the distinctive hypocercal tail of other mosasaurids suggesting good swimming ability, and they persisted alongside other mosasaurs until the end of the Cretaceous. The earliest known remains of halisaurines occur in rocks of Santonian age and the subfamily persists until the latest Maastrichtian. Halisaurines are known from North and South America, Europe, Asia and Africa, indicating a more or less global distribution in the Late Cretaceous. Four genera are currently recognized: Eonatator, Halisaurus, Phosphorosaurus and Pluridens.

<i>Eonatator</i> Extinct genus of lizards

Eonatator is an extinct genus of marine lizard belonging to the mosasaur family. It is a close relative of Halisaurus, and part of the same subfamily, the Halisaurinae. It is known from the Late Cretaceous of North America, Colombia and Sweden. Originally, this taxon was included within Halisaurus, but was placed in its own genus, which also led to the subfamily Halisaurinae being created for the two genera.

<i>Russellosaurus</i> Extinct genus of lizards

Russellosaurus is an extinct genus of tethysaurine mosasauroid from the Late Cretaceous of North America. The genus was described from a skull discovered in an exposure of the Arcadia Park Shale at Cedar Hill, Dallas County in the south-central part of the DFW Metroplex in Texas, United States. The skull was found in 1992 by a member of the Dallas Paleontological Society, who then donated to the museum. Other fragmentary specimens of Russelosaurus have been recovered from the slightly older Kamp Ranch Limestone at two other localities in the Dallas area.

Platecarpus is an extinct genus of aquatic lizards belonging to the mosasaur family, living around 84–81 million years ago during the middle Santonian to early Campanian, of the Late Cretaceous period. Fossils have been found in the United States and possible specimens in Belgium and Africa. A well-preserved specimen of Platecarpus shows that it fed on moderate-sized fish, and it has been hypothesized to have fed on squid, and ammonites as well. Like other mosasaurs, it was initially thought to have swum in an eel-like fashion, although another study suggests that it swam more like modern sharks. An exceptionally well-preserved specimen of P. tympaniticus known as LACM 128319 shows skin impressions, pigments around the nostrils, bronchial tubes, and the presence of a high-profile tail fluke, showing that it and other mosasaurs did not necessarily have an eel-like swimming method, but were more powerful, fast swimmers. It is held in the Natural History Museum of Los Angeles County. Isotopic analysis on teeth specimens has suggested that this genus and Clidastes may have entered freshwater occasionally, just like modern sea snakes.

<i>Prognathodon</i> Extinct genus of lizards

Prognathodon is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Mosasaurinae subfamily, alongside genera like Mosasaurus and Clidastes. Prognathodon has been recovered from deposits ranging in age from the Campanian to the Maastrichtian in the Middle East, Europe, New Zealand, and North America.

<i>Halisaurus</i> Extinct genus of lizards

Halisaurus is an extinct genus of marine reptile belonging to the mosasaur family. The holotype, consisting of an angular and a basicranium fragment discovered near Hornerstown, New Jersey, already revealed a relatively unique combination of features and prompted a new genus to be described. It was named by Othniel Charles Marsh in 1869 and means "ocean lizard". It was renamed by Marsh to Baptosaurus in 1870, since he believed the name to already be preoccupied by the fish Halosaurus. According to modern rules, a difference of a letter is enough and the substitute name is unneeded, making "Baptosaurus" a junior synonym.

Clidastes is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Mosasaurinae subfamily, alongside genera like Mosasaurus and Prognathodon. Clidastes is known from deposits ranging in age from the Coniacian to the early Campanian in the United States.

<i>Dolichorhynchops</i> Extinct genus of reptiles

Dolichorhynchops is an extinct genus of polycotylid plesiosaur from the Late Cretaceous of North America, containing three species, D. osborni, D. bonneri and D. tropicensis, as well as a questionably referred fourth species, D. herschelensis. Dolichorhynchops was a prehistoric marine reptile, but at least one species, D. tropicensis, likely entered rivers to collect gastroliths. Its Greek generic name means "long-nosed face". While typically measuring about 3 metres (9.8 ft) in length, the largest specimens of D. osborni and D. bonneri are estimated to have a total body length more than approximately 4.29 metres (14.1 ft) and 5.09 metres (16.7 ft), respectively.

The Mooreville Chalk is a geological formation in North America, within the U.S. states of Alabama and Mississippi, which were part of the subcontinent of Appalachia. The strata date back to the early Santonian to the early Campanian stage of the Late Cretaceous. The chalk was formed by pelagic sediments deposited along the eastern edge of the Mississippi embayment. It is a unit of the Selma Group and consists of the upper Arcola Limestone Member and an unnamed lower member. Dinosaur, mosasaur, and primitive bird remains are among the fossils that have been recovered from the Mooreville Chalk Formation.

<i>Ectenosaurus</i> Extinct genus of lizards

Ectenosaurus is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Plioplatecarpinae subfamily alongside genera like Angolasaurus and Platecarpus. Ectenosaurus is known from the Santonian and Campanian of Kansas, Alabama, and Texas.

<i>Angolasaurus</i> Extinct genus of lizards

Angolasaurus is an extinct genus of mosasaur. Definite remains from this genus have been recovered from the Turonian and Coniacian of Angola, and possibly the Coniacian of the United States, the Turonian of Brazil, and the Maastrichtian of Niger. While at one point considered a species of Platecarpus, recent phylogenetic analyses have placed it between the (then) plioplatecarpines Ectenosaurus and Selmasaurus, maintaining a basal position within the plioplatecarpinae.

<span class="mw-page-title-main">Paleobiota of the Niobrara Formation</span>

During the time of the deposition of the Niobrara Chalk, much life inhabited the seas of the Western Interior Seaway. By this time in the Late Cretaceous many new lifeforms appeared such as mosasaurs, which were to be some of the last of the aquatic lifeforms to evolve before the end of the Mesozoic. Life of the Niobrara Chalk is comparable to that of the Dakota Formation, although the Dakota Formation, which was deposited during the Cenomanian, predates the chalk by about 10 million years.

<i>Latoplatecarpus</i> Extinct genus of lizards

Latoplatecarpus is an extinct genus of plioplatecarpine mosasaur known from the Late Cretaceous of the northern Gulf of Mexico and the Western Interior Basin of North America. It was among the largest plioplatecarpine mosasaur, with L. nichollsae measuring over 8 metres (26 ft) in total body length.

<i>Plesioplatecarpus</i> Extinct genus of lizards

Plesioplatecarpus is an extinct genus of plioplatecarpine mosasaur known from the Late Cretaceous of the northern Gulf of Mexico and the Western Interior Basin of North America.

<span class="mw-page-title-main">Timeline of mosasaur research</span>

This timeline of mosasaur research is a chronologically ordered list of important fossil discoveries, controversies of interpretation, and taxonomic revisions of mosasaurs, a group of giant marine lizards that lived during the Late Cretaceous Epoch. Although mosasaurs went extinct millions of years before humans evolved, humans have coexisted with mosasaur fossils for millennia. Before the development of paleontology as a formal science, these remains would have been interpreted through a mythological lens. Myths about warfare between serpentine water monsters and aerial thunderbirds told by the Native Americans of the modern western United States may have been influenced by observations of mosasaur fossils and their co-occurrence with creatures like Pteranodon and Hesperornis.

References

  1. Kenneth R. Wright; Samuel Wayne Shannon (March 31, 1988), "A new plioplatecarpine mosasaur (Squamata, Mosasauridae) from Alabama", Journal of Vertebrate Paleontology , 8 (1): 102–107, doi:10.1080/02724634.1988.10011686, JSTOR   4523177
  2. Kiernan, Caitlin R. 2002. Stratigraphic distribution and habitat segregation of mosasaurs in the Upper Cretaceous of western and central Alabama, with an historical review of Alabama mosasaur discoveries. Journal of Vertebrate Paleontology 22 (1): 91–103.
  3. 1 2 Polcyn, M. J., and Everhart, M. J., 2008, Description and phylogenetic analysis of a new species of Selmasaurus (Mosasauridae: Plioplatecarpinae) from the Niobrara Chalk of western Kansas: In: Proceedings of the Second Mosasaur Meeting, edited by Everhart, M. J, Fort Hays Studies, Special Issue number 3, p. 13-28.
  4. "Selmasaurus johnsoni2". oceansofkansas.com. Retrieved 2017-09-29.
  5. Konishi, Takuya; Michael W. Caldwell (2011). "Two new plioplatecarpine (Squamata, Mosasauridae) genera from the Upper Cretaceous of North America, and a global phylogenetic analysis of plioplatecarpines". Journal of Vertebrate Paleontology. 31 (4): 754–783. doi:10.1080/02724634.2011.579023.