Thalattosaurus

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Thalattosaurus
Temporal range: Late Triassic
Thalattosauridae.jpg
Thalattosaurus with its relative Nectosaurus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Subclass: Diapsida
Order: Thalattosauria
Family: Thalattosauridae
Genus: Thalattosaurus
Merriam, 1904
Species
  • T. alexandrae
  • T. borealis

Thalattosaurus (pron.:"tha-la-to-SORE-us") meaning "ocean lizard" is an extinct genus of marine reptile in the family Thalattosauridae. They are aquatic diapsids that are known exclusively from the Triassic period, and have recently gained attention as a result of studies on general diapsid phylogeny.[1]

Thalattosaurus was a shellfish-eating reptile around 2 meters (7 ft) in length occurring in the Lower and Middle of British Columbia as well as Upper Triassic of California.[2]

Although originally described as four distinct species by Merriam in 1905, one was proven to be T. alexandrae upon further inspection and another has a missing type specimen. Currently it is believed to include two known species; Thalattosauridae alexandrae and Thalattosauridae borealis.

Both species have paddle-like limbs and a down-turned rostrum.

Etymology[edit]

Thalattosaurus alexandrae was named by Merriam in 1904, Thallattosaurus meaning "ocean lizard" and alexandrae in honor of Annie Alexander, an amateur paleontologist and patron to the University of California Museum of Paleontology.[3] Thalattosaurus borealis was named after its Northern appearance, with Thalattosaurus meaning "ocean lizard" and borealis coming from boreas (Greek word, βορέας) meaning "Northern."

Thalattosauroidea (which contains Clarazia and Thalattosaurus) have a relatively short rostrum, distinct from the elongate primitive condition, with convergent lateral margins that terminate in a pointed tip. It is also characteristic of their supratemporal to contact the frontal bone, having a heavy postorbital bar, diastema present that separates the premaxillary from the maxillary teeth, and a deep lower jaw. The Thalattosauroidea are easily distinguished by their down-turned snouts. In Clarazia and Thalattosaurus, the snouts taper to a narrow tip, with the premaxilla at the tip down-turned.[4]

Geology/Environment[edit]

Range: Early/middle (Olenekian/Anisian) to Late (Carnian) Triassic in a marine; shale and marl environment. Probably feeding on shelled animals. The Sulphur Mountain Formation consists of a series of marine siltstones, silty limestones, and fine gained sandstones. All of the thalattosaur specimens from Wapiti Lake are preserved in sandstones, suggestive of shallow water conditions. Thalattosaur limbs are not paddle-like, but Thalattosaurus limbs are.

Thalattosaurus alexandrae was about six feet long and an excellent swimmer.[5] T. alexandrae probably had a long flattened tail and claws possibly used to withstand the force of the surf when crawling up on shore.[6] It is likely that they often spent their time near the shore instead of deep-sea, open-water environments.

Since all of the known fossils come from Triassic and British Columbia (or one from California) it can be assumed that they were limited in both space and time.

Taxonomy[edit]

Thalattosaurus was originally described with the type species T. alexandrae. In 1993, Nicholls and Brinkman described T. borealis from fossils found at Wapiti Lake, British Columbia.[7]

Thalattosaurus borealis

The type species "T. borealis" specimen was found in a talus slope derived from the Middle Triassic Sulphur Mountain Formation, near Wapiti Lake, British Columbia. Discovered were the anterior part of skull, incomplete mandible, centra, isolated ribs, and left pterygoid.[5]

Phylogenetic interrelationships of the better known thalattosaur genera.[8]

Thalattosaurus alexandrae

Examination of the type specimen of Thalattosaurus alexandrae shows that the premaxilla is down-curved, more than originally illustrated by Merriam in 1905. In his reconstruction of the skull, Merriam illustrated the rostrum as being straight and showed six conical, striated teeth on the premaxilla. This reconstruction has since been used in various books and published journals but is not accurate and has been corrected after Nicholls published on the subject in 1999. The premaxilla of the type specimen of T. alexandrae is distinctly curved. The three "teeth" previously illustrated are not teeth at all, but more like bony extensions of the premaxilla-"pseudodont" teeth similar to those found in T. borealis. This pseudodont dentition possibly suggests a possibility of a beak being present as in turtles and birds.[5]

When comparing this specimen to the type of T. alexandrae, the most apparent difference is in size. Thalattosaurus borealis is much smaller, with the distance from the tip of the snout to the anterior edge of the orbit being less than 60mm. In T. alexandrae, however, this distance is almost 200mm. Initially it was difference in age (juvenile vs. adult) was suspected but because the bone in the T. borealis specimen is thoroughly ossified and the caudal vertebral neural arches being fused to the centra, it can be concluded that the specimen was not a juvenile, but a fully formed adult. The vomer of T. borealis also differs from the vomer of T. alexandrae in the type of dentition present. The vomer of T. alexandrae has two rows of teeth closely set anteriorly and divergent posteriorly with 10 teeth per row. These teeth are low crowned and bulbous which are set in sockets. The vomer of T. borealis, on the other hand, has six high, triangular teeth that are fused to the bone.

In this type specimen only a single row of teeth is present but the vomer must have developed as a paired structure, so it can be assumed that there must have been more than a single row of teeth. The bone is split sagittally and it is possible that an additional row of teeth was present but has broken away during preservation. However, there is no evidence of a paired, diverging tooth row like we see in T. alexandrae. The wide, button-like teeth on the dentary are characteristic of all three, Thalattosaurus, Clarazia, and Paralonectes. The posterior mandibular teeth of T. borealis differ from these genera, however, being set flush with the margin of the jaw. In both T. alexandrae and Clarazia the posterior, bulbous teeth are set slightly ventral and medial to the jaw margin. In all thalattosaurs found, the posterior end of the dentary bifurcates into two diverging processes (upper and lower). This can be distinguished from Clarazia in which these two process are almost equal in length whereas in T. borealis the ventral process is much longer than the upper.[2]

History/Discovery[edit]

The first thalattosaurs to be described were Thalattosaurus and Nectosaurus from the Upper Triassic of California by Merriam in 1904, 1905, and 1906. This material was recently reviewed by Nicholls in 1999.[9]

In the summer of 1903 Annie Alexander led an expedition with Miss Edna Wemple, Eustace Furlong, Merriam John C, W.B. Esterly, and Mr. F.S. Ray to Shasta County where they discovered what they initially thought was Shastasaurus. After Merriam’s further studies, however, it because the type specimen of Thalattosarus alexandrae.[6]

As noted by Merriam in 1905, the skull of the holotype was preserved in four pieces, originally connected by calcite vein-filled cracks, but they were separated during preparation.[10] Three of these four pieces found made up the rostrum. When aligned, the rostrum shape suggests dorsal curvature of the anterior end of the maxilla but ventral deflection in the anterior end of premaxilla. The prefrontal, however, comes down ventrally under the margin of the maxilla and contacts the anterior tip of the suborbital process [5] A line drawn from the preserved anterior alveolar margin of the maxilla to the lower edge of the prefrontal shows that the ventral margin of the maxilla was straight, very similar to the rostral structure known of Clarazia.

One of the fossils was found in the North Fork of Squaw Creek (Triassic of the United States) in Shasta County, California. It was estimated to be from the Hosselkus Limestone Formation, Carnian (235-221.5 Ma). The environment it was found in was shelly/skeletal limestone with the horizon composed largely of broken shells. It was collected by University of California -J.C. Merriam 1906.

Another fossil was found in Wapiti Lake, locality T cirque (Triassic of Canada) British Columbia, Canada. During Sulphur Mountain Formation, Anisian (247.2-235.0 Ma). The environment was also marine; shale and marl. This specimen was collected by Royal Tyrrell Museum of Palaeontoogy field crew.

Originally four subtaxa of Thalattosaurus were classified; Thalattosaurus alexandrae, Thalattosaurus borealis, Thalattosaurus perrini, and Thalattosaurus shastensis by Merriam [10] but later additional examination of the type of T. shastensis suggested that it does not belong in the genus Thalattosaurus. It is still currently under study. The type skull of T. perrini has not been located, but the vomer figured by Merriam in 1905 did not differ from the vomer of T. alexandrae.[11]

Thalattosaurus alexandrae Merriam A more recent review of the material of Thalattosaurus and Nectosaurus at the University of California Museum of Paleontology at Berkeley (UCMP) revealed a misinterpretation of the rostral structure that has misled previous analyses [1] The original reconstruction of Thalattosaurus alexandrae by Merriam incorrectly showed a straight rostrum [10] It has since been recognized to be distinctly curved.

Description/Paleobiology[edit]

Synapomorphies of Thalattosauroidea (Clarazia, Hescheleria, Thalattosaurus) include shorter rostrum with convergent lateral margins and pointed curved tip of, a diastema separating premaxillary from maxillary teeth, posterior dentary and maxillary teeth being bulbous, anterolateral process of frontal close to the posterior of the external naris, posterolateral process of frontal touching the supratemporal, postorbital and postfrontal fused, and quadrate with anterior flange.[8]

Thalattosaurus alexandrae Late Triassic ~210 mya, up to 2.5 m in length.[10] Thalattosaurus had a down-curled rostrum and a reduced postorbital region. The naris was raised closer to the top of the skull. The maxilla was longer. The upper temporal fenestrae much smaller (slit-like) with reduced jugal and postorbital. It had thecodont dentition. The lateral temporal fenestra was narrower than the orbit with a straight mandible. The Humerus and femur both appear shorter with paddle-like limbs .

In T. alexandrae the premaxilla was toothless, beaked, with “pseudodont dentition” of bony projections. The maxilla was long and slender and the vomer was large with strong crushing teeth.

T. alexandrae differs from other thalattosaurs by having 15-16 teeth on the mandible, a recurved symphyseal region of dentary, with massive, conical teeth, posterior dentary teeth bulbous, and a high & pointed coronoid process.[9]

In T. borealis the premaxilla is hooked anteriorly, small, extending into long rostrum, with a sharp horizontal ridge along anterior tip of premaxilla separating the external striated bone from smooth bone of "pseudodont dentition". The vomerine teeth are triangular in shape, posterior mandibular teeth are set flat against edge of mandible.

The fossil found by Annie Alexander in 1903 had much of original bone in preorbital area gone, vomer was exposed, an incomplete mandible, two dorsal ribs and centra, and three articulated caudal vertebrae pressed against the vomer. As described by Merriam, the snout was long, slender, distance between orbit and naris slightly longer than the length of the oval-shaped naris which was pointed anteriorly but bluntly rounded posteriorly.[7]

Upon further examination, the other characteristics found true of this fossil were a striated external surface of bone with smooth bone resembling "pseudodont teeth". The first of which was blunt, procumbent and short, the second was pointed and thinner, the third (although the tip was broken) the thick base implied a blunt tooth. There was an additional broken stump which may suggest a fourth tooth. There was also a premaxillary boundary between third and fourth pseudodont tooth which exposed vomerine dentition by creating a gap. Posteriorly premaxilla extended back to external naris where it split into upper and lower borders, upper part of premaxilla came to a thin point. The ventral process was found to be short and stout, contacting the maxilla and forming anteroventral border of external naris. Most of the maxilla was broken away exposing the vomer, curving inward and extending under and medial to the premaxilla.

References[edit]

  1. ^ a b Muller, Johannes (2007). "First Record of a Thalattosaur from the Upper Triassic of Austria". Journal of Vertebrate Paleontology. 27 (1): 236–240. doi:10.1671/0272-4634(2007)27[236:froatf]2.0.co;2. 
  2. ^ a b Muller, Johannes (2005). "The Anatomy of Askeptosaurus Italicus from the Middle Triassic of Monte San Giorgio and the Interrelationships of Thalattosaurs (reptilia, Diapsida).". Canadian Journal of Earth Sciences. 42 (7): 1347–1367. doi:10.1139/e05-030. 
  3. ^ Williams, Rianna M (1994). Annie Montague Alexander:explorer, naturalist, philanthropist. 
  4. ^ "Thalattosauria". www.paleos.com. 
  5. ^ a b c d Nicholls, Elizabeth L. (1999). A Reexamination of Thalattosaurus and Nectosaurus and the Relationships of the Thalattosauria (reptilia: Diapsida). Berkeley, CA: University of California, Museum of Paleontology. 
  6. ^ a b Hilton, Richard (2003). Dinosaurs and Other Mesozoic Reptiles of California. California: University of California Press. pp. 80–82. 
  7. ^ a b Merriam, John C. (1904). A New Marine Reptile from the Triassic of California. Berkeley, CA: University of California Press. 
  8. ^ a b Rieppel, Olivier; Liu, Johannes; Bucher, H (2000). "The First Record of a Thalattosaur Reptile from the Late Triassic of Southern China (guizhou Provence, Pr China)". Journal of Vertebrate Paleontology. 20 (3): 507–514. doi:10.1671/0272-4634(2000)020[0507:tfroat]2.0.co;2. 
  9. ^ a b Rieppel, Olivier; Nuller, Johannes; Liu, Jun (2005). "Rostral Structure in Thalattosauria (reptilia, Diapsida)". Canadian Journal of Earth Sciences. 42 (12): 2081–2086. doi:10.1139/e05-076. 
  10. ^ a b c d Merriam, John Campbell (1905). The Thalattosauria: a group of marine reptiles from the Triassic of California. The Academy. 
  11. ^ Nicholls, Elizabeth L.; Brinkman, Donald (1993). "New Thalattosaurs (reptilia: Diapsida) from the Triassic Sulphur Mountain Formationi of Wapiti Lake, British Columbia.". Journal of Paleontology. 67 (2): 263–278. 

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