Parvinatator

Parvinatator, from Latin, “parvus” little and “natator” swimmer, is an extinct genus of small ichthyopterygian marine reptile that lived during the Early to Middle Triassic. Its fossils have been found in British Columbia, Canada.[1]

Parvinatator Temporal range: Early Triassic–Middle Triassic PreꞒ Ꞓ O S D C P T J K Pg N
Restoration of its head
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Ichthyopterygia
Family:	†Parvinatatoridae McGowan and Motani, 2003
Genus:	†Parvinatator Nicholls and Brinkman, 1995
Type species
†Parvinatator wapitiensis Nicholls and Brinkman, 1995

Geological information

The only known Parvinatator fossil was located in an unknown horizon from the Sulfur Mountain Formation in a talus deposit, so its exact geological age is unknown. Best estimates place the fossil somewhere between the Olenekian and Ladinian age around 251-235 mya.[2] Other small ichthyosaurs have been found nearby including Grippia, Utatsusaurus, and Phalarodan.

Historical information / discovery

Parvinatator wapitiensis was discovered in the Sulfur Mountain Formation in British Columbia, Canada, by Elisabeth Nicholls and Don Brinkman in 1995. The only fossil recovered of this genus is a partial skull and two forefins.[1] The skull has been tectonically deformed, partially dis-articulated and broken with the posterior section rotated forward into the orbit. The left forefin is well preserved with only minor overlapping and breakage, while the right forefin is heavily overlapped and broken.[3]

Description and paleobiology

Based on analysis of tooth size relative to the skull width, Parvinatator is estimated to be less than 1 meter long,[2] approximately the same size and general shape of other basal ichthyosaurs, although it is unknown if Parvinatator lacked a dorsal fin like Utatsusaurus and Grippia. Also like other early ichthyosaurs, Parvinatator most likely used an eel-like lateral body movement for propulsion,[4] known as anguilliform. Based on jaw size and tooth shape, Parvinatator is thought to have feed on small fish and cephalopods similarly to other basal ichthyosaurs, as opposed to hard shelled organisms such as mollusks and clams.[3] Ichthyosaurs typically had large eyes suggesting they would hunt in the dark, either at night or in deep water.[5] After the skull's dis-articulated orbit was reconstructed, it matched the approximate shape and typical size of other ichthyosaurs eyes.[3]

Grippia longirostris from the early Triassic is a close relative.

Skull

The fossil of the skull is small, measuring approximately 15 cm in width by 10 cm in height. The skull had been dis-articulated, broken and tectonically deformed, but was reconstructed by Ryosuke Motani, leading to several differences from Nicholls and Brinkman's original description, including; the presence of the squamosal, quadratojugal, and a reduced supratemporal, a lack of contact between the prefrontal and postfrontal, as well as between the jugal and quadratojugal (which was previously identified as the squamosal).[3] Other diagnostic characteristics of the Parvinatator skull are as follows:

• The retroarticular process, composed of the articular, surangular, and angular, is present and well developed. This is suggested to be a derived trait for all ichthyosaurs.[1][6]
• The quadratojugal is high and narrow, laterally covering the quadrate.[7][6]
• The jugal and quadratojugal have a deep notch between them, agreeing with the lack of a lower temporal fenestra or lower zygomatic arch as in all other ichthyosaurs.[7][6]
• The postorbital is enlarged and overlaps the jugal along the posterior of the orbit, forming a natural articulation point.[8][3]
• The orbit is oval in shape, being longer than tall.[3]
• The external naris is enclosed by the premaxilla, maxilla and naris, separate from a well developed lacrimal.[1][6]

Forefin

Utatsusaurs, another close relative.

Unlike some basal ichthyosaurs, Parvinatator's forefin appears well adapted for aquatic life, although it was probably used for controlling pitch rather than locomotion.[9] On the well preserved left forefin of Parvinatator the ulna is reduced in both length and width and smaller than the radius. Several bones of digits 4 and 5 are fused together as well.[1][3]

Phylogeny

There is some disagreement about the exact location of Parvinatator among its relatives. The following phylogeny by Motani places Parvinatator with Utatsusaurus as basal ichthyopterygians but not true ichthyosaurs.[2]

Ichthyopterygia

Utatsusaurus     Parvinatator   Eoichthyosauria Grippidia   Chaohusaurus     Grippia     Ichthyosauria   Cymbospondylus       Mixosauria   Merriamosauria   Shastasauria   Euichthyosauria   Toretocnemus       Californosaurus   Parvipelvia   Macgowania       Hudsonelpidia       Suevoleviathan       Eurhinosauria     Temnodontosaurus   Thunnosauria   Stenopterygius       Ichthyosaurus     Ophthalmosauria

However, according to Maisch and Matzke in 2000, Parvinatator is a true ichthyosaur and more derived than Utatsusaurus and Grippia.[10]

Ichthyosauria	Thaisaurus       Utatsusaurus      Grippidia    Chaohusaurus     Grippia         Parvinatator       Quasianosteosaurus    Hueneosauria    Mixosauria    Longipinnati    Toretocnemidae       Cymbospondylidae     Merriamosauria

See also

• List of ichthyosaurs
• Timeline of ichthyosaur research

References

1. Nicholls, E. & Brinkman, D. (1995). A new ichthyosaur from the Triassic Sulphur Mountain formation of British Columbia. Sarjeant WAS (ed.): Vertebrate fossils and the evolution of scientific concepts: 521–535 London (Gordon & Breach).
2. Motani, R. (1999). Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology, 19:3, 473-496.
3. Motani, R. (1997). Phylogeny of the Ichthyosauria (Amniota: Reptilia) with special reference to Triassic forms. Ph.D. dissertation, University of Toronto, Toronto, Ontario, Canada.
4. Motani, R., You, H., & Mcgowan, C. (1996). Eel-like swimming in the earliest ichthyosaurs. Nature, 382, 347-348.
5. Motani, R.; Rothschild, B.M.; Wahl, W. (1999). "Large eyes in deep diving ichthyosaurs". Nature. 402: 747.
6. Maisch, M. (2010). Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity, 3, 151–214.
7. Maisch, M.W. & Matzke, A.T. (2002). The skull of a large Lower Triassic ichthyosaur from Spitzbergen and its implications for the origin of the Ichthyosauria. Lethaia, 35, 250–256.
8. Cuthbertson, R. S., Russell, A. P., & Anderson, J. S. (2013). Cranial morphology and relationships of a new grippidian (Ichthyopterygia) from the Vega-Phroso Siltstone Member (Lower Triassic) of British Columbia, Canada. Journal of Vertebrate Paleontology, 33, 831-847.
9. McGowan, C. (1973). Differential growth in three ichthyosaurs: Ichthyosaurus communis, I. breviceps, and Stenopterygius quadriscissus (Reptilia, Ichthyosauria). Life Sciences Contributions Royal Ontario Museum. 93: 1–21.
10. Maisch, M.W.; Matzke, A.T. (2000). "The Ichthyosauria". Stuttgarter Beitrage zur Naturkunde, Serie B (Geologie und Palaontologie). 298: 1–159.