Shastasaurus
Encyclopedia
Shastasaurus is an extinct genus
of ichthyosaur
from the middle and late Triassic
, and is the largest marine reptile
that has yet been found. Specimens have been reported from the United States
, Canada
, and China
. It has been interpreted as a specialised suction-feeder on unshelled cephalopods and fish.
period. S. sikanniensis is known from the Pardonet Formation British Columbia
, dating to the middle Norian
age (about 210 million years ago). The largest species, S. sikkanniensis, measured 21 metres (68.9 ft).
Shastasaurus was highly specialized, and differed considerably from other ichthyosaurs. It was very slender in profile. The largest specimens had a ribcage slightly less than 2 metres (6.6 ft) deep despite a distance of over 7 metres (23 ft) between its flippers.
Its skull bore an unusually short, toothless snout compared to the long, toothed, dolphin-like snouts of most ichthyosaurs. It is likely that this strange snout indicates that Shastasaurus was a suction feeder, feeding primarily on soft-bodied cephalopods which lacked shells. In S. liangae, the only species with several well preserved skulls, the skull measures only 8.3% of the total body length (9.3% in a juvenile specimen). Unlike the related Shonisaurus, even juvenile Shastasaurus completely lacked teeth. The snout was highly compressed via a unique arrangement of skull bones. Unlike almost all other reptiles, the nasal bone, which usually forms the mid part of the skull, extended to the very tip of the snout, and all bones of the snout tapered to abrupt points.
Shastasaurus was also traditionally depicted with a dorsal fin, a feature found in more advanced ichthyosaurs. However, other shastasaurids likely lacked dorsal fins, and there is no evidence to support the presence of such a fin in any species. The upper fluke of the tail was probably also much less developed than the shark-like tails found in later species.
of Shastasaurus is S. pacificus, from the late Carnian
of northern California
. It is known only from fragmentary remains, which have led to the assumption that is was a 'normal' ichthyosaur in terms of proportions, especially skull proportions. Several species of long-snouted ichthyosaur were referred to Shastasaurus based on his misinterpretation, but are now placed in other genera (including Callawayia
and Guizhouichthyosaurus
).
Shastasaurus liangae was the first species to definitively show the unusual short-snouted, toothless nature of Shastasaurus. It is known from several good specimens, and was originally placed in the separate genus Guanlingsaurus. The largest specimen (YIGMR SPCV03109) measures 8.3 metres (27.2 ft) long. A juvenile specimen (YIGMR SPCV03108) has also been found, measuring 3.74 metres (12.3 ft) in length.
S. sikkanniensis was originally described in 2004 as a large species of Shonisaurus. However, this classification was not based on any phylogenetic analysis, and the authors also noted similarities with Shastasaurus. The first study testing its relationships, in 2011, supported the hypothesis that it was indeed more closely related to Shastasaurus than to Shonisaurus, and it was reclassified as Shastasaurus sikkanniensis. Specimens belonging to S. sikanniensis have been found in the Pardonet Formation British Columbia
, dating to the middle Norian
age (about 210 million years ago).
In 2009, Shang & Li reclassified the species Guizhouichthyosaurus tangae as Shastasaurus tangae. However, later analysis showed that Guizhouichthyosaurus was in fact closer to more advanced ichthyosaurs, and so cannot be considered a species of Shastasaurus.
Dubious
species that have been referred to this genus include S. carinthiacus (Huene, 1925) from the Austrian Alps and S. neubigi (Sander, 1997) from the German Muschelkalk
Synonyms of S. liangae:
Guanlingsaurus liangae Yin et al., 2000
Synonyms of S. pacificus:
Shastasaurus alexandrae Merriam, 1902
Shastasaurus osmonti Merriam, 1902
Synonyms of S. sikkanniensis:
Shonisaurus sikkanniensis Nicholls & Manabe, 2004
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of ichthyosaur
Ichthyosaur
Ichthyosaurs were giant marine reptiles that resembled fish and dolphins...
from the middle and late Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
, and is the largest marine reptile
Marine reptile
Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semi-aquatic life in a marine environment.The earliest marine reptiles arose in the Permian period during the Paleozoic era...
that has yet been found. Specimens have been reported from the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, and China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. It has been interpreted as a specialised suction-feeder on unshelled cephalopods and fish.
Description
Shastasaurus lived during the late TriassicTriassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
period. S. sikanniensis is known from the Pardonet Formation British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, dating to the middle Norian
Norian
The Norian is a division of the Triassic geological period. It has the rank of an age or stage . The Norian lasted from 216.5 ± 2.0 to 203.6 ± 1.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.-Stratigraphic definitions:The Norian was named after the Noric Alps in...
age (about 210 million years ago). The largest species, S. sikkanniensis, measured 21 metres (68.9 ft).
Shastasaurus was highly specialized, and differed considerably from other ichthyosaurs. It was very slender in profile. The largest specimens had a ribcage slightly less than 2 metres (6.6 ft) deep despite a distance of over 7 metres (23 ft) between its flippers.
Its skull bore an unusually short, toothless snout compared to the long, toothed, dolphin-like snouts of most ichthyosaurs. It is likely that this strange snout indicates that Shastasaurus was a suction feeder, feeding primarily on soft-bodied cephalopods which lacked shells. In S. liangae, the only species with several well preserved skulls, the skull measures only 8.3% of the total body length (9.3% in a juvenile specimen). Unlike the related Shonisaurus, even juvenile Shastasaurus completely lacked teeth. The snout was highly compressed via a unique arrangement of skull bones. Unlike almost all other reptiles, the nasal bone, which usually forms the mid part of the skull, extended to the very tip of the snout, and all bones of the snout tapered to abrupt points.
Shastasaurus was also traditionally depicted with a dorsal fin, a feature found in more advanced ichthyosaurs. However, other shastasaurids likely lacked dorsal fins, and there is no evidence to support the presence of such a fin in any species. The upper fluke of the tail was probably also much less developed than the shark-like tails found in later species.
Species and synonyms
The type speciesType species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
of Shastasaurus is S. pacificus, from the late Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
of northern California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. It is known only from fragmentary remains, which have led to the assumption that is was a 'normal' ichthyosaur in terms of proportions, especially skull proportions. Several species of long-snouted ichthyosaur were referred to Shastasaurus based on his misinterpretation, but are now placed in other genera (including Callawayia
Callawayia
Callawayia is an extinct genus of ichthyosaur. It contains the species Callawayia neoscapularis and C. altispinus....
and Guizhouichthyosaurus
Guizhouichthyosaurus
Guizhouichthyosaurus is a genus of Ichthyosaur which existed during the lower Carnian stage of the Late Triassic in southwest China. In contains the species Guizhouichthyosaurus tangae. It had been referred to the genus Shastasaurus in the past, but later study showed that it is likely distinct,...
).
Shastasaurus liangae was the first species to definitively show the unusual short-snouted, toothless nature of Shastasaurus. It is known from several good specimens, and was originally placed in the separate genus Guanlingsaurus. The largest specimen (YIGMR SPCV03109) measures 8.3 metres (27.2 ft) long. A juvenile specimen (YIGMR SPCV03108) has also been found, measuring 3.74 metres (12.3 ft) in length.
S. sikkanniensis was originally described in 2004 as a large species of Shonisaurus. However, this classification was not based on any phylogenetic analysis, and the authors also noted similarities with Shastasaurus. The first study testing its relationships, in 2011, supported the hypothesis that it was indeed more closely related to Shastasaurus than to Shonisaurus, and it was reclassified as Shastasaurus sikkanniensis. Specimens belonging to S. sikanniensis have been found in the Pardonet Formation British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, dating to the middle Norian
Norian
The Norian is a division of the Triassic geological period. It has the rank of an age or stage . The Norian lasted from 216.5 ± 2.0 to 203.6 ± 1.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.-Stratigraphic definitions:The Norian was named after the Noric Alps in...
age (about 210 million years ago).
In 2009, Shang & Li reclassified the species Guizhouichthyosaurus tangae as Shastasaurus tangae. However, later analysis showed that Guizhouichthyosaurus was in fact closer to more advanced ichthyosaurs, and so cannot be considered a species of Shastasaurus.
Dubious
Nomen dubium
In zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...
species that have been referred to this genus include S. carinthiacus (Huene, 1925) from the Austrian Alps and S. neubigi (Sander, 1997) from the German Muschelkalk
Muschelkalk
The Muschelkalk is a sequence of sedimentary rock strata in the geology of central and western Europe. It has a Middle Triassic age and forms the middle part of the Germanic Trias, that further consists of the Buntsandstein and Keuper...
Synonyms of S. liangae:
Guanlingsaurus liangae Yin et al., 2000
Synonyms of S. pacificus:
Shastasaurus alexandrae Merriam, 1902
Shastasaurus osmonti Merriam, 1902
Synonyms of S. sikkanniensis:
Shonisaurus sikkanniensis Nicholls & Manabe, 2004