Thalattosaur
Encyclopedia
Thalattosaurs are a group of prehistoric marine reptile
s which lived during the mid-late Triassic
Period. Some species of thalattosaur grew to over 4 meters (13 feet) in length, including a long, flattened tail used in underwater propulsion. While they bore a superficial resemblance to lizard
s, the exact relationships of thalattosaurs is unknown; they are widely accepted as diapsid
s, but experts have variously placed them on the reptile family tree near ichthyosaur
s, archosaur
s, or Lepidosauromorpha
(lizards and their relatives), or as basal
neodiapsid
s.
Thalattosaurs have moderate adaptations to marine lifestyles, including long, paddle-like tails and slender bodies. Primitive features of thalattosaurs include the many teeth covering the palate
, single-headed ribs, and broad, plate-like coracoid bones. The most unusual features of thalattosaurs are their snouts. Members of one group, the Askeptosauroidea, have long, narrow, pointed skulls. The extended rostra
at the end of the skull push back the nostrils so that they are positioned closer to the eye sockets. Most have small teeth but one askeptosauroid, Endennasaurus
, is entirely toothless. Members of a second group, Thalattosauroidea, have more distinctive downturned snouts. Clarazia and Thalattosaurus
both have snouts that taper into a narrow tip. Most of the snout is straight, but premaxilla
e at the tip are downturned. Xinpusaurus also has downturned premaxilla
e, but the end of the maxilla
e are sharply upturned, forming a notch in the skull. In Hescheleria, Nectosaurus
, and Paralonectes, the premaxillae are abruptly downturned at the end of the snout, forming nearly a right angle with the rest of the jaw. In these forms, the end of the snout is a toothy hook separated from the rest of the jaw by a space called a diastema
.
) were named by American paleontologist John C. Merriam
in 1904 for a distinctive group of marine reptiles, at the time containing only the first known species, Thalattosaurus
. Merriam mentioned a jaw that was found from a Triassic deposit in California
and noted similarities between it and the ichthyosaur
Mixosaurus
, assigning the material to the genus. More remains were uncovered from Shasta County
in 1902 and 1903 by the University of California
. These specimens were found in marine limestone
s alongside the bones of ichthyosaurs, fish scales, and mollusk shells. The holotype
specimen of Thalattosaurus, the basis of Merriam's description of thalattosaurs, was found in four broken pieces separated by veins of rock. The rock was removed during preparation. In restoring the skull, Merriam considered the snout to be straight, like those of other reptiles. Round bone fragments near the limbs of Thalattosaurus were interpreted as phalanges, and the entire limb was considered to be a paddle. Later specimens with preserved limbs revealed that thalattosaurs retained functional digits.
, Nevada
, and British Columbia
. They are also present in Europe, with remains having been found from Switzerland
, Austria
, and Italy
. More recently, thalattosaur fossils have been found in China
.
A thalattosaur fossil was found on the shoreline of a southeast Alaska
n island in May, 2011. It included an exposed part of the tail found in calcareous shale in the intertidal zone
. Several blocks containing the rest of the skeleton were sawed from the surrounding rock. Impressions of the bivalve Halobia was found around the skeleton. The thalattosaur lived in tropical waters surrounding a volcanic atoll
that existed in the area during the Norian
stage of the Late Triassic
. A second thalattosaur fossil was found on the shoreline of south-central Alaska
near Anchorage in late July, 2011.
s. He compared them to Rhynchocephalia, which includes the living tuatara
. The classification of thalattosaurs has varied from study to study, especially since some paleontologists began including the askeptosaurid
s as true thalattosaurs. While some authors have placed the traditional thalattosaurs and the askeptosaurids within an expanded Order Thalattosauria, some cladistic definitions (see Phylogeny below) reserve the name Thalattosauria for a clade
comprising the traditional thalattosaurs, and name the larger thalattosaur-askeptosaurid group Thalattosauriformes. In studies that use a more inclusive Thalattosauria, the name Thalattosauroidea has been used for the more exclusive thalattosaur group.
is paraphyletic, as it includes the direct ancestors of the family Claraziidae.
Thalattosauriformes for the larger group. The phylogeny presented here again follows Haaramo (2004) and Wu et al. (2009).
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...
s which lived during the mid-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...
Period. Some species of thalattosaur grew to over 4 meters (13 feet) in length, including a long, flattened tail used in underwater propulsion. While they bore a superficial resemblance to lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s, the exact relationships of thalattosaurs is unknown; they are widely accepted as diapsid
Diapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...
s, but experts have variously placed them on the reptile family tree near ichthyosaur
Ichthyosaur
Ichthyosaurs were giant marine reptiles that resembled fish and dolphins...
s, archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s, or Lepidosauromorpha
Lepidosauromorpha
Lepidosauromorpha is a group of reptiles comprising all diapsids closer to lizards than to archosaurs . The only living sub-group is the Lepidosauria: extant lizards, snakes, and tuatara...
(lizards and their relatives), or as basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
neodiapsid
Neodiapsida
Neodiapsida is a clade or major branch of the reptilian family tree, that includes all Diapsids apart from some early primitive types known as Araeoscelidia....
s.
Description
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
, single-headed ribs, and broad, plate-like coracoid bones. The most unusual features of thalattosaurs are their snouts. Members of one group, the Askeptosauroidea, have long, narrow, pointed skulls. The extended rostra
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
at the end of the skull push back the nostrils so that they are positioned closer to the eye sockets. Most have small teeth but one askeptosauroid, Endennasaurus
Endennasaurus
Endennasaurus is an extinct genus of thalattosaurian from the Upper Triassic of Italy....
, is entirely toothless. Members of a second group, Thalattosauroidea, have more distinctive downturned snouts. Clarazia and Thalattosaurus
Thalattosaurus
Thalattosaurus is an extinct genus of marine reptile in the family Thalattosauridae that lived during the Triassic period. It includes two known species.Thalattosaurus was a shellfish-eating reptile around 2 meters in length....
both have snouts that taper into a narrow tip. Most of the snout is straight, but premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
e at the tip are downturned. Xinpusaurus also has downturned premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
e, but the end of the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
e are sharply upturned, forming a notch in the skull. In Hescheleria, Nectosaurus
Nectosaurus
Nectosaurus is a genus of marine diapsid reptile which lived during the Late Triassic of what is now California. The type species is N. halinus, described by John C. Merriam in 1905. A 2002 analysis of Nectosaurus classifies it as a thalattosaurian, one of a group of marine reptiles which lived...
, and Paralonectes, the premaxillae are abruptly downturned at the end of the snout, forming nearly a right angle with the rest of the jaw. In these forms, the end of the snout is a toothy hook separated from the rest of the jaw by a space called a diastema
Diastema (dentistry)
Diastema is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars.-In humans:...
.
History
The Thalattosauria (and family ThalattosauridaeThalattosauridae
Thalattosauridae is a family of extinct marine diapsid reptiles which lived along the Pacific coast of Late Triassic North America...
) were named by American paleontologist John C. Merriam
John C. Merriam
John Campbell Merriam was an American paleontologist. The first vertebrate paleontologist on the West Coast of the United States, he is best known for his taxonomy of vertebrate fossils at the La Brea Tar Pits in Los Angeles, particularly with the genus Smilodon, more commonly known as the...
in 1904 for a distinctive group of marine reptiles, at the time containing only the first known species, Thalattosaurus
Thalattosaurus
Thalattosaurus is an extinct genus of marine reptile in the family Thalattosauridae that lived during the Triassic period. It includes two known species.Thalattosaurus was a shellfish-eating reptile around 2 meters in length....
. Merriam mentioned a jaw that was found from a Triassic deposit in 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...
and noted similarities between it and the ichthyosaur
Ichthyosaur
Ichthyosaurs were giant marine reptiles that resembled fish and dolphins...
Mixosaurus
Mixosaurus
Mixosaurus is an extinct genus of ichthyosaur. Fossils of Mixosaurus have been found all over the world - China, Timor, Indonesia, Italy, Spitsbergen, Svalbard, Canada, Alaska, and Nevada. It was named in 1887 by George H. Baur...
, assigning the material to the genus. More remains were uncovered from Shasta County
Shasta County, California
Shasta County is a county located in the northern portion of the U.S. state of California. The county occupies the northern reaches of the Sacramento Valley, with portions extending into the southern reaches of the Cascade Range. As of the 2010 census, the population was 177,223, up from 163,256...
in 1902 and 1903 by the University of California
University of California
The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...
. These specimens were found in marine limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s alongside the bones of ichthyosaurs, fish scales, and mollusk shells. The holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
specimen of Thalattosaurus, the basis of Merriam's description of thalattosaurs, was found in four broken pieces separated by veins of rock. The rock was removed during preparation. In restoring the skull, Merriam considered the snout to be straight, like those of other reptiles. Round bone fragments near the limbs of Thalattosaurus were interpreted as phalanges, and the entire limb was considered to be a paddle. Later specimens with preserved limbs revealed that thalattosaurs retained functional digits.
Distribution
Thalattosaur fossils have been found in California, OregonOregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
, Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
, and 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...
. They are also present in Europe, with remains having been found from Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
, Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
, and Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
. More recently, thalattosaur fossils have been found in 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...
.
A thalattosaur fossil was found on the shoreline of a southeast Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
n island in May, 2011. It included an exposed part of the tail found in calcareous shale in the intertidal zone
Intertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...
. Several blocks containing the rest of the skeleton were sawed from the surrounding rock. Impressions of the bivalve Halobia was found around the skeleton. The thalattosaur lived in tropical waters surrounding a volcanic atoll
Atoll
An atoll is a coral island that encircles a lagoon partially or completely.- Usage :The word atoll comes from the Dhivehi word atholhu OED...
that existed in the area during the 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...
stage of the Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...
. A second thalattosaur fossil was found on the shoreline of south-central Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
near Anchorage in late July, 2011.
Systematics
Thalattosaurs were first classified by Merriam as diaptosaurs, or primitive diapsidDiapsid
Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...
s. He compared them to Rhynchocephalia, which includes the living tuatara
Tuatara
The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common...
. The classification of thalattosaurs has varied from study to study, especially since some paleontologists began including the askeptosaurid
Askeptosauridae
Askeptosauridae is a family of thalattosaurs within the superfamily Askeptosauroidea. Fossils have been found from Italy, Switzerland, and China. Askeptosaurids are distinguished from other thalattosaurs by their long necks and narrow skulls.-Classification:...
s as true thalattosaurs. While some authors have placed the traditional thalattosaurs and the askeptosaurids within an expanded Order Thalattosauria, some cladistic definitions (see Phylogeny below) reserve the name Thalattosauria for a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
comprising the traditional thalattosaurs, and name the larger thalattosaur-askeptosaurid group Thalattosauriformes. In studies that use a more inclusive Thalattosauria, the name Thalattosauroidea has been used for the more exclusive thalattosaur group.
Taxonomy
The taxonomy presented here includes genera listed in an online survey by Mikko Haaramo, with additional taxa added based on their referral in subsequent papers. Thalattosauria is used in its most inclusive sense after Cheng et al., 2007. Note that the family ThalattosauridaeThalattosauridae
Thalattosauridae is a family of extinct marine diapsid reptiles which lived along the Pacific coast of Late Triassic North America...
is paraphyletic, as it includes the direct ancestors of the family Claraziidae.
- Order Thalattosauria
- Superfamily AskeptosauroideaAskeptosauroideaAskeptosauroidea is a superfamily of thalattosaurs, a Triassic group of marine reptiles. Askeptosauroidea is one of two major subgroups of Thalattosauria, the other being Thalattosauroidea...
- Family AskeptosauridaeAskeptosauridaeAskeptosauridae is a family of thalattosaurs within the superfamily Askeptosauroidea. Fossils have been found from Italy, Switzerland, and China. Askeptosaurids are distinguished from other thalattosaurs by their long necks and narrow skulls.-Classification:...
- Genus AnshunsaurusAnshunsaurusAnshunsaurus is a genus of thalattosaurs within the family Askeptosauridae. Fossils have been found from Middle Triassic deposits in Guizhou, China. Three species are known: the type species A. huangguoshuensis , the slightly older species A. wushaensis , and the species A...
- Genus AskeptosaurusAskeptosaurusAskeptosaurus is an extinct genus of aquatic reptile related to the thalattosaurian group. Their remains have been found in Italy and Switzerland.Askeptosaurus was a very thin, elongated creature, that probably swam like an eel...
- Genus MiodentosaurusMiodentosaurusMiodentosaurus is a genus of thalattosaurian from the Late Triassic of China.-Sources:*...
- Genus Anshunsaurus
- Family Endennasauridae
- Genus EndennasaurusEndennasaurusEndennasaurus is an extinct genus of thalattosaurian from the Upper Triassic of Italy....
- Genus Endennasaurus
- Family Askeptosauridae
- Superfamily ThalattosauroideaThalattosauroideaThalattosauroidea is a superfamily of thalattosaurs, a Triassic group of marine reptiles. It was named in 1904 by paleontologist John Campbell Merriam to include the genus Thalattosaurus from California. Thalattosauroids are one of two groups of Thalattosauria, the other being Askeptosauroidea...
- Genus NectosaurusNectosaurusNectosaurus is a genus of marine diapsid reptile which lived during the Late Triassic of what is now California. The type species is N. halinus, described by John C. Merriam in 1905. A 2002 analysis of Nectosaurus classifies it as a thalattosaurian, one of a group of marine reptiles which lived...
- Genus Xinpusaurus
- Family Claraziidae
- Genus Clarazia
- Genus Hescheleria
- Family ThalattosauridaeThalattosauridaeThalattosauridae is a family of extinct marine diapsid reptiles which lived along the Pacific coast of Late Triassic North America...
*- Genus Agkistrognathus
- Genus Paralonectes
- Genus ThalattosaurusThalattosaurusThalattosaurus is an extinct genus of marine reptile in the family Thalattosauridae that lived during the Triassic period. It includes two known species.Thalattosaurus was a shellfish-eating reptile around 2 meters in length....
- Genus Nectosaurus
- Superfamily Askeptosauroidea
Phylogeny
While Thalattosauria has traditionally been used for the largest group containing all families of "thalattosaurs", several authors have recently used this name for what would otherwise be known as Thalattosauroidea, and used the cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
Thalattosauriformes for the larger group. The phylogeny presented here again follows Haaramo (2004) and Wu et al. (2009).