Champsosaurus
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''Champsosaurus'' is an
extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of crocodile-like
choristodere Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the late Miocene (168 to ...
reptile, known from the Late
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
and early
Paleogene The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...
periods of North America and Europe (
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...
-
Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
). The name ''Champsosaurus'' is thought to come from , () said in an Ancient Greek source to be an Egyptian word for "crocodiles", and , () Greek for "lizard". The morphology of ''Champsosaurus'' resembles that of
gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct b ...
s, with a long, elongated snout. It was native to freshwater environments where it likely preyed on fish, similar to living gharials.


History of research

''Champsosaurus'' was the first member of the
Choristodera Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the late Miocene (168 to 1 ...
to be described. ''Champsosaurus'' was named by
Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested ...
in 1876, from isolated vertebrae found in Late Cretaceous strata of the
Judith River Formation The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Judith River Group. It dates to the Late Cretaceous, between 79 and 75.3 million years ago, corresponding to the "Judithian" land vertebrate age. It ...
on the banks of the
Judith River The Judith River is a tributary of the Missouri River, approximately 124 mi (200 km) long, running through central Montana in the United States. It rises in the Little Belt Mountains and flows northeast past Utica and Hobson. It is ...
in Fergus County,
Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...
. Cope designated ''C. annectens'' as the
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
rather than the first named ''C. profundus'' due to the larger number of vertebrae he attributed to the species. ''C. annectens'' was based on 9 isolated vertebral centra (AMNH FR 5696) that were not figured in the paper of which two are now lost. Cope named several other species between 1876 and 1882, also based on isolated vertebrae.
Barnum Brown Barnum Brown (February 12, 1873 – February 5, 1963), commonly referred to as Mr. Bones, was an American paleontologist. Named after the circus showman P. T. Barnum, he discovered the first documented remains of ''Tyrannosaurus'' during a career ...
in 1905 described the first complete remains of ''Champsosaurus'', and noted that one of the species attributed to ''Champsosaurus'' by Cope in 1876, ''C. vaccinsulensis'' actually represented indeterminate
plesiosaur The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared ...
remains, and that the vertebrae that Cope used to diagnose his species of ''Champsosaurus'' were heavily eroded and the diagnostic features varied substantially along the spinal column, and were not diagnostic to species level, including the remains that Cope attributed to the type species ''C. annectens''.B. Brown. 1905
The osteology of Champsosaurus Cope
''American Museum of Natural History, Memoirs'' 9:1–264
The conclusion that ''C. annectens'' was undiagnostic was supported by William Parks in 1933.W. A. Parks. 1933. New species of Champsosaurus from the Belly River Formation of Alberta, Canada. ''Transactions of Royal Society of Canada'' 27:121–137 Brown in 1905 named two species of ''Champsosaurus.'' One was ''C. ambulator'', named from the specimen AMNH 983, a fragmentary skeleton with a partial skull found in the
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...
of Montana. The other was ''C. laramiensis'', named from AMNH 982, a nearly complete skeleton and skull, also found in the Hell Creek Formation. Parks in 1927 named ''C. albertensis'' from ROM 806, a partial skeleton lacking the skull, found in the
Horseshoe Canyon Formation The Horseshoe Canyon Formation is a stratigraphic unit of the Western Canada Sedimentary Basin in southwestern Alberta. It takes its name from Horseshoe Canyon, an area of badlands near Drumheller. The Horseshoe Canyon Formation is part of the ...
in Alberta, Canada. Parks in 1933 named the species ''C. natator'' from an incomplete skeleton with a fragmentary skull (TMP 81.47.1) found in the
Belly River Group The Belly River Group is a stratigraphical unit of Late Cretaceous age in the Western Canadian Sedimentary Basin. It takes the name from the Belly River, a tributary of the Oldman River in southern Alberta, and was first described in outcrop o ...
in the
Red Deer River The Red Deer River is a river in Alberta and a small portion of Saskatchewan, Canada. It is a major tributary of the South Saskatchewan River and is part of the larger Saskatchewan-Nelson system that empties into Hudson Bay. Red Deer River ...
valley in Alberta. In 1979,
Denise Sigogneau-Russell Denise Sigogneau-Russell (born ''c.'' 1941/42) is a French palaeontologist who specialises in mammals from the Mesozoic, particularly from France and the UK. She is currently based at the Muséum National d'Histoire Naturelle. Background Denise S ...
named the species ''C. dolloi'' from remains found in the Paleocene of Belgium. In 1972, Bruce Erickson named the species ''C. gigas'' from SMM P71.2.1, a partial skeleton and skull found in the
Sentinel Butte Formation The Sentinel Butte Formation is a geologic formation of Paleocene age in the Williston Basin of western North Dakota. It preserves significant assemblages of non-marine plant and animal fossil A fossil (from Classical Latin , ) is any pr ...
,
Golden Valley County Golden Valley County is the name of two counties in the United States: *Golden Valley County, Montana *Golden Valley County, North Dakota Golden Valley County is a county in the U.S. state of North Dakota. As of the 2020 census, the population ...
, North Dakota. Erickson subsequently in 1981 named the species ''C. tenuis'' from SMM P79.14.1, a partial skeleton and skull found in the Bullion Creek Formation, North Dakota. In 1998 K. Q. Gao and Richard Carr Fox described the species ''C. lindoei'' from UALVP 931, a nearly complete skeleton with skull and jaws from the
Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 76. ...
in Alberta. The publication also thoroughly reviewed ''Champsosaurus'', rediagnosing most species except for ''C. ambulator'' and ''C. laramiensis''. Fossils of ''Champsosaurus'' have been found in
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
(
Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...
,
Saskatchewan Saskatchewan ( ; ) is a Provinces and territories of Canada, province in Western Canada, western Canada, bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and on t ...
, Montana,
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...
,
Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...
,
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...
, and
Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
) and
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
(
Belgium Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in Northwestern Europe. The country is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeast, France to th ...
and
France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
), dating from the
Upper Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
to the late
Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
. Remains tenatively referred to ''Champsosaurus'' are known from the high
Canadian Arctic Northern Canada, colloquially the North or the Territories, is the vast northernmost region of Canada variously defined by geography and politics. Politically, the term refers to the three territories of Canada: Yukon, Northwest Territories and N ...
, dating to the
Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by t ...
Turonian The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stage in the Upper Cretaceous Series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded by t ...
, a time of extreme warmth.


Taxonomy

Sixteen species of ''Champsosaurus'' have been named, of which seven are presently considered valid. The
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
''Champsosaurus annectens'' Cope, 1876 is considered to be
dubious Doubt is a mental state in which the mind remains suspended between two or more contradictory propositions, unable to be certain of any of them. Doubt on an emotional level is indecision between belief and disbelief. It may involve uncertainty, ...
. The only named European species ''C. dolloi'' Sigogneau-Russell 1979 was considered to be too fragmentary to warrant a new species by Gao and Fox in 1998.


Description

Most species grew to about 1.50 m (5 ft) long,D.Lambert, D.Naish and E.Wyse 2001, "Encyclopedia of Dinosaurs and prehistoric life", p. 77, Dorling Kindersley Limited, London. though ''Champsosaurus gigas'', the largest species, reached 3–3.5 m (10–12 ft) in length.


Anatomy

The skull of ''Champsosaurus'' is
dorsoventrally Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
flattened, while the temporal arches are expanded posteriorly (towards the back of the skull) and laterally (away from the midline), giving the skull a heart shaped appearance when viewed from above. The snout is greatly elongated and
gharial The gharial (''Gavialis gangeticus''), also known as gavial or fish-eating crocodile, is a crocodilian in the family Gavialidae and among the longest of all living crocodilians. Mature females are long, and males . Adult males have a distinct b ...
-like, making up around half the length of the skull, and at least four times as long as it is wide, with the opening of the nostrils at the end of the snout. The openings of the ears are located on the underside of the skull. The body is flat and streamlined, with heavy
gastralia Gastralia (singular gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods. They are found between the sternum and pelvis, and do not articulate with the vertebrae. In these ...
(rib-like bones situated in the belly). Compared to other choristoderes, the
lacrimal bone The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...
is reduced in size to a small triangle, the
postorbital bone The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ve ...
does not form part of the orbit (eye socket), there is no contact between the
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...
and the
vomer The vomer (; lat, vomer, lit=ploughshare) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxill ...
bones, an internarial bone is present, the
choana The choanae (singular choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the throat in tetrapods, including humans and other mammals (as well as crocodilia ...
e are located posteriorly in correlation to the elongation of the vomer, the interpterygoid vacuity is small and completely enclosed by the
pterygoid bone The pterygoid is a paired bone forming part of the palate of many vertebrates, behind the palatine bone In anatomy, the palatine bones () are two irregular bones of the facial skeleton in many animal species, located above the uvula in the th ...
s and located near the posterior margin of the suborbital fenestra, the shape of the suborbital fenestra is shortened and kidney like, the articulation between the pterygoid and the
parasphenoid The parasphenoid is a bone which can be found in the cranium of many vertebrates. It is an unpaired dermal bone which lies at the midline of the roof of the mouth. In many reptiles (including birds), it fuses to the endochondral (cartilage-derived) ...
is fused, the joint between the skull and the lower jaws is anterior to level of the
occipital condyles The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra. The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...
, the neomorphic bone forms most of the border of the posttemporal fenestra, the paroccipital process is strongly deflected downwards, the basal tubera of the basisphenoid are wing-like in shape and expanded backwards and downwards, the
mandibular symphysis In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral halves ...
(connection of the two halves of the lower jaw) is elongated to over half the length of the tooth row, and the
splenial The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side (closest to the tongue) between the angular and surangular The suprangular or surangular is a jaw bone found in most land verteb ...
bone strongly intervenes in the mandibular symphysis.


Internal cranial anatomy

The
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, or brain-pan is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calvaria or skul ...
of ''Champsosaurus'' is poorly ossified at the front of the skull (anterior), but is well ossified in the rear (posterior) similar to other diapsids. The cranial endocast (space occupied by the brain in the
cranial vault The cranial vault is the space in the skull within the neurocranium, occupied by the brain. Development In humans, the cranial vault is imperfectly composed in newborns, to allow the large human head to pass through the birth canal. During bir ...
) is similar to that of basal
archosauromorphs Archosauromorpha (Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) rather than lepidosaurs (such as tuataras, liz ...
, being proportionally narrow in both dorsoventral and lateral axes, with an enlarged
pineal body The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycle ...
and
olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a grey matter, neural structure of the vertebrate forebrain involved in olfaction, the sense of odor, smell. It sends olfactory information to be further processed in the amygdala, the orbitof ...
s. The optic lobes and flocculi are small in size, indicating only average vision ability at best. The olfactory chambers of the nasal passages and olfactory stalks of the braincase are reasonably large, indicating that ''Champsosaurus'' probably had good
olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...
capabilities (sense of smell). The nasal passages lack bony
turbinates In anatomy, a nasal concha (), plural conchae (), also called a nasal turbinate or turbinal, is a long, narrow, curled shelf of bone tissue, bone that protrudes into the breathing passage of the nose in humans and various animals. The conchae ar ...
. The
semicircular canals The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The ...
are most similar to those of other aquatic reptiles. The expansion of the sacculus indicates that ''Champsosaurus'' likely had an increased sensitivity to low frequency sounds and vibrations. The absence of an
otic notch Otic notches are invaginations in the posterior margin of the skull roof, one behind each orbit. Otic notches are one of the features lost in the evolution of amniotes from their tetrapod ancestors. The notches have been interpreted as part of an ...
indicates that ''Champsosaurus'' lacked a tympanum, and probably had a poor ability to detect airborne sounds.


Teeth

''Champsosaurus'', like many of its fellow neochoristoderes, features teeth with striated enamel of the tooth crown with enamel infolding at the base. Anterior teeth are typically sharper and more slender than posterior teeth. Like other choristoderes, ''Champsosaurus'' possessed palatal teeth (teeth present on the bones of the roof of the mouth), with longitudinal rows present on the pterygoid,
palatine A palatine or palatinus (in Latin; plural ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman times.
and vomer, alongside a small row on the flange of the pterygoid. The palatine teeth of ''Champsosaurus'' are located on raised platforms of bone, though the wideness of the platforms, the sharpness and orientation of teeth vary between species. The orientation of the teeth varies in the jaw, with the posterior teeth being orientated backward. The palatal teeth, likely in combination with a fleshy tongue probably aided in gripping and swallowing prey.


Skin

Skin impressions of ''Champsosaurus'' have been reported. They consist of small (0.6-0.1 mm) pustulate and rhomboid scales, with the largest scales being located on the lateral sides of the body, decreasing in size dorsally, no
osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amp ...
s were present.


Classification

''Champsosaurus'' belongs to the
Neochoristodera Neochoristodera is a lineage of specialised crocodile-like fully aquatic choristodere reptiles. Noted for their long jaws and large size, these animals were predominant across the Northern Hemisphere, occurring in freshwater and coastal environme ...
, a clade within Choristodera, the members of which are characterised by elongated snouts and expanded temporal arches. The group first appeared during the Early Cretaceous in Asia, and are suggested to have evolved in the regional absence of aquatic crocodyliformes. While Neochoristodera is a well supported grouping, the relationships of the members of the group to each other are uncertain, with the clade having been recovered as a
polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...
in recent analyses. Phylogeny of Choristodera after Dong and colleagues (2020).


Paleobiology

''Champsosaurus'' is thought to have been highly specialised for aquatic life. Erickson 1985 suggested that the expanded temporal arches, which likely anchored powerful jaw muscles, and elongated snout allowed ''Champsosaurus'' to prey on fish akin to modern gharials, with these adaptions allowing rapid movement of the head and jaws for prey capture. A study in 2021 found that the middle and posteror neck vertebrae of ''Champsosaurus'' were adapted for lateral movement, and that ''Champsosaurus'' may have fed by laterally sweeping its head, using its slender jaws to grab individual fish from shoals, akin to how modern gharials feed. The mechanism of head movement is different from that of gharials, where the lateral movement occurs at the head-neck joint. It is unlikely that ''Champsosaurus'' fed by intertial feeding (where the prey is temporarily let go and the head moved forwards in order to force the prey deeper into the throat), but that the prey was moved down the throat by the tongue in combination with the palatal dentition. Erickston 1985 proposed that the position of the nostrils at the front of the snout allowed ''Champsosaurus'' to spend large amounts of time at the bottom of water bodies, with the head being angled upwards to allow the snout to act like a snorkel when the animal needed to breathe. However, later studies suggested that the neck vertebrae of ''Champsosaurus'' only had a limited ability to flex upwards. ''Champsosaurus'' co-existed with similarly sized aquatic
crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
ns and at some Paleocene localities with fellow neochoristodere ''
Simoedosaurus ''Simoedosaurus'' is an extinct reptile known from the Paleocene of North America, Europe and western Asia, and a member of the Choristodera, a group of aquatic reptiles that lived in the Northern Hemisphere from the Jurassic to the early Cenoz ...
'', though in assemblages where ''Champsosaurus'' occurs longirostrine (long snouted) gharial-like crocodilians are absent, suggesting that there was
niche differentiation In ecology, niche differentiation (also known as niche segregation, niche separation and niche partitioning) refers to the process by which competing species use the environment differently in a way that helps them to coexist. The competitive excl ...
. Previously, two species of ''Champsosaurus'' were identified from the
Tullock Formation The Tullock Formation is a Formation (geology), geologic formation in Montana. It preserves fossils dating back to the Paleogene Period (geology), period. See also * List of fossiliferous stratigraphic units in Montana * Paleontology in Montana ...
in
Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...
. However, these differences are now thought to be
sexually dimorphic Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
, with presumed females possessing robust limb bones. Non-deformation related fusion of the sacral vertebrae is also observed in specimens with robust limb bones. These are hypothesised to be related to breeding behaviour, with the more robust limb bones and fused sacrals of the females allowing them to move themselves onto land to lay eggs.


See also

*
Tullock Formation The Tullock Formation is a Formation (geology), geologic formation in Montana. It preserves fossils dating back to the Paleogene Period (geology), period. See also * List of fossiliferous stratigraphic units in Montana * Paleontology in Montana ...
*
Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The formation stretches over portions of ...
*
Paleontology in Montana Paleontology in Montana refers to paleontological research occurring within or conducted by people from the U.S. state of Montana. The fossil record in Montana stretches all the way back to the Precambrian. During the Late Precambrian, western Mo ...


References

{{Taxonbar, from=Q860015 Paleontology in Alberta Fossils of Canada Choristodera Prehistoric marine reptiles Prehistoric reptile genera Maastrichtian genera Campanian genus first appearances Late Cretaceous reptiles of Europe Paleocene reptiles of Europe Late Cretaceous reptiles of North America Paleocene reptiles of North America Taxa named by Edward Drinker Cope Fossil taxa described in 1877