Hyphalosaurus
Encyclopedia
Hyphalosaurus is a genus
of freshwater
aquatic
reptile
, and a major part of the Jehol Biota
. It lived during the early Cretaceous
period (Aptian
age), about 122 million years ago. It contains two species, H. lingyuanensis and H. baitaigouensis, both from the Yixian Formation
of Liaoning Province, China
. It is among the best-known animals from the Jehol Biota, with thousands of fossil
specimens representing all growth stages in scientific and private collections.
, though subsequent study showed that the fossil beds it was found in probably also belong to the Yixian, though from younger rocks and a different region than H. lingyuanensis. H. lingyuanensis and H. baitaigouensis were largely similar in anatomy, both achieved a maximum adult body size of about 0.8 meters. They had small heads with numerous needle-like teeth, and extremely long tails with more than 55 vertebrae. The primary difference between the two species is the number of vertebrae in the neck. H. lingyuanensis had 19 neck vertebrae, while H. baitaigouensis had 26.
.
Hyphalosaurus was covered mostly in small, irregularly patterned polygonal scales, though these varied across the body. The scales of the hind legs were smaller, finer and more irregular than those of the torso, while the scales of the tail were nearly square and arranged in more regular rows. In addition to the small scales, two rows of large, round scutes with shallow keels ran along the animals sides. One row ran directly along the flank, with the other either slightly higher or lower and composed of scutes only 1/4 the size of the flank scutes. The flank row of larger scutes extended all the way to the base of the tail, and remained uniform in size across the entire row.
The tail itself has preserved soft tissue extending well beyond the margins of the skeleton. This, combined with the already flattened appearance of the tail vertebrae, suggests that a ridge of skin may have extended from the top and bottom of the tail creating a small fin. The feet and hands also appear to have been webbed.
.
In the holotype
specimen of H. baitaigouensis, several eggs containing embryos were preserved in and around the body. These eggs appeared to lack mineralized shells, which Ji and colleagues later interpreted as evidence that Hyphalosaurus gave birth to live young and that egg shells never fully developed inside the body of the mother. However, in 2006, Ji and colleagues re-examined the specimen specimen and noted clearly defined, though thin and leathery, shells. They agreed, however, that these eggs must have developed inside the mother which would later have given live birth (a reproductive method sometimes called ovoviviparity
), but were likely expelled from the body when the mother died. Hou and colleagues (2010) also described several eggs with flexible shells, containing H. baitaigouensis embryos. These shells were soft, and more similar to the eggs of lizards than to those of crocodiles, but nonetheless possessed a thin mineralized shell.
Another fossil specimen of H. baitaigouensis, described by Ji and colleagues in 2010, appeared to be pregnant, containing 18 fully developed embryos arranged in pairs. One of the rear-most embryos was positioned in a reverse, head-first position, a complication which may have killed the mother. This confirmed that Hyphalosaurus and other choristoderans were viviparous, the only known Mesozoic fresh-water reptiles to give birth to live young.
Both Hyphalosaurus species were aquatic, a lifestyle reflected by their long necks and tails and relatively small limbs. Superficially, they resembled miniature plesiosaur
s, though this resemblance arose convergently and does not reflect a close relationship. Hyphalosaurus was among the most aquatically adapted choristoderans, with smoother, flatter scales than its relatives, a tall and flattened tail for swimming, a long neck and webbed feet. Because the torso was fairly inflexible and the limbs were not particularly adapted for aquatic life, Hyphalosaurus probably swam using mainly its tall, flattened tail. The chest was barrel-shaped and made up of thick, heavy rib bones which would have helped Hyphalosaurus stay submerged.
Hyphalosaurus appears to have exclusively inhabited deep-water lake
s. All specimens are preserved in silt characteristic of the deepest part of the lake environment, and are often preserved alongside deep-water fish and crustaceans. Hyphalosaurus is also conspicuously absent from the aquatic sediments of the Jiufotang Formation, which preserved a more swamp
y, shallow-water ecosystem.
Hyphalosaurus is the most abundant tetrapod
(four-limbed vertebrate) in the Yixian Formation, and probably played an important role in the aquatic food chain. It's long and highly flexible neck and small, flattened skull indicates that it captured small prey animals like fish or arthropods using a sideways-strike, similar to modern aquatic predators with flattened skulls. Unlike other choristoderans, Hyphalosaurus was likely an active predator, rather than one that used a "sit and wait" ambush strategy. Its fossils are often found preserved along side the small fish Lycoptera
, which may have been a prey item, and at least one specimen preserved fish rubs as stomach contents. However, the lack of preserved stomach contents among the thousands of known specimens may indicate that they ate mainly soft-bodied prey.
The slab and counterslab of the holotype
specimen of H. lingyuanensis were given to different groups of researchers in Beijing
, one from the Institute of Vertebrate Paleontology and Paleoanthropology
and the other from the Beijing Natural History Museum. Each team described the taxon and published their results independently in January 1999, giving the animal two different names: Hyphalosaurus lingyuanensis and Sinohydrosaurus lingyuanensis. It was quickly recognized that Sinohydrosaurus and Hyphalosaurus were mirror images of one another and in fact represented different halves of the same specimen. The ICZN
, which governs the naming of animals, mandates that the older name is valid. However, in June 2001 paleontologists Joshua Smith and Jerry Harris noted that since both were published at almost exactly the same time, a third party needed to select which name would better serve as the objective senior synonym
. Smith and Harris took the opportunity to do so, selecting Hyphalosaurus as the senior synonym because the manuscript for its description had apparently been submitted (though not published) earlier. They therefore made Sinohydrosaurus a junior synonym of Hyphalosaurus.
Hyphalosaurus is related to the large, crocodile-like Champsosaurus
and the smaller, lizard-like Monjurosuchus
. Its closest relative was the similarly built species, Shokawa ikoi
, from the Early Cretaceous of Japan. The choristoderes were a clade
of aquatic reptiles that survived the end-Cretaceous
extinction along with crocodilians, turtle
s, lizard
s and snake
s. The choristoderes became extinct by the Miocene
.
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 freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
, and a major part of the Jehol Biota
Jehol Biota
The Jehol Biota includes all the living organisms - the ecosystem - of northeastern China between 133 to 120 million years ago. This is the Lower Cretaceous ecosystem which left fossils in the Yixian Formation and Jiufotang Formation. It is also believed to have left fossils in the Sinuiju series...
. It lived during the early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period (Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
age), about 122 million years ago. It contains two species, H. lingyuanensis and H. baitaigouensis, both from the Yixian Formation
Yixian Formation
The Yixian Formation is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period...
of Liaoning Province, 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 is among the best-known animals from the Jehol Biota, with thousands of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
specimens representing all growth stages in scientific and private collections.
Description and biology
Hyphalosaurus fossils are relatively widespread in the Jehol beds, which represent a series of freshwater lakes. Several specimens of H. lingyuanensis and thousands of H. baitaigouensis specimens are known from the Yixian Formation, including entire growth series from embryos in eggs to fully grown adults. H. baitaigouensis was originally reported from the younger Jiufotang FormationJiufotang Formation
The Jiufotang Formation is an Early Cretaceous geological formation in Chaoyang, Liaoning which has yielded fossils of feathered dinosaurs, primitive birds, pterosaurs, and other organisms. . It is a member of the Jehol group. The exact age of the Jiufotang has been debated for years, with...
, though subsequent study showed that the fossil beds it was found in probably also belong to the Yixian, though from younger rocks and a different region than H. lingyuanensis. H. lingyuanensis and H. baitaigouensis were largely similar in anatomy, both achieved a maximum adult body size of about 0.8 meters. They had small heads with numerous needle-like teeth, and extremely long tails with more than 55 vertebrae. The primary difference between the two species is the number of vertebrae in the neck. H. lingyuanensis had 19 neck vertebrae, while H. baitaigouensis had 26.
Skin
Two specimens of Hyphalosaurus have been described with clear impressions of the skin. One specimen represents H. lingyuanensis, and the other (with clearer impressions) cannot be assigned to a species because part of the neck (the length of which is a key indicator of species) was destroyed when fossil dealers grafted a skull from a different specimen onto the slab. However, both specimens show nearly identical patterns of scalesScale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
.
Hyphalosaurus was covered mostly in small, irregularly patterned polygonal scales, though these varied across the body. The scales of the hind legs were smaller, finer and more irregular than those of the torso, while the scales of the tail were nearly square and arranged in more regular rows. In addition to the small scales, two rows of large, round scutes with shallow keels ran along the animals sides. One row ran directly along the flank, with the other either slightly higher or lower and composed of scutes only 1/4 the size of the flank scutes. The flank row of larger scutes extended all the way to the base of the tail, and remained uniform in size across the entire row.
The tail itself has preserved soft tissue extending well beyond the margins of the skeleton. This, combined with the already flattened appearance of the tail vertebrae, suggests that a ridge of skin may have extended from the top and bottom of the tail creating a small fin. The feet and hands also appear to have been webbed.
Reproduction
Numerous embryonic and/or newborn specimens of Hyphalosaurus have been recovered from the Yixian Formation, dating 122 million years ago. In 2007, one of these specimens was even found with two heads, the oldest known case of polycephalyPolycephaly
Polycephaly is a condition of having more than one head. The term is derived from the Greek stems poly- meaning 'much' and kephali- meaning "head", and encompasses bicephaly and dicephaly . A variation is an animal born with two faces on a single head, a condition known as diprosopus...
.
In 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 H. baitaigouensis, several eggs containing embryos were preserved in and around the body. These eggs appeared to lack mineralized shells, which Ji and colleagues later interpreted as evidence that Hyphalosaurus gave birth to live young and that egg shells never fully developed inside the body of the mother. However, in 2006, Ji and colleagues re-examined the specimen specimen and noted clearly defined, though thin and leathery, shells. They agreed, however, that these eggs must have developed inside the mother which would later have given live birth (a reproductive method sometimes called ovoviviparity
Ovoviviparity
Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...
), but were likely expelled from the body when the mother died. Hou and colleagues (2010) also described several eggs with flexible shells, containing H. baitaigouensis embryos. These shells were soft, and more similar to the eggs of lizards than to those of crocodiles, but nonetheless possessed a thin mineralized shell.
Another fossil specimen of H. baitaigouensis, described by Ji and colleagues in 2010, appeared to be pregnant, containing 18 fully developed embryos arranged in pairs. One of the rear-most embryos was positioned in a reverse, head-first position, a complication which may have killed the mother. This confirmed that Hyphalosaurus and other choristoderans were viviparous, the only known Mesozoic fresh-water reptiles to give birth to live young.
Ecology
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...
s, though this resemblance arose convergently and does not reflect a close relationship. Hyphalosaurus was among the most aquatically adapted choristoderans, with smoother, flatter scales than its relatives, a tall and flattened tail for swimming, a long neck and webbed feet. Because the torso was fairly inflexible and the limbs were not particularly adapted for aquatic life, Hyphalosaurus probably swam using mainly its tall, flattened tail. The chest was barrel-shaped and made up of thick, heavy rib bones which would have helped Hyphalosaurus stay submerged.
Hyphalosaurus appears to have exclusively inhabited deep-water lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
s. All specimens are preserved in silt characteristic of the deepest part of the lake environment, and are often preserved alongside deep-water fish and crustaceans. Hyphalosaurus is also conspicuously absent from the aquatic sediments of the Jiufotang Formation, which preserved a more swamp
Swamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...
y, shallow-water ecosystem.
Hyphalosaurus is the most abundant tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
(four-limbed vertebrate) in the Yixian Formation, and probably played an important role in the aquatic food chain. It's long and highly flexible neck and small, flattened skull indicates that it captured small prey animals like fish or arthropods using a sideways-strike, similar to modern aquatic predators with flattened skulls. Unlike other choristoderans, Hyphalosaurus was likely an active predator, rather than one that used a "sit and wait" ambush strategy. Its fossils are often found preserved along side the small fish Lycoptera
Lycoptera
Lycoptera is a genus of fish that lived from the late Jurassic to Cretaceous periods in present-day China, Korea, Mongolia and Siberia. It is known from abundant fossils representing sixteen species, and is an important index fossil used to date geologic formations in China...
, which may have been a prey item, and at least one specimen preserved fish rubs as stomach contents. However, the lack of preserved stomach contents among the thousands of known specimens may indicate that they ate mainly soft-bodied prey.
Classification and species
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 H. lingyuanensis were given to different groups of researchers in Beijing
Beijing
Beijing , also known as Peking , is the capital of the People's Republic of China and one of the most populous cities in the world, with a population of 19,612,368 as of 2010. The city is the country's political, cultural, and educational center, and home to the headquarters for most of China's...
, one from the Institute of Vertebrate Paleontology and Paleoanthropology
Institute of Vertebrate Paleontology and Paleoanthropology
The Institute of Vertebrate Paleontology and Paleoanthropology of China is a prominent research institution and collections repository for fossils, including many dinosaur and pterosaurand cat poo specimens...
and the other from the Beijing Natural History Museum. Each team described the taxon and published their results independently in January 1999, giving the animal two different names: Hyphalosaurus lingyuanensis and Sinohydrosaurus lingyuanensis. It was quickly recognized that Sinohydrosaurus and Hyphalosaurus were mirror images of one another and in fact represented different halves of the same specimen. The ICZN
International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...
, which governs the naming of animals, mandates that the older name is valid. However, in June 2001 paleontologists Joshua Smith and Jerry Harris noted that since both were published at almost exactly the same time, a third party needed to select which name would better serve as the objective senior synonym
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
. Smith and Harris took the opportunity to do so, selecting Hyphalosaurus as the senior synonym because the manuscript for its description had apparently been submitted (though not published) earlier. They therefore made Sinohydrosaurus a junior synonym of Hyphalosaurus.
Hyphalosaurus is related to the large, crocodile-like Champsosaurus
Champsosaurus
Champsosaurus is an extinct genus of diapsid reptile belonging to the order Choristodera. It grew to about 1.50 m long....
and the smaller, lizard-like Monjurosuchus
Monjurosuchus
Monjurosuchus is a genus of choristoderan reptile that lived in what is now China and Japan during the Early Cretaceous. It has large eyes, a rounded skull, robust legs with short claws, and a long, thin tail. Fossils have been found that preserve soft tissue, showing that it had soft skin and...
. Its closest relative was the similarly built species, Shokawa ikoi
Shokawa
Shokawa ikoi was a 1.8 m long choristoderan diapsid reptile closely resembling and closely related to the smaller choristoderan, Hyphalosaurus. It lived during the Lower Cretaceous in what is now Japan....
, from the Early Cretaceous of Japan. The choristoderes were 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...
of aquatic reptiles that survived the end-Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
extinction along with crocodilians, turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s, 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 and snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s. The choristoderes became extinct by the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
.