''Mixosaurus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

(

Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...

, about 250-240 Mya)

ichthyosaur Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...

. Its fossils have been found near the

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

–

Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...

border and in

South China South China ( zh, s=, p=Huá'nán, j=jyut6 naam4) is a geographical and cultural region that covers the southernmost part of China. Its precise meaning varies with context. A notable feature of South China in comparison to the rest of China is ...

. The genus was named in 1887 by George H. Baur. The name means "Mixed Lizard", and was chosen because it appears to have been a

transitional form A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross ...

between the

eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 20 families, 164 genera, and about 1000 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...

-shaped ichthyosaurs such as ''

Cymbospondylus ''Cymbospondylus'' (meaning "cupped vertebrae") is an extinct genus of large ichthyosaurs, of which it is among the oldest representatives, that lived from the Lower to Middle Triassic in what are now North America and Europe. The first known fo ...

'' and the later

dolphin A dolphin is an aquatic mammal in the cetacean clade Odontoceti (toothed whale). Dolphins belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontopori ...

-shaped ichthyosaurs, such as ''

Ichthyosaurus ''Ichthyosaurus'' (derived from Greek () meaning 'fish' and () meaning 'lizard') is a genus of ichthyosaurs from the Early Jurassic (Hettangian - Pliensbachian) of Europe (Belgium, England, Germany and Portugal). Some specimens of the ichthy ...

''. Baur named ''Mixosaurus'' as a new genus because its forefin was sufficiently different from that of ''Ichthyosaurus''. Numerous species have been described within this genus, which was once considered as the most common genus of Triassic ichthyosaurs. The

type species In International_Code_of_Zoological_Nomenclature, 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 spe ...

is ''M. cornalianus'', and the definitive number of species is debated among researchers.

Description

''Mixosaurus'' was a small ichthyosaur, measuring long and weighing . The largest specimens would have measured over in length. It possessed a long tail with a low fin, suggesting it wasn’t as fast as Jurassic Ichthyosaurs, but also possessed a

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

for stability in the water. The paddle-like limbs were made up of five toes each, unlike the three toes found in later Ichthyosaurs. Noteworthy, however, is that each toe had more individual bones than is usual in reptiles, and the front limbs were longer than the back limbs, both adaptations typical of later ichthyosaurs. The jaws were narrow, with several sharp teeth, that would have been ideal for catching fish. They had relatively large skulls compared to their bodies, unlike the basal ichthyosaurs, but resembled fish-shaped ichthyosaurs that appeared later. They had around 50 vertebrae in front of the pelvic girdle, around twice as many as terrestrial

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...

s. Recent studies suggest that genus ''Mixosaurus'' may have lived near shore or in a shelf-like habitat as it possesses more compact

spongy bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and ...

within its long bones than other Ichthyosaurs.

Species

A 2024 study which described the new species ''M. luxiensis'' showed that the identity of species within ''Mixosaurus'' is debated among researchers except for ''M. cornalianus'' and ''M. kuhnschnyderi''. ''M. xindianensis'' has been suggested as ''

species inquirenda In biological classification, a ''species inquirenda'' is a species of doubtful identity requiring further investigation. The use of the term in English-language biological literature dates back to at least the early nineteenth century. The ter ...

'', and it may more likely belong to ''

Phalarodon ''Phalarodon'' is an extinct genus of mixosaurid ichthyosaur known from the Middle Triassic (247.2-242.0 million years ago). Its name is derived from the Greek φάλαρα (''phálara'') ("boss on the cheek plate of a helmet") and ''odon'' ("to ...

''. Known species of ''Mixosaurus'' share many similar characteristics throughout the cranial and post cranial with the main differences

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

occurring in the dental region. Examples of the dental variation are the extent of the dental groove in the upper jaw, the shape and size of the teeth and the number of rows of teeth Previous authors assigned other species to the genus, including ''M. atavus'' (Quenstedt, 1852), ''M. callawayi'' Schmitz ''et al.'', 2004, ''M. panxianensis'' Jiang ''et al.'', 2006 and ''M. yangjuanensis'' Liu & Yin, 2008. These are now included in ''

Contectopalatus ''Phalarodon'' is an extinct genus of mixosaurid ichthyosaur known from the Middle Triassic (247.2-242.0 million years ago). Its name is derived from the Greek φάλαρα (''phálara'') ("boss on the cheek plate of a helmet") and ''odon'' ("to ...

'', ''

'', '' Barracudasauroides'' and ''

Nothosaurus ''Nothosaurus'' ('false lizard', from the Ancient Greek and ) is an extinct genus of sauropterygian reptile from the Triassic period, approximately 245–228 million years ago, with fossils being distributed throughout the former Tethys Ocean, ...

'', respectively. ''Mixosaurus'' species declared as ''nomen dubium'', meaning the description was insufficient to fully classify them as a species, are ''M. maotaiensis'', ''M. helveticus'', ''M. timorensis'', ''M. major'', and ''M. nordenskioeldii''.

''Mixosaurus cornalianus''

Many specimens of ''Mixosaurus cornalianus'' have been found from the Middle

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

Monte San Giorgio Monte San Giorgio is a Swiss mountain and UNESCO World Heritage Site near the border between Switzerland and Italy. It is part of the Lugano Prealps, overlooking Lake Lugano in the Swiss Canton of Ticino. Monte San Giorgio is a wooded mountai ...

and the Tessin areas on the border of

and

. ''Mixosaurus cornalianus'' is the only Triassic ichthyosaur for which completely articulated skeletons have been found. Many specimens have been collected but ''Mixosaurus cornalianus'' is not well studied, this is because all of the known specimens have been compressed during the preservation process. ''Mixosaurus cornalianus'' has a

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are excepti ...

associated with the expansion of the upper temporal

fenestra A fenestra (fenestration; : fenestrae or fenestrations) is any small opening or pore, commonly used as a term in the biology, biological sciences. It is the Latin word for "window", and is used in various fields to describe a pore in an anatomy, ...

. This indicates that it had exceptionally strong jaw muscles. The current name-bearing specimen is the

neotype In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to anchor or centralizes ...

(PIMUZ T 2420), which was approved by the ICZN in 2001 after a 1999 petition.

''Mixosaurus luxiensis''

In 2024, Ye-Wei Fang and coauthors named a new species, ''M. luxiensis'', with HFUT HL-21-08-002 being the holotype and only known specimen. This specimen, a largely complete but disarticulated skeleton, was found in the Upper Member of the Guanling Formation in Luxi County,

Yunnan Province Yunnan; is an inland Provinces of China, province in Southwestern China. The province spans approximately and has a population of 47.2 million (as of 2020). The capital of the province is Kunming. The province borders the Chinese provinces ...

, China; the species name refers to this county.

''Mixosaurus panxianensis''

''Mixosaurus panxianensis'' was discovered in the Middle Triassic of the

Guizhou Province ) , image_skyline = , image_caption = , image_map = Guizhou in China (+all claims hatched).svg , mapsize = 275px , map_alt = Map showing the location of Guizhou Province , map_caption = Map s ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. The specimens have been found in the

Guanling Formation The Guanling Formation is a Middle Triassic (Anisian or Pelsonian in the regional chronostratigraphy) geologic Formation (geology), formation in southwestern China. Geology The formation encompasses two members. The first member is primarily c ...

, which consists of thinly bedded bituminous

limestones Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Limestone forms when these ...

and marls. The specimens found have important Mixosaurid characteristics such as a long sagittal crest along the top of the

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

but is seen as a different species because there is no external contact between the

jugal The jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anatomy ...

and the

quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...

. Articulated skeletons have been found and the centra of the

vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

are higher than they are long. This is evidence for its transitional position between basal early Triassic ichthyosaurs and more derived Jurassic and Cretaceous species; who have disc shaped circular centra of the vertebrae. This species has been moved to its own genus, '' Barracudasauroides''. The 2024 study included ''M. panxianensis'' as a species of ''Mixosaurus'' based on phylogenetic analyses.

Classification

In recent years the

taxonomy image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...

and

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

of mixosaurid ichthyosaurs has been a controversial topic. Most recently, Mixosauridae has been separated into Mixosaurinae and the sister group Phalarondontinae. ''Mixosaurus'' contains ''M. cornalianus'', ''M. kuhnschnyderi'' and ''M. xindianensis'', '' Barracudasauroides'' contains ''B. panxianensis'', ''

'' contains ''P. fraasi'' and ''P. callawayi'', and ''

'' contains ''C. atavus''. Mixosaurids are characterised by a relatively short and wide humerus and ''Phalarodon'' are characterised by the lack of a dental groove in the upper jaw. ''Phalarodon'' fossils are found in every major Mixosaur locality. It was suggested that ''Tholodus schmidi'' should be included in Mixosauridae but only dental material has been found so it is difficult to assign it to a genus.

Cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

following Jiang and colleagues, 2006. Cladogram following the preferred tree of Moon, 2017.

References

{{Taxonbar, from=Q134753 Middle Triassic ichthyosaurs Middle Triassic reptiles of Asia Middle Triassic reptiles of Europe Middle Triassic reptiles of North America Guanling Formation Taxa named by Georg Baur Fossil taxa described in 1887 Ichthyosauromorph genera