''Umoonasaurus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of plesiosaur belonging to the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Leptocleididae Leptocleididae is a family of small-sized plesiosaurs that lived during the Early Cretaceous period (early Berriasian to early Albian stage). They had small bodies with small heads and short necks. ''Leptocleidus'' and '' Umoonasaurus'' had ro ...

. This genus lived approximately 115 million years ago during the

Late 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'', ...

period ( Aptian-

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

stages), in shallow seas covering parts of what is now Australia. It was a relatively small animal around long. An identifying trait of ''Umoonasaurus'' is three crest-ridges on its skull.

Discovery and naming

The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

of ''Umoonasaurus demoscyllus'' is AM F99374, an opalized skeleton that has been nicknamed "Eric". It was discovered in the Zorba Extension Opal Field near the town of

Coober Pedy Coober Pedy () is a town in northern South Australia, north of Adelaide on the Stuart Highway. The town is sometimes referred to as the "opal capital of the world" because of the quantity of precious opals that are mined there. Coober Pedy is ...

, and is very well preserved, representing the most complete opalized

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

of a

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

known. Other specimens have also been referred to this

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

. SAM P2381, discovered in the Andamooka opal fields, is another opalized specimen. SAM 31050 was discovered in the Curdimurka area near

Lake Eyre Lake Eyre ( ), officially known as Kati Thanda–Lake Eyre, is an endorheic lake in east-central Far North South Australia, some north of Adelaide. The shallow lake is the depocentre of the vast endorheic Lake Eyre basin, and contains th ...

, and SAM P410550, a juvenile specimen, comes from the Neales River region, near the town of

Oodnadatta Oodnadatta is a small, remote outback town and locality in the Australian state of South Australia, located north-north-west of the state capital of Adelaide by road or direct, at an altitude of . The unsealed Oodnadatta Track, an outback roa ...

. Another juvenile specimen, SAM P15980, was later referred to this species. All known specimens come from the Bulldog Shale in

South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...

, although material very similar to ''Umoonasaurus'' has been found in the

Darwin Formation Darwin may refer to: Common meanings * Charles Darwin (1809–1882), English naturalist and writer, best known as the originator of the theory of biological evolution by natural selection * Darwin, Northern Territory, a territorial capital city i ...

in the

Northern Territory The Northern Territory (commonly abbreviated as NT; formally the Northern Territory of Australia) is an Australian territory in the central and central northern regions of Australia. The Northern Territory shares its borders with Western Aust ...

. The generic name is a combination of the Antakirinja name for the Coober Pedy region, ''Umoona'', and the Greek word ''sauros'', meaning "lizard." The specific name comes from the Greek words and , meaning "of the people" and "sea monster," respectively, referring to the public donations used to acquire the holotype.

Description

''Umoonasaurus'' is a small plesiosaur, a four-flippered marine reptile. The holotype is estimated to have measured long and weighed , while the juvenile specimen SAM P15980 was only long at maximum. ''Umoonasaurus'' is unusual in possessing a combination of primitive and derived characters. ''Umoonasaurus'' had a small, triangular

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

with a length of and a width of . A tall, narrow crest is present along the middle of the anterior (front) end of the skull. Two other ridges are present above the

orbits In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

(openings for the eyes). 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 ...

bears a dental rosette with five

tooth socket Dental alveoli (singular ''alveolus'') are sockets in the jaws in which the roots of teeth are held in the alveolar process with the periodontal ligament. The lay term for dental alveoli is tooth sockets. A joint that connects the roots of the ...

s. These would have held long, sharp

teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, t ...

. Each

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

bears at least ten tooth sockets. The teeth in positions 4 to 6 would have been very large, while the rest of the teeth would have been smaller and more gracile. The external nares (openings for the nostrils) are very small and positioned close to the orbits. While incompletely preserved, the

temporal fenestrae The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

would have likely occupied one third of the length of the skull. The anterior skull roof (composed of the parietals) is lanceolate (shaped like a lance head). The pineal foramen (a depression between the orbits and the temporal fenestrae) is triangular and has raised edges. 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 max ...

palatal 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 separ ...

bone close to the tip of the snout) barely extends posteriorly past the internal nares (opening for the nostrils on the inside of the skull). The region of the

pterygoid Pterygoid, from the Greek for 'winglike', may refer to: * Pterygoid bone, a bone of the palate of many vertebrates * Pterygoid processes of the sphenoid bone ** Lateral pterygoid plate ** Medial pterygoid plate * Lateral pterygoid muscle * Medi ...

(another of the palatal bones) that borders the quadrate is squared off. The interpterygoid vacuity (a region enclosed by the left and right pterygoids) is separated into the anterior and posterior interpterygoid vacuities, the former of which is particularly large. The parasphenoid (one of the bones in the braincase) runs over the midline of the narrower posterior interpterygoid vacuity. The surrounding surface of the pterygoid is concave. The basicranium (braincase floor) has gracile paroccipital processes (extensions that connect to other bones in the skull). The basioccipital has robust protuberances and extends behind the pterygoid. The

postcranium Postcrania (postcranium, adjective: postcranial) in zoology and vertebrate paleontology is all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g. of dinosaurs or other extinct tetrapods, consist of partial or isolated s ...

of ''Umoonasaurus'' is not very specialized, but does have some derived traits. The

cervical In anatomy, cervical is an adjective that has two meanings: # of or pertaining to any neck. # of or pertaining to the female cervix: i.e., the ''neck'' of the uterus. *Commonly used medical phrases involving the neck are **cervical collar **cerv ...

(neck)

centra Centra is a convenience shop chain that operates throughout Ireland. The chain operates as a symbol group owned by Musgrave Group, the food wholesaler, meaning the stores are all owned by individual franchisees. The chain has three different ...

(vertebral bodies) are taller than wide, and the width of the zygapophyses (projections on the

neural arch The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

involved in vertebral articulation) is roughly equal to that of the centrum. The

neural spine The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

s are laterally (side to side) compressed and blade-like in shape. The ribs are single-headed. The epipodials (the bones that form the

forearm The forearm is the region of the upper limb between the elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, a word which is most often used to describe the entire appendage of the upper limb, but which in ...

and shin) are wider than long. At least five posterior caudal vertebrae are fused into a structure that resembles a

pygostyle Pygostyle describes a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main compone ...

. The function of this structure is unclear, but it may have been involved in the support of a

dermal The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided ...

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

Classification

Prior to their description, the specimens that would later be assigned to ''Umoonasaurus'' were thought to represent a new species of ''

Leptocleidus ''Leptocleidus'' is an extinct genus of plesiosaur, belonging to the family (biology), family Leptocleididae.Smith AS, Dyke GJ. 2008. The skull of the giant predatory pliosaur ''Rhomaleosaurus cramptoni'': implications for plesiosaur phylogenetic ...

''. In 2006, Kear and colleagues found ''Umoonasaurus'' to belong to the superfamily

Pliosauroidea Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive t ...

and be the most basal member of the family Rhomaleosauridae. They found the latter surprising, as ''Umoonasaurus'' was also identified as the last surviving member of that family. ''

'' was recovered as a more derived rhomaleosaurid, although it was still considered plausible that the two might be close relatives. In 2008, Smith and Dyke found ''Umoonasaurus'' to belong to

instead of Rhomaleosauridae, although still within Pliosauroidea. A 2009 study by Druckenmiller and Russel also recovered ''Umoonasaurus'' as a pliosauroid, this time as a possible member of

Polycotylidae Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. ...

. In 2010, Ketchum and Benson found ''Umoonasaurus'' to be a member of Leptocleididae, although they recovered that family as a member of

Plesiosauroidea Plesiosauroidea (; Greek: 'near, close to' and 'lizard') is an extinct clade of carnivorous marine reptiles. They have the snake-like longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceous period ...

instead of Pliosauroidea. ''Umoonasaurus'' was also recovered as a leptocleidid by Druckenmiller and Knutsen in 2012, who found Leptocleididae to belong to Pliosauroidea once again. A 2013 study by Benson and colleagues found ''Umoonasaurus'' to be a close relative of ''Leptocleidus'' and to belong to Leptocleidia within Plesiosauroidea. In 2015, Parrilla-Bel and Canudo found ''Umoonasaurus'' to be a leptocleidid, and in turn finding Leptocleididae to belong to Leptocleidia, which was once again recovered as a member of Plesiosauroidea. Subsequent analyses have recovered a placement for ''Umoonasaurus'' similar to the previous two studies. Topology recovered by Druckenmiller and Russell (2009) Topology of Leptocleidia recovered by Parrilla-Bel and Canudo (2015)

Paleobiology

The crests of ''Umoonasaurus'' were likely covered by

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

in life, which would have made the crests considerably taller. Since the crests seem too delicate for defense, intraspecific combat, or skull reinforcement, they likely were used for communication. These structures could have been used for both

species recognition Intra-species recognition is the recognition by a member of a species of a conspecific (another member of the same species). In many species, such recognition is necessary for procreation. Different species may employ different methods, but all ...

and

courtship display A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), ...

s, and may have been boldly colored in life. Preserved gut contents of ''Umoonasaurus'' include

gastrolith A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In oth ...

s and

teleost Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tele ...

vertebrae. The latter indicates a preference for small prey items, a view reinforced by the lack of

hypercarnivorous A hypercarnivore is an animal which has a diet that is more than 70% meat, either via active predation or by scavenging. The remaining non-meat diet may consist of non-animal foods such as fungi, fruits or other plant material. Some extant exampl ...

adaptations in ''Umoonasaurus''.

Paleoenvironment

All known ''Umoonasaurus'' come from the Bulldog Shale, a member of the Marree Subgroup located in the

Eromanga Basin The Eromanga Basin is a large Mesozoic sedimentary basin in central and northern Australia. It covers parts of Queensland, the Northern Territory, South Australia, and New South Wales, and is a major component of the Great Artesian Basin. The E ...

. The sediments of the Bulldog Shale were deposited from the early Aptian to early

stages of the

Lower Cretaceous Lower may refer to: *Lower (surname) *Lower Township, New Jersey *Lower Receiver (firearms) *Lower Wick Lower Wick is a small hamlet located in the county of Gloucestershire, England. It is situated about five miles south west of Dursley, eig ...

, with ''Umoonasaurus'' coming from Aptian deposits laid down approximately 115 million years ago. Some horizons of the Bulldog Shale, including those around Coober Pedy, contain rich opal deposits. Fossilized

macroinvertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate s ...

and pollen have been used to recover an early Aptian to early

age for this formation. The deposits of the Bulldog Shale represent a

coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in ...

al region of a shallow epicontinental sea. Dark layers of shale rich in organic material with low

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

ic diversity are indictive of dysaerobic waters (those with a low amount of dissolved oxygen) near the bottom. This sea would have been located at a

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

of approximately 70° S, within the polar regions. The presence of glendonite, ice-rafted boulders, and coniferous

driftwood __NOTOC__ Driftwood is wood that has been washed onto a shore or beach of a sea, lake, or river by the action of winds, tides or waves. In some waterfront areas, driftwood is a major nuisance. However, the driftwood provides shelter and fo ...

with dense

growth rings Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...

indicate a seasonal climate with near-freezing temperatures. The average temperature of this region has been estimated at approximately . This suggests that ''Umoonasaurus'' was able to tolerate these cold conditions. The Bulldog Shale has yielded fossils of

plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...

invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

, and reptiles. The macroinvertebrate fauna of this formation includes several molluscs, such as

belemnite Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...

s, gastropods, and bivalves. Fish are represented by chimaeras (such as '' Edaphodon'') and bony fish (including

teleosts Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Teleo ...

and a

lungfish Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...

Shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s are conspicuously absent in the Bulldog Shale. Among the reptiles are other plesiosaurs, including the possible aristonectine

elasmosaurid Elasmosauridae is an extinct family of plesiosaurs, often called elasmosaurs. They had the longest necks of the plesiosaurs and existed from the Hauterivian to the Maastrichtian stages of the Cretaceous, and represented one of the two groups of ...

'' Opallionectes'', other indeterminate elasmosaurids, the giant

pliosaurid Pliosauridae is a family of plesiosaurian marine reptiles from the Latest Triassic to the early Late Cretaceous (Rhaetian to Turonian stages) of Australia, Europe, North America and South America. The family is more inclusive than the archetypal ...

Kronosaurus ''Kronosaurus'' ( ; meaning "lizard of Kronos") is a potentially dubious genus of extinct short-necked pliosaur. With an estimated length of , it was among the largest pliosaurs, and is named after the leader of the Greek Titans, Kronos. It ...

'', and polycotylid-like specimens. The ichthyosaur '' Platypterigius'' is also known from the Bulldog Shale.

References