''Priosphenodon'' is an extinct, large herbiviorous eilenodontine

rhynchocephalia Rhynchocephalia (; ) is an order of lizard-like reptiles that includes only one living species, the tuatara (''Sphenodon punctatus'') of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a speciose g ...

n known from the mid-Cretaceous (

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...

) of

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

. It is one of the largest known sphenodontians.

Taxonomy

The type species of ''Priosphenodon, P. avelasi'', was described in 2003 from the Late Cretaceous (

Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...

) aged

Candeleros Formation The Candeleros Formation is a geologic formation that outcrop, crops out in the Rio Negro Province, Río Negro, Neuquen Province, Neuquén, and Mendoza Province, Mendoza provinces of northern Patagonia, Argentina. It is the oldest formation in the ...

of Argentina. In 2014, a second smaller species, ''P. minimus'' was described from the same formation, though from an earlier unit suggested to be Early Cretaceous (

) in age. Other authors have disputed the use of the genus ''Priosphenodon'', with some authors treating ''P. avelasi'' and ''P. minimus'' as members of the previously named genus ''

Kaikaifilusaurus ''Kaikaifilusaurus'' is an extinct genus of rhynchocephalians in the family Sphenodontidae from the Late Cretaceous of South America. Fossils of the genus were found in Cenomanian sediments of the Candeleros Formation and Turonian layers of the ...

'' instead.

Description

With some individuals reaching over in total length, ''Priosphenodon avelasi'' is the largest known terrestrial sphenodontian. The skull is around long. The front of the upper jaw has a large beak-like structure probably formed from fused teeth on 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 mammals h ...

. The teeth are densely packed with a cone-in-cone structure, and have prismatic enamel structure similar to those of mammals and the lizard ''

Uromastyx ''Uromastyx'' is a genus of lizards in the family Agamidae. The genus is native to Africa and the Middle East (West Asia). Member species are commonly called spiny-tailed lizards, uromastyces, mastigures, or dabb lizards. Lizards in the genus ...

,'' which was likely an adaptation to wear resistance in the absence of tooth replacement. There is a large elongate tooth row on the palatine bones on the roof of the mouth that runs parallel to the maxillary tooth row. The

ungual An ungual (from Latin ''unguis'', i.e. ''nail'') is a highly modified distal toe bone which ends in a hoof, claw, or nail. Elephants and ungulates have ungual phalanges, as did the sauropod Sauropoda (), whose members are known as sauropods (; ...

phalanges The phalanges (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...

are square in shape, and are expanded towards their

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

ends, a condition unlike those known in other lepidosaurs, which typically have pointed ungual phalanges. ''P. avelasi'' exhibited variable but generally slow growth rates, with a probably

sexually mature Sexual maturity is the capability of an organism to reproduce. In humans, it is related to both puberty and adulthood. ''Puberty'' is the biological process of sexual maturation, while ''adulthood'', the condition of being socially recognized as ...

individual confirmed to be at least 13 years old by bone histology was still growing and only around 40% maximum size at the time of death. ''P. minimus'' differs from ''P. avelasi'' by having a proportionally shorter skull in addition to a considerably smaller body size, among a variety of differences in the arrangement and shape of the skull bones.

Ecology

During feeding the dentary teeth of ''Priosphenodon'' slotted between the maxillary and palatine tooth rows, which in turn with backward and forward (propalinal) motion of the jaw, served to shred plant material. The Candeleros Formation is suggested to have been deposited in an arid environment. Remains of ''P. avelasi'' are the most abundant of any terrestrial vertebrate at the sites it is found, and the lack of other herbivores found at the localities suggests that it may have been the only resident herbivore.

References

{{Taxonbar, from=Q21368826 Prehistoric lepidosaurs Prehistoric reptile genera Cenomanian life Turonian life Late Cretaceous reptiles of South America Cretaceous Argentina Fossils of Argentina Candeleros Formation Cerro Barcino Formation Fossil taxa described in 2003 Rhynchocephalia