''Lamarquesaurus'' 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 ...

sphenodontian Rhynchocephalia (; ) is an order (biology), 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 ...

from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent 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 ''cre ...

(

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. Campa ...

) of

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

. It is known from a single

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, ''Lamarquesaurus cabazai''. This genus and species is represented by MML-PV-42, a well-preserved right

maxillary bone In vertebrates, the maxilla (: maxillae ) 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. The two maxillar ...

(including 10 teeth of varying completeness). This bone was found at the Cerro Tortuga site near Lamarque, Argentina, which preserves fossils from the Cretaceous

Allen Formation The Allen Formation is a geological formation in Argentina whose strata date back to the Late Cretaceous (middle Campanian to early Maastrichtian.Salgado et al., 2007 Dinosaur remains are among the fossils that have been recovered from the forma ...

. The discoverer of the Cerro Tortuga locality, Tito Cabaza, is the namesake of the species.

Description

The maxilla is in length; including missing portions it would have been about long if complete. The inner surface of the maxilla is deeply concave but the outer surface has several scattered pits as well as deep facets above the third and fifth teeth. These facets are reminiscent of those which sphenodonts possess on the lower jaw due to abrasion from teeth of the upper jaw, but this cannot be the case in ''Lamarquesaurus'''s maxilla. This is because the lower jaw of sphenodonts contacts the inner edge of the maxilla when the mouth is closed; the outer surface of the maxilla would not be eroded by teeth because it never contacts the lower jaw. Therefore, these concavities (referred to as "false wear marks") must have had a different origin than true wear marks. The outer surface of the maxilla of ''Lamarquesaurus'' also has a characteristic longitudinal ridge which is large and roughly-textured at the front part of the bone and forks into two ridges at the rear part of the bone. The teeth are widely spaced and roughly conical, with ridges on their outer surface and slanted wear facets on their inner surface. These wear facets indicate that ''Lamarquesaurus'' was an advanced sphenodont capable of propalinal (front-to-back) jaw movement. Although they do not closely resemble the wide, rectangular teeth of eilenodontine opisthodonts, there are no qualities which preclude a relationship between ''Lamarquesaurus'' and basal opisthodonts such as '' Opisthias'', or alternatively members of the family

Sphenodontidae Sphenodontidae is a family within the reptile group Rhynchocephalia, comprising taxa most closely related to the living tuatara (''Sphenodon punctatus''). Historically the taxa included within Sphenodontidae have varied greatly between analyses, ...

such as the modern

tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...

(''Sphenodon''). ''Lamarquesaurus'''s discovery bolsters the theory that eupropalinal sphenodonts continued to dominate the "small reptile" niche of

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

n ecosystems during the Cretaceous and early

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

, while other sphenodonts went extinct and were replaced by

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

s in the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

References

{{Portal bar, Paleontology, Reptiles Rhynchocephalia Campanian life Late Cretaceous reptiles of South America Cretaceous Argentina Fossils of Argentina Allen Formation Fossil taxa described in 2007