''Lamarquesaurus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

sphenodontian 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 diverse g ...

from 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'', the ...

(

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. Campanian s ...

) of

Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and gl ...

. It is known from a single

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 ...

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

maxillary bone 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. The tw ...

(including 10 teeth of varying completeness). This bone was found at the Cerro Tortuga site near

Lamarque, Argentina Lamarque is a city in the Avellaneda Department of Río Negro Province, Argentina. It is located within the Río Negro (Black River) valley, about from the city of Choele Choel in the southeast of Isla Grande de Choele Choel. According to the 201 ...

, 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 formati ...

. 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 ''Opisthias'' is a genus of sphenodont reptile. The type species, ''Opisthias rarus'', is known from the Late Jurassic (Kimmeridgian-Tithonian) Morrison Formation of western North America, present in stratigraphic zones 2 and 4–6. Distributio ...

'', 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 of the genus ''Sphenodon''. Historically the taxa included within Sphenodontidae have varied greatly between analyses ...

such as the modern

tuatara Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...

(''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 relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

n ecosystems during the Cretaceous and early

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

, while other sphenodonts went extinct and were replaced by

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...

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 as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

References

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