''Zapatadon'' 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 n ...

sphenodontid 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 gro ...

reptile from the end of the

Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma ...

in the lower part of

La Boca Formation The La Boca Formation is a geological formation in Tamaulipas state, northeast Mexico. It was thought to date back to the Early Jurassic, concretely the Pliensbachian stage epoch. Although, the latest studies had proven that the local Vulcanism, r ...

Tamaulipas Tamaulipas (), officially the Free and Sovereign State of Tamaulipas ( es, Estado Libre y Soberano de Tamaulipas), is a state in the northeast region of Mexico; one of the 31 states which, along with Mexico City, comprise the 32 Federal Entiti ...

Mexico Mexico ( Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guate ...

.Marisol Montellano, James A. Hopson and James M. Clark (2008)
Late Early Jurassic Mammaliaforms from Huizachal Canyon, Tamaulipas, México
''Journal of Vertebrate Paleontology'', Vol. 28, No. 4 (Dec. 12, 2008), pp. 1130-1143. Is known from a nearly complete

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, t ...

with

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bon ...

of a post-hatchling individual (the specimen IGM 3497, in the Instituto de Geologia, of the

Universidad Nacional Autónoma de Mexico The National Autonomous University of Mexico ( es, Universidad Nacional Autónoma de México, UNAM) is a public research university in Mexico. It is consistently ranked as one of the best universities in Latin America, where it's also the bigges ...

), and is one of the smallest skulls between the sphenodontians, with an estimated total length of 11.3 millimetres, a bit smaller than the hatchling individuals observed in the modern tuatara (''

Sphenodon 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 ...

''); features like the oblique mandibular

symphysis A symphysis (, pl. symphyses) is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint. # A symphysis is an amphiarthrosis, a slightly movable joint. # A growing togethe ...

suggests that the holotype is from an individual in a relatively mature stage of ontogenic development. ''Zapatadon'' is diagnosed by their hatchling tooth series located in a depression in the anterior part of the

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

bone, the

prefrontal Prefrontal may refer to: *Prefrontal bone, a skull bone in some tetrapods *Prefrontal cortex, a region of the brain of a mammal *Prefrontal scales The prefrontal scales on snakes and other reptiles are the scales connected to the frontals, toward ...

bone surrounding the dorsal process of the

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. The ...

and the broad

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. Anat ...

that extends over the maxillary suborbital process, been almost excluded of the

orbit 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 a ...

. ''Zapatadon'' was first described and named by Víctor-Hugo Reynoso and James M. Clark in 1998, and the name of the genus is a homage to the Mexican revolutionary leader

Emiliano Zapata Emiliano Zapata Salazar (; August 8, 1879 – April 10, 1919) was a Mexican revolutionary. He was a leading figure in the Mexican Revolution of 1910–1920, the main leader of the people's revolution in the Mexican state of Morelos, and the in ...

, added to the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

sufix -''odon'', "tooth", common in other sphenodontian taxa; the name of the type species, ''ejidoensis'' is in gratitude to the people of the

ejido An ''ejido'' (, from Latin ''exitum'') is an area of communal land used for agriculture in which community members have usufruct rights rather than ownership rights to land, which in Mexico is held by the Mexican state. People awarded ejidos ...

(communal land area) of El Huizachal, that allow the investigation of the fossils. In the La Boca Formation, where the fossils of ''Zapatadon'' were collected, also have been found fossils of another sphenodontian taxa like '' Cynosphenodon huizachalensis'' and the possibly venomous '' Sphenovipera jimmysjoyi'', the primitive diapsid '' Tamaulipasaurus morenoi'', the primitive pterosaur '' Dimorphodon weintraubi'', the

tritylodont Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details o ...

'' Bocatherium mexicanum'' and the

mammaliaforms Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent comm ...

'' Bocanodon tamaulipensis'', '' Victoriaconodon inaequalis'' and '' Huasteconodon wiblei'', along with fragmentary cranial and postcranial remains of

crocodyliforms Crocodyliformes is a clade of crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the only pseu ...

, and teeth of

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ...

and

ornithischian Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek ...

dinosaurs.

Classification

The

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

analysis of the original description of ''Zapatadon'' found it as a part of a clade that contains to the subfamilies Eilenodontidae (''

Toxolophosaurus ''Toxolophosaurus'' was a sphenodont from the Early Cretaceous-age Kootenai Formation of Montana. Discovery ''Toxolophosaurus cloudi'' was found by Olson in 1960 in the Kootenai Formation, 223.75 feet above the base of the Kootenai. Classific ...

'', '' Eilenosaurus'') and Sphenodontinae (''

'', ''

Cynosphenodon ''Cynosphenodon'' ( ; "Dog Sphenodontian") is an extinct genus of the family Sphenodontidae from the Middle Jurassic La Boca Formation The La Boca Formation is a geological formation in Tamaulipas state, northeast Mexico. It was thought to da ...

''), in an unresolved

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...

with the genus ''

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. Distribut ...

'' and this subfamilies, within 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 ...

. This inclusion is supported by have the great length of the

supratemporal fenestra 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, the ...

, more than a fourth of the skull length, the single palatine row of teeth and an orbit less than a third of the skull length. Although certain features like the great length of the lower temporal fenestra and the enlarged quadrate-quadratojugal

foramen In anatomy and osteology, a foramen (;Entry "foramen"
in

are shared with ''Sphenodon'', suggesting a close relationship, the authors noted that the immature nature of the holotype make a mixture of advanced and primitive characters that do not allow make more clear their phylogenetic relationships.

References

* Reynoso-Rosales, V. H. 1992. ''Descripción de los esfenodontes del Jurásico temprano o medio del Cañon del Huizachal, Tamaulipas, México''. M.S. thesis, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, México, 86 pp. {{Taxonbar, from=Q8066388 Sphenodontia Jurassic lepidosaurs Early Jurassic reptiles of North America Prehistoric reptile genera