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Sapheosaurs are 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 ...
group of
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 reptiles from the
Late Jurassic The Late Jurassic is the third Epoch (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...
period. "Sapheosaurs" is an informal name for a group of rhynchocephalians closely related to the genus '' Sapheosaurus''. It was first recognized as a group containing multiple genera by Hoffstetter in 1955. The group has sometimes been given a formal taxonomic name as the
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
Sapheosauridae, although in some analyses this group belongs to 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, ...
(which also contains the
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 ...
, the only living rhynchocephalian) and thus cannot be assigned its own family. They were fairly advanced rhynchocephalians which may have had semiaquatic habits.


Classification

The most well-known members of the group are ''Sapheosaurus'' and '' Kallimodon'', and most
phylogenetic analyses In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as Computational phylogenetics, phylogenetic inference. It infers the relationship among organ ...
on rhynchocephalians only study these two
genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
as representatives of sapheosaurs. Although a few early phylogenies in the 1990s did not find that these two formed a natural clade, the relation between these two is now considered to be more stable, and has been found in practically every major analysis of rhynchocephalians since Apesteguía & Novas (2003). However, the relation between sapheosaurs as a whole and other rhynchocephalians is less clear. Although they are clearly members of the group Sphenodontia like almost all other rhynchocephalians, the construction of their jaw joints means that they were unable to move their jaws in a front-to-back chewing movement. This excludes them from the clade of sphenodonts which are capable of such movement, the so-called "eupropalinal sphenodonts" such as the tuatara, (formally known as ''Sphenodon''), its close relatives, and the herbivorous opisthodonts. They are also generally considered to be more derived (as in closer to ''Sphenodon'') than
clevosaurs Clevosaurs are an extinct group of rhynchocephalian reptiles from the Triassic and Jurassic periods. History and definition Although members of this group have been known since 1910, only recently has the group received a formal name. In the ...
and basal genera such as ''
Godavarisaurus ''Godavarisaurus'' is an extinct genus of sphenodontian reptile from the Early-Middle Jurassic Kota Formation of Andhra Pradesh, India. It is known from jaw fragments (including the maxilla, premaxilla, and parts of the dentary). It was a small s ...
'' and '' Sphenocondor''. The in-group relations of sphenodonts are inconsistent between analyses, so although sapheosaurs may be the
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
of eupropalinal sphenodonts under some methodologies, other potentially more derived taxon include ''
Homoeosaurus ''Homoeosaurus'' is an extinct genus of rhynchocephalian reptile, known from the Late Jurassic-earliest Cretaceous of Europe, with specimens being reported from France ( Canjuers Lagerstatte), England (Purbeck Group) and Germany ( Solnhofen Lime ...
'', '' Pamizinsaurus'', ''
Ankylosphenodon ''Ankylosphenodon'' is an extinct genus of sphenodontian known from multiple specimens recovered from the Early Cretaceous deposits of the Tlayúa Formation, near Tepexi de Rodríguez, Tepexi de Rodriguez, Mexico. It is likely part of a sphenodo ...
'', the ''
Sphenovipera ''Sphenovipera jimmysjoyi'' is an extinct species of sphenodontian dated from the Jurassic. If was discovered in the lower part of the La Boca Formation located in Tamaulipas, Mexico. Only the lower jaw of this organism has been discovered and s ...
''+''
Theretairus ''Theretairus'' is a Late Jurassic genus of sphenodont reptile from the Morrison Formation of western North America,Foster, J. (2007). "Table 2.1: Fossil Vertebrates of the Morrison Formation." ''Jurassic West: The Dinosaurs of the Morrison For ...
'' clade, and pleurosaurs. A 2017 study utilizing both parsimony and
bayesian Thomas Bayes ( ; c. 1701 – 1761) was an English statistician, philosopher, and Presbyterian minister. Bayesian ( or ) may be either any of a range of concepts and approaches that relate to statistical methods based on Bayes' theorem Bayes ...
analyses found some support for a clade including sapheosaurs, '' Vadasaurus'', and pleurosaurs. Although the bayesian analysis placed this clade in a large
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 tree ...
with various other rhynchocephalian groups and genera, the parsimony analysis actually placed it among the eupropalinal sphenodonts. If this phylogeny is accurate, this would mean that an ancestor of this clade lost front-to-back chewing which was present in an even earlier ancestor, rather than never having it in the first place. A 2022 study found a clade containing pleurosaurs, ''Vadasaurus'', sapheosaurs, ''Homoeosaurus, Kallimodon'' and '' Leptosaurus,'' to the exclusion of other neosphenodontians like
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, ...
. This clade was called Leptorhynchia.


Description

All known members of the group lived in coastal environments of the late Jurassic in what is now
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
and
France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...
. They are very similar to each other, only noticeably differing in certain proportions of the skull. Evans (1988) and Ahmad (1993) have even considered that they may all belong to 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), ...
, although a shortage of good studies and descriptions focusing on this group in particular means that any conclusions on the relationships between different sapheosaurs are uncertain at best.


Skull

The postorbital region of the skull (behind the eyes) is lengthened and expanded in this group to such a point that it is longer than the preorbital region (in front of the eyes). Although clevosaurs also had skulls which were largest in the postorbital region, they evolved such a feature through different means, namely the preorbital region being shortened. The upper temporal fenestrae (a pair of large holes on the top of the rear part of the skull) are long but fairly thin, a feature also known in ''Vadasaurus'' and ''
Palaeopleurosaurus ''Palaeopleurosaurus'' (meaning "old side lizard") is an extinct genus of diapsid reptiles belonging to the group Sphenodontia.Dupret, V. (2004). The pleurosaurs: anatomy and phylogeny. ''Revue de Paléobiologie'', 9: 61-8/ref> ''Palaeopleuros ...
,'' potential relatives of the group. Each upper temporal arch, which separates the upper temporal fenestrae from the lower temporal fenestrae (on the sides of the skull), is broad. Sapheosaurs, like other sphenodonts, had acrodont teeth which grew directly from the bone and could not be replaced. Historically ''Sapheosaurus'' was thought to be toothless. However, it was later shown that ''Sapheosaurus'' and ''Oenosaurus'' possessed tooth plates on the upper and lower jaws, formed by the fusion of
maxilla 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 maxil ...
ry and dentary teeth. Tooth plates are now thought to be characteristic of all sapheosaurs. They are thought to have been used for crushing hard shelled organisms (
durophagy Durophagy is the eating behavior of animals that consume Seashell, hard-shelled or exoskeleton-bearing organisms, such as corals, shelled mollusks, or crabs. It is mostly used to describe fish, but is also used when describing reptiles, including ...
).


Other features

The centra (main body) of each caudal (tail)
vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
is flattened from the side in sapheosaurs. Sapheosaur vertebrae also had swollen neural arches (the area above the spinal cord) and zygapophyses (connecting joint plates), features also present in ''Ankylosphenodon''. Some have proposed that sapheosaurs were at least partially aquatic due to some similarities and/or possible close relations to ''Ankylosphenodon'' (now believed to be convergently evolved) or ''Vadasaurus'' and pleurosaurs. However, they do not share many of the adaptations that these other taxa possess, and some researchers are not convinced by this hypothesis.


References

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