Paramacellodidae is an extinct
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
of
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 that first appeared in the
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...
around 170 million years ago (Ma) and became extinct at the end of the Cretaceous around 66 Ma. It was one of the earliest groups of lizards to have undergone an
evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity that is caused by elevated rates of speciation, that may or may not be associated with an increase in morphological disparity. Radiations may affect one clade or many, and be rapid ...
, with members found across the supercontinent
Laurasia
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
. The
phylogenetic relationship
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 ...
s and constituent species of Paramacellodidae are uncertain.
Many studies regard them to be
scincomorphs, a large group that includes
skink
Skinks are lizards belonging to the family Scincidae, a family in the infraorder Scincomorpha. With more than 1,500 described species across 100 different taxonomic genera, the family Scincidae is one of the most diverse families of lizards. Ski ...
s and their closest extinct relatives, and possibly also to
Cordyoidea, a group that includes
spinytail lizards and relatives. Like modern skinks, paramacelloidids had rectangular bony plates called
osteoderm
Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amp ...
s covering most of their bodies, including their backs, undersides, and tails. They also had short and robust limbs.
Paramacellodids are distinguished from other lizards by the combination two traits in their dentition, the teeth are labiolingually expanded at their bases, and the tooth apices are lingually concave.
The family was named in 1983 to include two well-known genera, ''
Paramacellodus'' and ''
Becklesius'', from the
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987.
In European lithostratigraphy, the name ...
and Early Cretaceous of Europe. A third genus, ''
Sharovisaurus'', was named in 1984 from the Late Jurassic of Kazakhstan, and a fourth, ''
Mimobecklesisaurus'', in 1985 from the Late Jurassic of China. Remains of ''Paramacellodus'' were later described from the
Morrison Formation
The Morrison Formation is a distinctive sequence of Late Jurassic, Upper Jurassic sedimentary rock found in the western United States which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandsto ...
in Utah. Possible paramacellodid remains have also been found in Late Cretaceous deposits in Mongolia, as well as the Late Jurassic
Tendaguru Formation
The Tendaguru Formation, or Tendaguru Beds are a highly fossiliferous formation and Lagerstätte located in the Lindi Region of southeastern Tanzania. The formation represents the oldest sedimentary unit of the Mandawa Basin, overlying Neoproter ...
in Tanzania, which would indicate that the family was also present in
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
. In 2020, this was confirmed with the discovery of the first paramacellodid from Brazil, ''Neokotus sanfranciscanus'', from the Early Cretaceous Sanfranciscana Basin.
Three other early scincomorphs—''
Pseudosaurillus'', ''
Saurillodon'', and ''
Saurillus''—have also commonly been referred to Paramacellodidae, although some recent phylogenetic studies find them to be non-paramacellodid scincomorphs. Collectively, paramacellodids and taxa formerly referred to Paramacellodidae may represent a
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
grade
Grade most commonly refers to:
* Grade (education), a measurement of a student's performance
* Grade, the number of the year a student has reached in a given educational stage
* Grade (slope), the steepness of a slope
Grade or grading may also ref ...
of basal scincomorphs closely related to Scincoidea. In 2002, the newly named genus ''
Atokasaurus'' from the Early Cretaceous
Antlers Formation
The Antlers Formation is a stratum which ranges from Arkansas through southern Oklahoma into northeastern Texas. The stratum is thick consisting of silty to sandy mudstone and fine to coarse grained sandstone that is poorly to moderately sorted. ...
in Oklahoma was described as a "paramacellodid-grade" scincomorph to reflect this phylogenetic ambiguity.
[ In the Late Cretaceous, paramacellodids are only known from the European Archipelago.]
References
{{Taxonbar, from=Q16587035
Jurassic lizards
Cretaceous lizards
Lizard families
Prehistoric reptile families
Middle Jurassic reptiles
Late Jurassic reptiles
Early Cretaceous reptiles
Middle Jurassic first appearances
Early Cretaceous extinctions