Tarjadia
Encyclopedia
Tarjadia is an extinct genus
of Triassic
archosauriform. It is known from a single species, T. ruthae, first described in 1998 from the Middle Triassic
Los Chañares Formation
in Argentina
. Partial remains have been found from deposits that are Anisian
-Ladinian
in age. The genus is known mostly from osteoderm
s, as well as several vertebrae and fragments of the skull. Tarjadia predates the Late Triassic
aetosaur
s and phytosaur
s, two early groups of crurotarsans with heavy plating, making it one of the first heavily-armored archosaurs. However, its classification within Archosauria is uncertain, as the known remains are fragmentary.
T. ruthae was named in honor of Ruth Romer, wife of the American
paleontologist Alfred Romer
, who led a Harvard expedition to Los Chañares in 1964 and 1965. Ruth went with Alfred on the expedition and was the first woman to work in the locality.
because of their large size and similar appearance to scutes of other rauisuchians. In 1990, more complete osteoderms were found from the formation, as well as associated vertebrae. Three years later, in Romer's collections at Harvard University
, remains of a skull were found in association with an osteoderm that was similar to the ones described in 1971 and 1990. This allowed Tarjadia to be erected as a new genus, distinct from Luperosuchus.
s, the most common material that has been found of the genus. The paramedian osteoderms, which overlie the back to either side of the midline, are thick and rectangular. Their medial edges are serrated, allowing the two rows to suture tightly together. Smaller, more rounded osteoderms are thought to have been placed to the sides of the paramedians, although no articulated remains bearing these lateral osteoderms have been found to prove this. Both the paramedian and lateral osteoderms are deeply pitted. The paramedian osteoderms are thickest at the center and medial edges, with spongy bone between the compact outer layers.
The bones of the skull table are very thick. Like the osteoderms, they are covered in coarse pitting. The surfaces of these bones also bear perforations for blood vessels, especially around the edges of the orbits
, or eye sockets. The parietal bone
s, which lie between two openings on the skull table called supratemporal fenestrae, have a distinctive sagittal crest
. On the underside of the parietals, there is a depression for the olfactory bulb
of the brain
, responsible for the perception of smell. An olfactory channel leads up to this depression and can be seen on the underside of the frontal bone
s.
The fragmentary occipital
area of the skull (the base of the skull) shows part of the boundary of the foramen magnum (through which the spinal cord
enters the skull), channels for the vestibular system
(part of the inner ear
responsible for balance), and holes for the semicircular canal (also part of the vestibular system). These holes and channels are found on the supraoccipital bone. The exoccipitals and opisthotics are also known in Tarjadia, and form the paraoccipital processes, two projections for the attachments of muscles that open the lower jaw. These processes form notches that may be tympanic fossae, cavities of the middle ear
.
The vertebrae of Tarjadia have centra, or central bodies, that are about as long as they are high. The centra have concave ventral surfaces and depressed lateral surfaces. The neural spines
that project upward from the centra are laterally compressed, but have distal portions that expand into a flattened table with a groove on its upper surface. Above the flat tables of the neural arches lie the paramedian osteoderms, which also form a flat surface. Thick transverse processes on some vertebrae suggest that they make up the sacrum
, the area of the spine that attaches to the pelvis
. The six vertebrae known from Tarjadia probably represent the posterior dorsals, sacrals, and first caudals, comprising the end of the back vertebrae and the beginning of the tail vertebrae.
and Euparkeria
, as well as the archosauriform family Proterochampsidae
, all have heavy armor over the dorsal vertebrae and existed around the same time as Tarjadia. Proterochampsids do not have ornamented osteoderms like Tarjadia, nor do they have two rows of osteoderms on either side of the back (most proterochampsids, with the exception of Cerritosaurus
and Chanaresuchus
, have only a single row on either side). While Euparkeria has a pair of osteoderms overlying each vertebra, similar to the condition seen in Tarjadia, its osteoderms aren't ornamented. Ornamented paired osteoderms are seen as derived
conditions in Doswellia and crurotarsan archosaurs. Some studies have not considered Tarjadia to be a close relative of Doswellia because of differences in the structure of the vertebrae. Moreover, Tarjadia possesses a prefrontal bone
in the skull which is absent in Doswellia.
Among crurotarsans, vertebrae that are each overlain by a single row of pitted, paired osteoderms as in Tarjadia are seen in aetosaurs, phytosaurs, and Crocodylomorpha
. Tarjadia has been distinguished from aetosaurs by its apparent lack of an anterior articular lamina (a depressed region along the front of each osteoderm), and clear differences in the skull tables. Tarjadia differs from sphenosuchia
ns and proterosuchia
ns, the two main Triassic crocodylomorph groups, in that its osteoderms lack any clear structures on the anterior edges of the osteoderms. In the case of sphenosuchians, the anterior edge of the osteoderm forms a process or "lappet", while in protosuchians, the anterior edge has a depressed band similar to those of aetosaurs. Out of all Triassic crurotarsans, the osteoderms of Tarjadia bear the closest resemblance to those of phytosaurs; they have a similar shape and are also heavily pitted. Moreover, the skull roof of is also pitted in phytosaurs. However, Tarjadia can be distinguished from all phytosaurs in that it has differently shaped parietal bones. The strong sagittal crest on the parietals of Tarjadia is not seen in any phytosaur.
When it was first erected in 2011, the family Doswelliidae
was proposed to include Doswellia, Archeopelta
, and Tarjadia. Synapomorphies, or unique features of the group, include coarsely pitted and incised osteoderms and an anterior articular lamina.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
archosauriform. It is known from a single species, T. ruthae, first described in 1998 from the Middle Triassic
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided. It spans the time between 245 ± 1.5 Ma and 228 ± 2 Ma...
Los Chañares Formation
Chañares Formation
The Chañares Formation in La Rioja Provence, Argentina, dates to the Anisian stage of the early Middle Triassic.Some localities are well known for their abundance of tetrapods. Theraspids include the kannemeyeriid Dinodontosaurus, and cynodonts such as Probainognathus and Massetognathus, the latter...
in Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. Partial remains have been found from deposits that are Anisian
Anisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from 245 million years ago until 237 million years ago, approximately...
-Ladinian
Ladinian
The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between 237 ± 2 Ma and 228 ± 2 Ma...
in age. The genus is known mostly from osteoderm
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
s, as well as several vertebrae and fragments of the skull. Tarjadia predates the Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...
aetosaur
Aetosaur
Aetosaurs are an extinct order of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. They have small heads, upturned snouts, erect limbs, and a body covered by plate-like scutes. All aetosaurs belong to the family Stagonolepididae...
s and phytosaur
Phytosaur
Phytosaurs are an extinct group of large semi-aquatic Late Triassic archosaurs. Phytosaurs belong to the family Phytosauridae and the order Phytosauria. They were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodiles in size, appearance, and lifestyle, an example...
s, two early groups of crurotarsans with heavy plating, making it one of the first heavily-armored archosaurs. However, its classification within Archosauria is uncertain, as the known remains are fragmentary.
Etymology
The genus name Tarjadia is derived from Sierra de los Tarjados, the closest mountain range to the outcrops of the Los Chañares Formation where remains have been found. The type speciesType species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
T. ruthae was named in honor of Ruth Romer, wife of the American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
paleontologist Alfred Romer
Alfred Romer
Alfred Sherwood Romer was an American paleontologist and comparative anatomist and a specialist in vertebrate evolution.-Biography:...
, who led a Harvard expedition to Los Chañares in 1964 and 1965. Ruth went with Alfred on the expedition and was the first woman to work in the locality.
History
The first remains of Tarjadia from the Los Chañares Formation were mentioned by Alfred Romer in 1971. He recognized two types of osteoderms from the formation, which he tentatively attributed to the rauisuchian LuperosuchusLuperosuchus
Luperosuchus is an extinct genus of crurotarsan, a group which includes crocodilians and their ancestors. It lived during the Anisian stage of the Middle Triassic, and is known from Argentina.-References:* in the Paleobiology Database...
because of their large size and similar appearance to scutes of other rauisuchians. In 1990, more complete osteoderms were found from the formation, as well as associated vertebrae. Three years later, in Romer's collections at Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
, remains of a skull were found in association with an osteoderm that was similar to the ones described in 1971 and 1990. This allowed Tarjadia to be erected as a new genus, distinct from Luperosuchus.
Description
Tarjadia has been diagnosed on the basis of its osteoderms, or bony scuteScute
A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...
s, the most common material that has been found of the genus. The paramedian osteoderms, which overlie the back to either side of the midline, are thick and rectangular. Their medial edges are serrated, allowing the two rows to suture tightly together. Smaller, more rounded osteoderms are thought to have been placed to the sides of the paramedians, although no articulated remains bearing these lateral osteoderms have been found to prove this. Both the paramedian and lateral osteoderms are deeply pitted. The paramedian osteoderms are thickest at the center and medial edges, with spongy bone between the compact outer layers.
The bones of the skull table are very thick. Like the osteoderms, they are covered in coarse pitting. The surfaces of these bones also bear perforations for blood vessels, especially around the edges of the orbits
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
, or eye sockets. The parietal bone
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
s, which lie between two openings on the skull table called supratemporal fenestrae, have a distinctive sagittal crest
Sagittal crest
A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull of many mammalian and reptilian skulls, among others....
. On the underside of the parietals, there is a depression for the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...
of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
, responsible for the perception of smell. An olfactory channel leads up to this depression and can be seen on the underside of the frontal bone
Frontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
s.
The fragmentary occipital
Occipital
The word occipital, in zoology, pertains to the occiput .Occipital is a descriptor for several areas of animal & human anatomy.*External occipital protuberance* Internal occipital crest* Greater occipital nerve...
area of the skull (the base of the skull) shows part of the boundary of the foramen magnum (through which the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
enters the skull), channels for the vestibular system
Vestibular system
The vestibular system, which contributes to balance in most mammals and to the sense of spatial orientation, is the sensory system that provides the leading contribution about movement and sense of balance. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of...
(part of the inner ear
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...
responsible for balance), and holes for the semicircular canal (also part of the vestibular system). These holes and channels are found on the supraoccipital bone. The exoccipitals and opisthotics are also known in Tarjadia, and form the paraoccipital processes, two projections for the attachments of muscles that open the lower jaw. These processes form notches that may be tympanic fossae, cavities of the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
.
The vertebrae of Tarjadia have centra, or central bodies, that are about as long as they are high. The centra have concave ventral surfaces and depressed lateral surfaces. The neural spines
Spinous process
The spinous process of a vertebra is directed backward and downward from the junction of the laminae , and serves for the attachment of muscles and ligaments. In animals without an erect stance, the process points upward and may slant forward or backward...
that project upward from the centra are laterally compressed, but have distal portions that expand into a flattened table with a groove on its upper surface. Above the flat tables of the neural arches lie the paramedian osteoderms, which also form a flat surface. Thick transverse processes on some vertebrae suggest that they make up the sacrum
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
, the area of the spine that attaches to the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
. The six vertebrae known from Tarjadia probably represent the posterior dorsals, sacrals, and first caudals, comprising the end of the back vertebrae and the beginning of the tail vertebrae.
Classification
Tarjadia has rarely been given a specific phylogenetic placement within Archosauria. Comparisons between it and other Triassic archosaurs were made in its initial 1998 description. The archosauriforms DoswelliaDoswellia
Doswellia is an extinct genus of archosauriform. It has been placed in its own family, Doswellidae, and its own suborder, Doswelliina. It was a terrestrial carnivore that existed in eastern North America during the Carnian stage of the Late Triassic. Fossils have been found from the Poor Farm...
and Euparkeria
Euparkeria
Euparkeria was a small African reptile of the early Triassic period between 248-245 million years ago, close to the ancestry of the archosaurs.- Palaeobiology :...
, as well as the archosauriform family Proterochampsidae
Proterochampsidae
Proterochampsidae is a family of archosauriforms. Proterochampsids may have filled an ecological niche similar to modern crocodiles, and had a general crocodile-like appearance. They lived in what is now South America in the Middle and Late Triassic....
, all have heavy armor over the dorsal vertebrae and existed around the same time as Tarjadia. Proterochampsids do not have ornamented osteoderms like Tarjadia, nor do they have two rows of osteoderms on either side of the back (most proterochampsids, with the exception of Cerritosaurus
Cerritosaurus
Cerritosaurus is a genus of proterochampsid archosauromorph from the Upper Triassic. It has been found in the Santa Maria Formation, in the Geopark of Paleorrota. It is represented by one species....
and Chanaresuchus
Chanaresuchus
Chanaresuchus is an extinct genus of proterochampsian archosauriform. It was of modest size for a proterochampsian, being on average just over a meter in length. Fossils have been found from the Chañares Formation in La Rioja Provence, Argentina, dating back to the Ladinian stage of the Middle...
, have only a single row on either side). While Euparkeria has a pair of osteoderms overlying each vertebra, similar to the condition seen in Tarjadia, its osteoderms aren't ornamented. Ornamented paired osteoderms are seen as derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
conditions in Doswellia and crurotarsan archosaurs. Some studies have not considered Tarjadia to be a close relative of Doswellia because of differences in the structure of the vertebrae. Moreover, Tarjadia possesses a prefrontal bone
Prefrontal bone
The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern and extinct lungfish, amphibians and reptiles...
in the skull which is absent in Doswellia.
Among crurotarsans, vertebrae that are each overlain by a single row of pitted, paired osteoderms as in Tarjadia are seen in aetosaurs, phytosaurs, and Crocodylomorpha
Crocodylomorpha
The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were...
. Tarjadia has been distinguished from aetosaurs by its apparent lack of an anterior articular lamina (a depressed region along the front of each osteoderm), and clear differences in the skull tables. Tarjadia differs from sphenosuchia
Sphenosuchia
Sphenosuchia is a suborder of basal crocodylomorphs that first appeared in the Triassic and occurred into the Late Jurassic. Most were small, gracile animals with an erect limb posture. They are now thought to be ancestral to crocodyliforms, which include all living crocodilians.-Stratigraphic...
ns and proterosuchia
Proterosuchia
Proterosuchia refers to one of the suborders of Thecodontia; specifically the most primitive and ancestral forms. These were primitive, vaguely crocodile-like, archosaurs that mostly lived during the Early Triassic period....
ns, the two main Triassic crocodylomorph groups, in that its osteoderms lack any clear structures on the anterior edges of the osteoderms. In the case of sphenosuchians, the anterior edge of the osteoderm forms a process or "lappet", while in protosuchians, the anterior edge has a depressed band similar to those of aetosaurs. Out of all Triassic crurotarsans, the osteoderms of Tarjadia bear the closest resemblance to those of phytosaurs; they have a similar shape and are also heavily pitted. Moreover, the skull roof of is also pitted in phytosaurs. However, Tarjadia can be distinguished from all phytosaurs in that it has differently shaped parietal bones. The strong sagittal crest on the parietals of Tarjadia is not seen in any phytosaur.
When it was first erected in 2011, the family Doswelliidae
Doswelliidae
Doswelliidae is a family of carnivore archosauriforms. Doswelliids existed in North and South America during the Middle–Late Triassic period and were among the most derived non-archosaurian archosauriforms. The family was named by R. E. Weems in 1980 and it has been placed in its own suborder,...
was proposed to include Doswellia, Archeopelta
Archeopelta
Archeopelta is an extinct genus of carnivorous archosauriform from the late Middle or early Late Triassic period . It is a doswelliid which lived in what is now southern Brazil. It is known from the holotype CPEZ-239a, which consists of partial skeleton and braincase...
, and Tarjadia. Synapomorphies, or unique features of the group, include coarsely pitted and incised osteoderms and an anterior articular lamina.
External links
- Tarjadia in the Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...