Arachana
Encyclopedia
Arachana is an extinct genus
of rhinesuchid-like temnospondyl known from the Late Permian or Early Triassic
of northeastern Uruguay
. Arachana was first named by Graciela Piñeiro, Alejandro Ramos and Claudia Marsicano in 2011
and the type species
is A. nigra. It shares characteristics with both rhinesuchids and lydekkerinids, making it a transitional form between basal and more advanced stereospondyls.
FC-DPV 1369, a three-dimensionally preserved nearly complete skull
that has not been distorted. It was collected in the Colonia Orozco locality from the Buena Vista Formation of the Norte Basin. The formation spans the boundary between the Permian
and the Triassic
, and it is uncertain whether FC-DPV 1369 was found in rocks above or below the boundary.
Arachana shares several characters with rhinesuchids. For example, it has a large head with orbits
or eye sockets placed slightly behind the mid-length of the skull. The surface of the skull is deeply pitted and the margins of the cheek regions are smooth and convex. The basioccipital bone at the back of the skull is small. On the palate
, the pterygoid bone is connected to the vomer
by a projection called the palatal ramus. The ramus separates the interpterygoid vacuities, two large holes in the palate. In many temnospondyls, the palatine
and ectopterygoid bones also touch the vacuities, but in Arachana and rhinesuchids, the palatine ramus excludes these bones from the vacuity margins.
Arachana also has characteristics of a more derived
group of stereospondyls called lydekkerinids. In both Arachana and lydekkerinids, the otic notch
in the back of the skull is shallow and is not touched by the supratemporal bone as is otherwise the case in temnospondyls. Sensory sulci
, grooves in the skull that held a lateral line
system in life, run across the lacrimal bone
but are not angled sharply as in other temnospondyls. On the palates of both Arachana and lydekkerinids, rows of palatine teeth are relatively small and the main body of the pterygoid bone is covered in small projections. None of these features are present in rhinesuchids, suggesting that Arachana was a transitional form between basal stereospondyls like rhinesuchids and more advanced forms like lydekkerinids.
The transitional features of Arachana place it as part of an entire transitional fauna that existed around the Permo-Triassic boundary. While it is uncertain whether Arachana lived before or after the Permian-Triassic extinction event
, its transitional position shows that some temnospondyls were diversifying rather than diminishing across the boundary.
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 rhinesuchid-like temnospondyl known from the Late Permian or Early Triassic
Early Triassic
The Early Triassic is the first of three epochs of the Triassic period of the geologic timescale. It spans the time between 251 ± 0.4 Ma and 245 ± 1.5 Ma . Rocks from this epoch are collectively known as the Lower Triassic, which is a unit in chronostratigraphy...
of northeastern Uruguay
Uruguay
Uruguay ,officially the Oriental Republic of Uruguay,sometimes the Eastern Republic of Uruguay; ) is a country in the southeastern part of South America. It is home to some 3.5 million people, of whom 1.8 million live in the capital Montevideo and its metropolitan area...
. Arachana was first named by Graciela Piñeiro, Alejandro Ramos and Claudia Marsicano in 2011
2011 in paleontology
-Lobopods:-Arachnids:-Insects:-Newly named jawless vertebrates:-Newly named acanthodian:-Newly named cartilaginous fishes:-Newly named bony fishes:-Newly named lepospondyls:-Newly named temnospondyls:-Newly named lissamphibians:...
and the type species
Type 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...
is A. nigra. It shares characteristics with both rhinesuchids and lydekkerinids, making it a transitional form between basal and more advanced stereospondyls.
Description
Arachana is known from the holotypeHolotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
FC-DPV 1369, a three-dimensionally preserved nearly complete skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
that has not been distorted. It was collected in the Colonia Orozco locality from the Buena Vista Formation of the Norte Basin. The formation spans the boundary between the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
and the 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...
, and it is uncertain whether FC-DPV 1369 was found in rocks above or below the boundary.
Arachana shares several characters with rhinesuchids. For example, it has a large head with 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 placed slightly behind the mid-length of the skull. The surface of the skull is deeply pitted and the margins of the cheek regions are smooth and convex. The basioccipital bone at the back of the skull is small. On the palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
, the pterygoid bone is connected to the vomer
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...
by a projection called the palatal ramus. The ramus separates the interpterygoid vacuities, two large holes in the palate. In many temnospondyls, the palatine
Palatine bone
The palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...
and ectopterygoid bones also touch the vacuities, but in Arachana and rhinesuchids, the palatine ramus excludes these bones from the vacuity margins.
Arachana also has characteristics of a more 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...
group of stereospondyls called lydekkerinids. In both Arachana and lydekkerinids, the otic notch
Otic notch
Otic notches are invagination in the posterior margin of the skull roof, one behind each orbit. Such notches are found in labyrinthodonts and some of their immediate ancestors, but not their reptilian descendants...
in the back of the skull is shallow and is not touched by the supratemporal bone as is otherwise the case in temnospondyls. Sensory sulci
Sulci
This article refers to Sulci the city. For the anatomical feature, see Sulcus .Sulci |Paus.]]), was one of the most considerable cities of ancient Sardinia, situated in the southwest corner of the island, on a small island, now called Isola di Sant'Antioco, which is, however, joined to the mainland...
, grooves in the skull that held a lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...
system in life, run across the lacrimal bone
Lacrimal bone
The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.-Lateral or orbital surface:...
but are not angled sharply as in other temnospondyls. On the palates of both Arachana and lydekkerinids, rows of palatine teeth are relatively small and the main body of the pterygoid bone is covered in small projections. None of these features are present in rhinesuchids, suggesting that Arachana was a transitional form between basal stereospondyls like rhinesuchids and more advanced forms like lydekkerinids.
The transitional features of Arachana place it as part of an entire transitional fauna that existed around the Permo-Triassic boundary. While it is uncertain whether Arachana lived before or after the Permian-Triassic extinction event
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...
, its transitional position shows that some temnospondyls were diversifying rather than diminishing across the boundary.