''Triopticus'' is a genus of
archosauriform reptile
Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
from the
Late Triassic
The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
of
Texas
Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
,
United States
The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
. It contains a single species, ''Triopticus primus'', described in 2016 by Stocker ''et al''. It has an unusually domed head reminiscent of the later
pachycephalosauria
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachyceph ...
n
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s in an example of
convergent evolution
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
.
Description
The portion of the head that is preserved shows remarkable similarities to the specialized heads of
pachycephalosaurs
Pachycephalosauria (; from Ancient Greek, Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, m ...
. Five bosses, or rounded protuberances of bone are visible on the fossil, with one on the
frontal bone
In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...
and two pairs behind (respectively on the
postorbital bone
The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...
and the
squamosal
The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...
/
parietal bone
The parietal bones ( ) are two bones in the skull which, when joined at a fibrous joint known as a cranial suture, form the sides and roof of the neurocranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four bord ...
s) separated by a shallow groove. The backmost pair of bosses form a thick shelf that stretches outwards and backwards over the rear end of the skull, comparable to the domes of pachycephalosaurs and the frills of
ceratopsia
Ceratopsia or Ceratopia ( or ; Ancient Greek, Greek: "horned faces") is a group of herbivore, herbivorous, beaked dinosaurs that thrived in what are now North America, Asia and Europe, during the Cretaceous Period (geology), Period, although ance ...
ns.
The bosses have very roughly textured surfaces, suggesting that they would have been covered by keratin in life.
CT scan
A computed tomography scan (CT scan), formerly called computed axial tomography scan (CAT scan), is a medical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are called radiographers or ...
s show that the bosses internally have three distinct layers of bone, with a dense innermost layer which resembles the typical skull roof in texture, followed by a spongy, porous layer filled with blood vessels, and the least dense third layer close to the surface. Pachycephalosaurs like ''
Stegoceras
''Stegoceras'' is a genus of Pachycephalosauria, pachycephalosaurid (dome-headed) dinosaur that lived in what is now North America during the Late Cretaceous Period (geology), period, about 77.5 to 74 million years ago (mya). The first specim ...
'' also similarly have three layers of bone inside its dome,
but the second layer is the least dense as opposed to the third. However, this difference may arise from differential deposition of minerals in the skull.
The deep pit in the back of the skull of ''Triopticus'' likely arises from the two back pairs of bosses growing around this region, not unlike the holes of
doughnut
A doughnut or donut () is a type of pastry made from leavened fried dough. It is popular in many countries and is prepared in various forms as a sweet snack that can be homemade or purchased in bakeries, supermarkets, food stalls, and fran ...
s. It is also possible that the pit is an opening for the
pineal gland
The pineal gland (also known as the pineal body or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep, sleep patterns following the diurnal c ...
, as it is in approximately the right position, but this is less likely because no other archosauriforms have such an opening,
and the opening does not appear to be connected to any internal structures. The pit has the same texture as the bosses, suggesting that it was also potentially covered in keratin.
The eye socket is very large, and it is surrounded by
mineralized cartilage, a trait only seen otherwise in pachycephalosaurs. CT scans also revealed large
optic nerve
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual system, visual information from the retina to the brain. In humans, the optic nerve i ...
s as well as long
semicircular canals
The semicircular canals are three semicircular interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the lateral, anterior and posterior semicircular canals. They are the part of the bony labyrinth, ...
and a relatively large
cerebellar flocculus (both involved in stabilizing gaze,
although long semicircular canals are also associated with bipedal stances
). These traits suggest that ''Triopticus'' was a very visually-oriented animal. Otherwise, the structure of the brain is generally similar to other archosauriforms, although it superficially resembles that of ''Stegoceras''.
Despite the heavy similarity of ''Triopticus'' to pachycephalosaurs, there are a number of traits that indicate its archosauriform identity. In ''Triopticus'', the
parabasisphenoid bone is horizontal, the posttemporal fenestrae at the back of the braincase are not sealed off at all, and the region where the
squamosal bone
The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral ...
and
quadrate bone
The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids ( reptiles, birds), and early synapsids.
In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms up ...
come into contact is not externally visible, all of which indicate that it is not a basal dinosaur.
Additionally, a narrow strip of smooth bone on the
lacrimal bone
The lacrimal bones are two small and fragile bones of the facial skeleton; they are roughly the size of the little fingernail and situated at the front part of the medial wall of the orbit. They each have two surfaces and four borders. Several bon ...
in front of the eye sockets forms the margin of the
antorbital fenestra
An antorbital fenestra (plural: fenestrae) is an opening in the skull that is in front of the eye sockets. This skull character is largely associated with Archosauriformes, archosauriforms, first appearing during the Triassic Period. Among Extant ...
and the associated antorbital fossa, which are distinctly archosauriform characteristics.
Discovery and naming
The only known specimen of ''Triopticus'',
TMM 31100-1030, consists of a
braincase
In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
containing part of the
eye socket
In anatomy, the orbit is the cavity or socket/hole 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. In the adult human, the volume of the orbit is ...
. It was discovered in Quarry 3 of the Otis Chalk in
Howard County, Texas, which is part of the
Dockum Group. While the Otis Chalk itself has not yet been dated, comparisons with the animal fauna of other sites (including the
Pekin Formation of
North Carolina
North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...
) suggest a potential age between 229 Ma and 226 Ma.
The genus name of ''Triopticus'' is derived from the Latin ''tri'' ("three") and ''optic'' ("vision"), in reference to a large pit in the back of the skull reminiscent of a third eye socket. The species name, ''primus'', is the Latin word for "first".
Classification
A
phylogenetic analysis
In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
was conducted in 2016 in an attempt to elucidate the relationships of ''Triopticus''. The
phylogenetic tree
A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...
recovered by the analysis, reproduced below, found ''Triopticus'' in a
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 other
basal archosauriforms.
Paleoecology
In the Otis Chalk assemblage, ''Triopticus'' lived alongside a number of other archosauriforms including the
allokotosaurs ''
Trilophosaurus'' and ''
Malerisaurus''; the
doswelliid ''
Doswellia''; the
phytosaur
Phytosaurs (Φυτόσαυροι in Greek, meaning 'plant lizard') are an extinct group of large, mostly semiaquatic Late Triassic archosauriform or basal archosaurian reptiles. Phytosaurs belong to the order Phytosauria and are sometimes ref ...
s ''
Parasuchus
''Parasuchus'' is an extinct genus of basal phytosaur known from the Late Triassic (late Carnian to early Norian stage) of Andhra Pradesh and Madhya Pradesh, India. At its most restricted definition, ''Parasuchus'' contains a single species, '' ...
'' and ''
Angistorhinus''; the
aetosaur
Aetosaurs () are heavily armored reptiles belonging to the extinct order (biology), order Aetosauria (; from Ancient Greek, Greek, (aetos, "eagle") and (, "lizard")). They were medium- to large-sized Omnivore, omnivorous or Herbivore, herbivor ...
s ''
Longosuchus'', ''
Lucasuchus'', and ''
Coahomasuchus''; the
shuvosaurid ''
Effigia''; the
silesaurid ''
Silesaurus''; the
lagerpetid dinosauromorph ''
Dromomeron
''Dromomeron'' (meaning "running femur") is a genus of Lagerpetidae, lagerpetid Avemetatarsalia, avemetatarsalian which lived around 220 to 211.9 ± 0.7 million years ago. The genus contains species known from Late Triassic-age rocks of the South ...
'', and the
coelophysoid theropod
Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
dinosaur ''
Lepidus
Marcus Aemilius Lepidus (; 89 BC – late 13 or early 12 BC) was a Roman general and statesman who formed the Second Triumvirate alongside Octavian and Mark Antony during the final years of the Roman Republic. Lepidus had previously been ...
''. Some of these also exhibit evolutionary convergence upon archosaurs living after the
Triassic–Jurassic extinction event
The Triassic–Jurassic (Tr-J) extinction event (TJME), often called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, . It represents one of five major extinction events during the Phanerozoic, profoundly ...
- the phytosaurs resemble the
thalattosuchians,
gavialoids, and
spinosaurids, the aetosaurs resemble the
ankylosaurs
Ankylosauria is a group of herbivorous dinosaurs of the clade Ornithischia. It includes the great majority of dinosaurs with Armour (zoology), armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short ...
, and the shuvosaurids resemble the
ornithomimosaurs
Ornithomimosauria ("bird-mimic lizards") are theropod dinosaurs which bore a superficial resemblance to the modern-day ostrich. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period (geology), Period of Laurasia (now Asia ...
.
References
External links
An annotated 3D model of the ''Triopticus'' skull.A transparent 3D model of the ''Triopticus'' skull, showing its brain structure.
{{Taxonbar, from=Q28432240
Archosauriformes
Prehistoric reptile genera
Late Triassic reptiles of North America
Paleontology in Texas
Fossil taxa described in 2016