Avicranium

Last updated

Avicranium
Temporal range: Late NorianRhaetian
~213–203  Ma
Avicranium skull views.jpg
A 3D reconstruction of Avicranium's skull in multiple views.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Drepanosauromorpha
Family: Drepanosauridae
Genus: Avicranium
Pritchard & Nesbitt, 2017
Type species
Avicranium renestoi
Pritchard & Nesbitt, 2017

Avicranium is a genus of extinct drepanosaur reptile known from the Chinle Formation of the late Triassic. The type species of Avicranium is Avicranium renestoi. "Avicranium" is Latin for "bird cranium", in reference to its unusual bird-like skull, while "renestoi" references Silvio Renesto, a paleontologist known for studies of Italian drepanosaurs. [1]

Contents

AMNH FARB 30834, mostly obscured by rock. Avicranium fossil.jpg
AMNH FARB 30834, mostly obscured by rock.

Discovery

The holotype and only known specimen of Avicranium is AMNH FARB 30834, a disarticulated skull attached to a few cervical (neck) vertebrae. This specimen hails from the Coelophysis (or Whitaker) quarry at Ghost Ranch in New Mexico, USA. This quarry belongs to the 'siltstone member' of the Chinle formation, which corresponds to the late Norian to early Rhaetian stages of the Triassic. Various unprepared blocks from this locality were excavated by American Museum of Natural History field parties during the 1940s. Long believed to only contain multiple specimens of the early dinosaur Coelophysis, these blocks are now known to contain a multitude of different Triassic reptiles. [2] AMNH FARB 30834 was uncovered during the preparation of a block containing the holotype of Effigia okeeffeae , a large shuvosaurid pseudosuchian. Multiple other drepanosaur vertebrae and limb fragments have also been found in the block, although it is unclear whether they belong to the same specimen as AMNH FARB 308344. [1]

Despite AMNH FARB 30834 being disarticulated and partially covered by the rock matrix, many of its bones are well-preserved and the specimen as a whole was micro-CT scanned, revealing elements hidden under rock. This scan allowed the skull to be reconstructed in 3D using digital software. An endocast of the brain was also digitally constructed from the skull. Avicranium has the most well-preserved skull material of any drepanosaur. [1]

Description

The micro-CT scan of AMNH FARB 30834 Avicranium 3D scan.jpg
The micro-CT scan of AMNH FARB 30834

AMNH FARB 30834 can be recognized as a drepanosaur due to several traits of its vertebrae. The articular surfaces of its vertebral centres are heterocoelous (saddle-shaped). The neural spines are anteroposteriorly short and strongly anterodorsally inclined. These features closely resemble those of Drepanosaurus .

Avicranium skull drawing.jpg
Avicranium reconstruction Buffa 2024.jpg
Drawings of the skull of Avicranium as reconstructed by Pritchard & Nesbitt (2017), top, and Buffa et al. (2024), bottom.

However, the most notable feature of AMNH FARB 30834 is its skull, which as initially reconstructed by Pritchard and Nesbitt shares many similarities with the skulls of modern birds. The snout is slender and tapering, similar to that of Megalancosaurus , although it differs from Megalancosaurus by being completely toothless. The frontal, postfrontal, and postorbital bones (above and behind the eyes) project outwards, forcing the orbits to face forwards. The frontal and parietal bones are both broad and thick, creating a high and domed cranium. According to the digital endocast of the specimen, this domed cranium held an enlarged brain with large optic lobes. [1]

On the other hand, the skull also possesses many primitive features. The squamosal is tall and anteroposteriorly broad, and possesses bony plates which frame the quadrate from the side and rear. This is similar to the condition in captorhinids and Araeoscelis , but unlike that of neodiapsids, which only have the quadrate framed from the side. The quadrate itself is also stouter than that of neodiapsids and lacks a concave rear edge and a dorsal extension which articulates with the squamosal. The occipital condyle of the braincase possesses a deep pit, while the foramen ovale is very large. [1]

The stapes is massive, similar to the condition in other early reptiles. Combined with a straight quadrate without a tympanic crest, this indicates that Avicranium (and presumably other drepanosaurs) lacked a tympanic membrane (eardrum), and that the drepanosauromorph lineage evolved before more advanced diapsids (such as saurians) acquired tympanic membranes. [1]

Avicranium can be distinguished from Hypuronector , Megalancosaurus, and Vallesaurus (the only other drepanosaurs with preserved skulls) due to the following features:

An alternative reconstruction of the skull was proposed in 2024 by Valentin Buffa and colleagues, who reidentified many of the bones in Pritchard and Nesbitt's original reconstruction. Most notably, they did not identify any of the preserved bones as those of the rostrum and so could not confirm if Avicranium was indeed toothless. They also reinterpreted several of the bones around the eyes, namely identifying the maxilla as a jugal and the former jugal as a lacrimal. This noticeably decreases the size of the orbit from the original reconstruction. They also altered the position of the postorbital, placing it more behind the postfrontal than laterally, decreasing the overall width of the rear skull. This correspondingly pushes back the squamosal and quadrate and creates a steeper incline down to the jaw joint. [3]

Paleobiology

The binocular eyes and enlarged brain of Avicranium are likely adaptations to an arboreal lifestyle, as such a lifestyle requires advanced sensory processing, such as depth perception and three-dimensional navigation. It is very likely that these features convergently evolved with those of pterosaurs and modern birds, which would require similar sensory processing during flight. An arboreal lifestyle has been previously proposed for other members of the drepanosauromorph lineage. [1]

Related Research Articles

<i>Protoavis</i> Extinct genus of reptiles

Protoavis is a problematic taxon known from fragmentary remains from Late Triassic Norian stage deposits near Post, Texas. The animal's true classification has been the subject of much controversy, and there are many different interpretations of what the taxon actually is. When it was first described, the fossils were described as being from a primitive bird which, if the identification is valid, would push back avian origins some 60-75 million years.

The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians.

<span class="mw-page-title-main">Archosauromorpha</span> Infraclass of reptiles

Archosauromorpha is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs rather than lepidosaurs. Archosauromorphs first appeared during the late Middle Permian or Late Permian, though they became much more common and diverse during the Triassic period.

<span class="mw-page-title-main">Neodiapsida</span> Clade of reptiles

Neodiapsida is a clade, or major branch, of the reptilian family tree, typically defined as including all diapsids apart from some early primitive types known as the araeoscelidians. Modern reptiles and birds belong to the neodiapsid subclade Sauria.

<span class="mw-page-title-main">Avicephala</span> Extinct clade of neodiapsid reptiles

Avicephala is a potentially polyphyletic grouping of extinct diapsid reptiles that lived during the Late Permian and Triassic periods characterised by superficially bird-like skulls and arboreal lifestyles. As a clade, Avicephala is defined as including the gliding weigeltisaurids and the arboreal drepanosaurs to the exclusion of other major diapsid groups. This relationship is not recovered in the majority of phylogenetic analyses of early diapsids and so Avicephala is typically regarded as an artificial or unnatural grouping. However, the clade was recovered again in 2021 following a redescription of Weigeltisaurus, raising the possibility that the clade may be valid after all, although subsequent analyses have not supported this result.

<i>Hovasaurus</i> Extinct genus of reptiles

Hovasaurus is an extinct genus of basal diapsid reptile. It lived in what is now Madagascar during the Late Permian and Early Triassic, being a survivor of the Permian–Triassic extinction event and the paleontologically youngest member of the Tangasauridae. Fossils have been found in the Permian Lower and Triassic Middle Sakamena Formations of the Sakamena Group, where it is amongst the commonest fossils. Its morphology suggests an aquatic ecology.

<i>Megalancosaurus</i> Extinct genus of reptiles

Megalancosaurus is a genus of extinct reptile from the Late Triassic Dolomia di Forni Formation and Zorzino Limestone of northern Italy, and one of the best known drepanosaurids. The type species is M. preonensis; a translation of the animal's scientific name would be "long armed reptile from the Preone Valley."

<i>Drepanosaurus</i> Extinct genus of reptiles

Drepanosaurus is a genus of arboreal (tree-dwelling) reptile that lived during the Triassic Period. It is a member of the Drepanosauridae, a group of diapsid reptiles known for their prehensile tails. Drepanosaurus was probably an insectivore, and lived in a coastal environment in what is now modern day Italy, as well as in a streamside environment in the midwestern United States.

<span class="mw-page-title-main">Drepanosaur</span> Extinct clade of reptiles

Drepanosaurs are a group of extinct reptiles that lived between the Carnian and Rhaetian stages of the late Triassic Period, approximately between 230 and 210 million years ago. The various species of drepanosaurid were characterized by specialized grasping limbs and often prehensile tails, adaptions for arboreal (tree-dwelling) and fossorial (digging) lifestyles, with some having also been suggested to be aquatic. Fossils of drepanosaurs have been found in Arizona, New Mexico, New Jersey, Utah, England, and northern Italy. The name is taken from the family's namesake genus Drepanosaurus, which means "sickle lizard," a reference to their strongly curved claws.

<span class="mw-page-title-main">Kuehneosauridae</span> Extinct family of reptiles

Kuehneosauridae is an extinct family of small, lizard-like gliding diapsids known from the Triassic period of Europe and North America.

<i>Vancleavea</i> Extinct genus of reptiles

Vancleavea is a genus of extinct, armoured, non-archosaurian archosauriforms from the Late Triassic of western North America. The type and only known species is V. campi, named by Robert Long & Phillip A Murry in 1995. At that time, the genus was only known from fragmentary bones including osteoderms and vertebrae. However, since then many more fossils have been found, including a pair of nearly complete skeletons discovered in 2002. These finds have shown that members of the genus were bizarre semiaquatic reptiles. Vancleavea individuals had short snouts with large, fang-like teeth, and long bodies with small limbs. They were completely covered with bony plates known as osteoderms, which came in several different varieties distributed around the body. Phylogenetic analyses by professional paleontologists have shown that Vancleavea was an archosauriform, part of the lineage of reptiles that would lead to archosaurs such as dinosaurs and crocodilians. Vancleavea lacks certain traits which are present in most other archosauriforms, most notably the antorbital, mandibular and supratemporal fenestrae, which are weight-saving holes in the skulls of other taxa. However, other features clearly support its archosauriform identity, including a lack of intercentra, the presence of osteoderms, an ossified laterosphenoid, and several adaptations of the femur and ankle bones. In 2016, a new genus of archosauriform, Litorosuchus, was described. This genus resembled both Vancleavea and more typical archosauriforms in different respects, allowing Litorosuchus to act as a transitional fossil linking Vancleavea to less aberrant archosauriforms.

<i>Daemonosaurus</i> Genus of reptiles (fossil)

Daemonosaurus is an extinct genus of possible theropod dinosaur from the Late Triassic of New Mexico. The only known fossil is a skull and neck fragments from deposits of the latest Triassic Chinle Formation at Ghost Ranch. Daemonosaurus was an unusual dinosaur with a short skull and large, fang-like teeth. It lived alongside early neotheropods such as Coelophysis, which would have been among the most common dinosaurs by the end of the Triassic. However, Daemonosaurus retains several plesiomorphic ("primitive") traits of the snout, and it likely lies outside the clade Neotheropoda. It may be considered a late-surviving basal theropod or non-theropod basal saurischian, possibly allied to other early predatory dinosaurs such as herrerasaurids or Tawa.

<i>Cosesaurus</i> Extinct genus of reptiles

Cosesaurus is a genus of archosauromorph reptiles likely belonging to the family Tanystropheidae. It is known from fossil imprints of a single small skeleton, MGB V1, which was found in Muschelkalk outcrops near the municipalities of Mont-ral and Alcover in Spain. These outcrops are dated to the Ladinian age of the middle Triassic about 242 to 237 million years ago. The specimen is stored at the Museu Martorell, which is now part of the Museu de Ciències Naturals de Barcelona. The poor preservation and likely juvenile nature of the specimen has led to the anatomy of Cosesaurus being misidentified by several different sources. For example, Paul Ellenberger claimed that it was an ancestor to birds in the 1970s, while David Peters claimed that it was a pterosaur ancestor in 2000. Both of these claims contrast with mainstream scientific theories on the origins of either group, and other paleontologists who study the specimen are unable to find the features which Ellenberger or Peters reported to be present. The Ellenberger and Peters hypotheses are thus considered fringe theories with questionable scientific soundness due to their low reproducibility. Mainstream hypotheses for the relations of Cosesaurus generally agree that it is a "protorosaur", specifically a tanystropheid closely related to long-necked reptiles such as Macrocnemus, Tanytrachelos, Tanystropheus, or Langobardisaurus.

<i>Jesairosaurus</i> Extinct genus of reptiles

Jesairosaurus is an extinct genus of early archosauromorph reptile known from the Illizi Province of Algeria. It is known from a single species, Jesairosaurus lehmani. Although a potential relative of the long-necked tanystropheids, this lightly-built reptile could instead be characterized by its relatively short neck as well as various skull features.

<span class="mw-page-title-main">Protorosauria</span> Extinct order of reptiles

Protorosauria is an extinct, likely paraphyletic group of basal archosauromorph reptiles from the latest Middle Permian to the end of the Late Triassic of Asia, Europe and North America. It was named by the English anatomist and paleontologist Thomas Henry Huxley in 1871 as an order, originally to solely contain Protorosaurus. Other names which were once considered equivalent to Protorosauria include Prolacertiformes and Prolacertilia.

<i>Palatodonta</i> Extinct genus of reptiles

Palatodonta is an extinct genus of neodiapsid reptile known from the early Middle Triassic of the Netherlands. It was initially described in 2013 as a basal placodontiform closely related to a group of marine reptiles called placodonts, characterized by their crushing teeth and shell-like body armor. Under this interpretation, Palatodonta is transitional between placodonts and less specialized reptiles. Like placodonts, it has a row of large teeth on its palate, but while these teeth are thick and blunt in placodonts, Palatodonta has palatal teeth that are thin and pointed. A 2023 study instead classified it as a sauropterygomorph and the sister taxon to Eusaurosphargis. In other words, it is close to, but not within, Sauropterygia.

<i>Pamelaria</i> Extinct genus of reptiles

Pamelaria is an extinct genus of allokotosaurian archosauromorph reptile known from a single species, Pamelaria dolichotrachela, from the Middle Triassic of India. Pamelaria has sprawling legs, a long neck, and a pointed skull with nostrils positioned at the very tip of the snout. Among early archosauromorphs, Pamelaria is most similar to Prolacerta from the Early Triassic of South Africa and Antarctica. Both have been placed in the family Prolacertidae. Pamelaria, Prolacerta, and various other Permo-Triassic reptiles such as Protorosaurus and Tanystropheus have often been placed in a group of archosauromorphs called Protorosauria, which was regarded as one of the most basal group of archosauromorphs. However, more recent phylogenetic analyses indicate that Pamelaria and Prolacerta are more closely related to Archosauriformes than are Protorosaurus, Tanystropheus, and other protorosaurs, making Protorosauria a polyphyletic grouping.

<span class="mw-page-title-main">Allokotosauria</span> Extinct clade of reptiles

Allokotosauria is a clade of early archosauromorph reptiles from the Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of specialized herbivorous archosauromorphs was recovered by Sterling J. Nesbitt, John J. Flynn, Adam C. Pritchard, J. Michael Parrish, Lovasoa Ranivoharimanana and André R. Wyss in 2015. The name Allokotosauria is derived from Greek meaning "strange reptiles" in reference to unexpected grouping of early archosauromorph with a high disparity of features typically associated with herbivory.

<span class="mw-page-title-main">Azendohsauridae</span> Extinct family of reptiles

Azendohsauridae is a family of allokotosaurian archosauromorphs that lived during the Middle to Late Triassic period, around 242-216 million years ago. The family was originally named solely for the eponymous Azendohsaurus, marking out its distinctiveness from other allokotosaurs, but as of 2022 the family now includes four other genera: the basal genus Pamelaria, the large horned herbivore Shringasaurus, and two carnivorous genera grouped into the subfamily-level subclade Malerisaurinae, Malerisaurus and Puercosuchus, and potentially also the dubious genus Otischalkia. Most fossils of azendohsaurids have a Gondwanan distribution, with multiple species known across Morocco and Madagascar in Africa as well as India, although fossils of malerisaurine azendohsaurids have also been found in the southwestern United States of North America.

Boreopricea is an extinct genus of archosauromorph reptile from the Early Triassic of arctic Russia. It is known from a fairly complete skeleton discovered in a borehole on Kolguyev Island, though damage to the specimen and loss of certain bones has complicated study of the genus. Boreopricea shared many similarities with various other archosauromorphs, making its classification controversial. Various studies have considered it a close relative of Prolacerta, tanystropheids, both, or neither. Boreopricea is unique among early archosauromorphs due to possessing contact between the jugal and squamosal bones at the rear half of the skull.

References

  1. 1 2 3 4 5 6 7 Pritchard, Adam C.; Nesbitt, Sterling J. (2017-10-01). "A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida". Royal Society Open Science. 4 (10): 170499. Bibcode:2017RSOS....470499P. doi:10.1098/rsos.170499. ISSN   2054-5703. PMC   5666248 . PMID   29134065.
  2. Nesbitt, Sterling J; Norell, Mark A (2006-05-07). "Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda)". Proceedings of the Royal Society B: Biological Sciences. 273 (1590): 1045–1048. doi:10.1098/rspb.2005.3426. ISSN   0962-8452. PMC   1560254 . PMID   16600879.
  3. Buffa, V.; Frey, E.; Steyer, J.-S.; Laurin, M. (2024). "'Birds' of two feathers: Avicranium renestoi and the paraphyly of bird-headed reptiles (Diapsida: 'Avicephala')". Zoological Journal of the Linnean Society. zlae050. doi: 10.1093/zoolinnean/zlae050 .