| Incertovenator | |
|---|---|
 | |
| Axis and third cervical of the holotype ofIncertovenator (PVSJ 397) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Eucrocopoda |
| Clade: | Archosauria (?) |
| Genus: | †Incertovenator Yáñez et al.,2021 |
| Species: | †I. longicollum |
| Binomial name | |
| †Incertovenator longicollum Yáñez et al., 2021 | |
Incertovenator (meaning "uncertain hunter") is anextinctgenus ofarchosauriform reptile, likely anarchosaur, of uncertain affinities. Its unstable position is a result of possessing a number features found in both thebird-lineavemetatarsalian archosaurs and thecrocodylian-linepseudosuchians. Thetype and only knownspecies isI. longicollum, which is known from single specimen discovered in theLate Triassic (Carnian aged)Ischigualasto Formation ofArgentina.Incertovenator is known almost entirely by itsvertebral column. This indicates that it had a relatively long neck, leading to its uncertain classification due to theconvergent evolution of elongated neck vertebrae in both avemetatarsalian and pseudosuchian archosaurs.[1]
Thetype and only known specimen ofIncertovenator, PVSJ 397, was discovered in the Cancha de Bochasmember of theIschigualasto Formation. The specimen was found at the Hoyada de Ischigualasto locality in the southern region ofIschigualasto Provincial Park,stratigraphically positioned at roughly mid-level in the Cancha de Bochas member, approximately 120 metres (390 ft) above the base of the Ischigualasto Formation. The specimen was formally described in 2021 by Imanol Yáñez, Diego Pol, Juan Martín Leardi, Oscar A. Alcober, and Ricardo N. Martínez, and is held in the Museo de Ciencias Naturales at theNational University of San Juan, Argentina.[1]
The specimen is almost entirely known by its vertebral column (axial skeleton): fivecervical vertebrae (including theaxis), ten articulated mid-to-posteriordorsal vertebrae, twosacral vertebrae, and sixcaudal vertebrae (five of which are complete). The only other bones found with the specimen are a completeilium and two unidentifiable bone fragments. The specimen is preserved partially articulated. However, the overall preservation is poor, with a dark reddish-brown to grey colouration indicative of ahaematite coating andpermineralization. Additionally, the deformation has distorted the shape of the vertebrae and parts of them have been cracked and weathered away.[1]
Thegeneric name comes from theLatin wordsincerto- (uncertain) andvenator (hunter), referencing its uncertain phylogenetic relationships to other archosaurs and its presumed predatory lifestyle. Likewise, thespecific name is from Latinlongus andcollum, meaning "long neck", in recognition of its notably elongatedcervical vertebrae.[1]
As it is known almost entirely from vertebrae, little can be confidently said about the life appearance ofIncertovenator, apart from it having a relatively long neck. However, it was likely a quadruped, similar to the majority of other early-diverging archosaurs, including potential relatives (aphanosaurs and early-diverging loricatans).
The axial skeleton ofIncertovenator is well-known, with vertebrae from all the major regions of the spine (cervicals, dorsals, sacrals and caudals) as well as associatedribs. The cervicals are characteristically elongated, withcentra (the main body of the vertebra) 2.5 times as long as they are wide. Theneural spines are similarly longer than high, and overhang each centrum at the front with tips that are expanded and rugosely textured.[1]
The dorsal vertebrae are shorter than the cervicals, but are still relatively long compared with other archosaurs; their centra are both longer than they are tall, and taller than wide. Unlike the cervicals, they lack a keel on their underside and have no hollowed out fossae on their surfaces. The neural spines, however, are similar to the cervicals' in that they are long and low, roughly similar in height to the centra. This trait is unusual for aphanosaurs, but similar to some dinosauromorphs, crocodylomorphs andMandasuchus.[1]
Incertovenator has only two sacral vertebrae, the ancestral condition for archosaurs, and they are unfused. Their neural spines are low, like the dorsals, although the first sacral spine has a slight forward slant and the second resembles that of the axis, with an arched upper margin. The caudal centra are also longer than they are high, although they gradually become shorter moving down the tail. The neural spines are also similar to those of the dorsals, although they shift from a roughly central position from the first caudal towards the back of the centrum by the fifth caudal.[1]
The only other known part of the skeleton is theilium, the large hip bone that forms the top part of thepelvis. The most notable feature of the ilium is the elongated preacetabular process projecting forward beyond theacetabulum (hip socket). This is an uncommon feature of early archosaurs, and is otherwise only known to occur in poposauroids,crocodylomorphs, pterosaurs and dinosaurs. The ilium also has a concave upper margin when seen from the side. Theacetabulum (hip socket) is completely closed, and has a prominent roof formed by a supraacetabular crest, a ridge of bone bordering the top and front of the acetabulum. This relatively long supraacetabular crest is similar to some early crocodylomorphs (e.g.Trialestes) but not others (e.g.Terrestrisuchus). The joint between the peduncles for the attachment of thepubis andischium is triangular, with an angle of roughly 140° between them. This is typical of most pseudosuchians and early avemetatarsalians, but unlike the concave margin of dinosaurs,poposauroids and most crocodylomorphs.[1]
When introduced to a dataset of Triassicarchosauriforms,Incertovenator was able to occupy multiple equallyparsimonious positions in the resultingcladogram (shown below), making it difficult to determine its true evolutionary relationships.Incertovenator was able to occupy these differing positions due to it possessing various traits associated with distinct lineages of early-diverging archosaurs, namely convergent traits related to an elongated neck. For example, it shares with variousaphanosaurs mid-cervicals longer than its mid-dorsals, cervical neural spines with expanded tips and a roughened texture, dorsal neural spines with expanded, rounded tips, and cervical neural spines that slant forwards at their front edge (a trait previously interpreted as an unambiguous synapomorphy of aphanosaurs). Alternatively, a position in Pseudosuchia related toMandasuchus is supported by sharing a convex dorsal margin of the axial neural spine and a forward slant to the front face of their cervicals.[1]
The cladogram below is reproduced and simplified from Yáñez et al., 2021:
| Archosauria |
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Additional suboptimal positions were also identified on the cladogram, including as the sister taxon topterosaurs orgracilisuchids (one step longer), or various positions withinDinosauromorpha, Poposauroidea and elsewhere among earlysuchians, early diverging crocodylomorphs, and in one analysis even as the sister taxon to the non-archosaurproterochampsidTropidosuchus. Some of these positions were also associated with the elongation of the cervicals, although notably the shape of the ilium is similar to early crocodylomorphs, derived poposauroids, pterosaurs and dinosaurs, namely the long preacetabular process. These conflicting signals cloud its relationship to other archosaurs even further.[1]
As indicated by its name,Incertovenator was presumed to be a predator, largely due to the convergent evolution of similar long-necks being associated with small-sized predatory archosaurs.[1] The Cancha de Brochas member is correlated with theScaphonyx-Exaeretodon-Herrerasaurus Biozone, in whichIncertovenator coexisted with the dinosaursEoraptor,Herrerasaurus,Sanjuansaurus andChromogisaurus, as well as the pseudosuchiansSaurosuchus,Sillosuchus andTrialestes. It also coexisted with various other reptiles, including the abundantrhynchosaurHyperodapedon (="Scaphonyx"), theaetosaurAetosauroides, thesilesauridIgnotosaurus, and the two proterochampsidsProterochampsa andPseudochampsa. Coevalsynapsids include the largedicynodontIschigualastia and thecynodontsChiniquodon,Diegocanis,Ecteninion, andExaeretodon.[2]
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