| Asilisaurus | |
|---|---|
 | |
| Skeletal cast PR 3062 on display at theField Museum of Natural History, Chicago, Illinois, USA. | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria (?) |
| Clade: | †Ornithischia (?) |
| Family: | †Silesauridae |
| Genus: | †Asilisaurus Nesbittet al.,2010 |
| Species: | †A. kongwe |
| Binomial name | |
| †Asilisaurus kongwe Nesbittet al., 2010 | |
Asilisaurus (/ɑːˌsiːliːˈsɔːrəs/ah-SEE-lee-SOR-əs); fromSwahili,asili ("ancestor" or "foundation"), andGreek,σαυρος (sauros, "lizard") is an extinctgenus ofsilesauridarchosaur. Thetype species isAsilisaurus kongwe.Asilisaurus fossils were uncovered in theManda Beds ofTanzania, a formation typically dated to theAnisian; however, some authors controversially assert that the formation may be earlyCarnian instead.[1] If it truly is Anisian,Asilisaurus would be one of the oldest known members of theAvemetatarsalia (animals on thedinosaur/pterosaur side of the archosaurian family tree). It was the first non-dinosauriandinosauriform recovered fromAfrica. The discovery ofAsilisaurus has provided evidence for a rapid diversification of avemetatarsalians during the Middle Triassic, with the diversification of archosaurs during this time previously only documented inpseudosuchians (crocodylian-line archosaurs).[2][3]
Asilisaurus is known from a relatively large amount of fossils compared to most non-dinosaurdinosauromorphs. This has allowed it to provide important information for the evolution of other silesaurids and the origin of dinosaurs. It had several unique features compared to its close relatives, such as a lack of teeth at the front of thepremaxilla and a lower jaw which was not only toothless at the tip, but also downturned. These indicate that it probably had a small beak on the tip of the snout. It was fairly basal among silesaurids, and retained some dinosaur-like features absent in advanced silesaurids, such as a supratemporal fossa and aquadrate which overlaps thesquamosal. On the other hand, it also retains primitive features contrasting with dinosaurs, including twohip vertebrae, a closedhip socket, acrurotarsal ankle, and a non-vestigial fifth toe of the foot.[2][3]
Asilisaurus was described in 2010 by a team of researchers from theUnited States,Germany, andSouth Africa, in the journalNature. Thegeneric name is derived fromasili, theSwahili word for "ancestor" or "foundation", as well asσαυρος (sauros), theGreek word for "lizard". This refers to its position as an early avemetatarsalian which helps provide details for the evolution of dinosaurs. Thespecific namekongwe is in line with this naming scheme, as it is the Swahili word for "ancient".[2]The first remains ofAsilisaurus were found in 2007 at "locality Z34", abonebed near the town of Litumba Ndyosi inTanzania. This bonebed is part of the Lifua Member of theManda Beds, which preserves a middle Triassic lake ecosystem. At least 14 individuals were present at the site, includingNMT RB9, theholotypedentary. Fossils likely belonging toAsilisaurus are known from throughout the Manda Beds. Numerous fragments of small individuals have been found at another site, "locality Z90". NMT RB159, a well-preserved and articulated partial specimen, was found at "locality Z137" and described in 2019.[4][2][3]
Asilisaurus was a lightly built animal with a fairly long neck (by early archosaur standards), a short snout tipped with a beak, and slender limbs. It probably walked on all four legs based on the length of its limbs.Asilisaurus specimens have been estimated to measure from 1 to 3 metres (3 to 10 ft) long and 0.5 to 1 metre (2 to 3 ft) high at the hip, and may have weighed 10 to 30 kilograms (20 to 70 lb).[4] The ~3 meter upper length estimate is based on NHMUK R16303, an incomplete silesaurid femur found in the Manda Beds. This femur had an estimated length of 29.6-39.6 cm (11.7-15.6 in), more than twice the length of the next largestAsilisaurus femurs. The specimen belonged to one of the largest known pre-Norian avemetatarsalians, only exceeded by someHerrerasaurus specimens. Uncertainty over thestratigraphic position of the specimen and a lack of unambiguouslyAsilisaurus-like traits means that it is not certain that NHMUK R16303 belongs toAsilisaurus.[5]
The overall skull was short, with a smallnasal opening and a large and roundorbit. Thepremaxilla is large and primarily toothless, with a sharp lower edge converging towards a pointed front tip. There is only a single tooth at the rear of the bone, near the contact with the maxilla. Other basaldinosauromorphs (even other silesaurids) had teeth along the entirety of the premaxilla. Other aspects, such as the shape of the posterodorsal process and the loose contact between adjacent premaxillae, are similar to other silesaurids. This is also the case with thepalatal process of themaxilla, which has a flat surface separated from the rest of the bone via a ridge and pit. However,Asilisaurus's palatal process of the maxilla is much taller than other silesaurids. Theantorbital fossa of the maxilla is shallow and poorly defined. There were at least 10 teeth in the maxilla, possibly up to 12 assuming that the rear part of the maxilla (which was not preserved) was similar to that ofSilesaurus andSacisaurus. Like other silesaurids, the teeth were ankylothecodont, set in sockets but also fused to the bone through small ridges. The teeth are conical, pointed, and slightly recurved. Some specimens' teeth have poorly developed serrations, while others are more prominent.[3]
The rest of the skull generally resemblesSilesaurus andLewisuchus, with some exceptions. Theprefrontal is large and has a pronounced lateral "swelling" in front of the eye. The rear of thefrontal has a pronounced supratemporal fossa likeTeleocrater and dinosaurs, but unlike other silesaurids. Just lateral to the fossa, the frontal has a straight (rather than embayed) contact with either thepostfrontal orpostorbital. Neither preservation norphylogenetic bracketing are stable enough to determine whether a postfrontal bone was present, or instead lost (which is the situation in dinosaurs). Also like dinosaurs, thequadrate partially overlaps part of thesquamosal in lateral view. The front tip of thejugal is pointed, wedging between a wide lower branch of thelacrimal and a presumably sloping rear portion of the maxilla. Theectopterygoid has a curved jugal process and a deep ventral fossa, similar toLewisuchus andtheropod dinosaurs.[3]
Like the premaxilla, the front of thedentary is toothless and curves into a point. The tip bends down, in contrast to other silesaurids where it bends up. In addition, advanced silesaurids have a longitudinal groove at the lower edge of the inner portion of the dentary, while that ofAsilisaurus is positioned higher.Asilisaurus has fewer teeth in the dentary (8-10) than other silesaurids, and the tooth row occupies a smaller portion of the bone than its relatives. The rear of the lower jaw is similar to other silesaurids, with a long retroarticular process and weakly definedsurangular ridge. The middle portion of the lower jaw is not known, so there is ambiguity about the overall shape of the jaw.[2][3]
Theneck vertebrae are parallelogram-shaped and moderately elongated, proportionally similar to other silesaurids. Most of the depressions and ridges on the neck vertebrae ofAsilisaurus were not as well-developed as those ofSilesaurus. However, thepostzygapophyses of a few vertebrae do have small projections identified asepipophyses, which are present inaphanosaurs and dinosaurs but not advanced silesaurids. Thedorsal vertebrae are shorter and more complex, lacking epipophyses and instead possessinghyposphenes, another dinosaur-like feature. There were twosacral vertebrae, each with its own sacral ribs. This contrasts withSilesaurus, which has three or four sacral vertebrae which share sacral ribs between each other. Like otherdinosauriforms, the tail vertebrae increase in length further down the tail.[2][3]
Thescapula is long and narrow, expanded into a triangular structure at the tip and lacking any perforations on theacromion process. Theglenoid is directed backwards, and overlies a strongly developed knob-like process on thecoracoid. The shaft of thehumerus is straight and only slightly twisted. Several aspects of the humerus, such as the small deltopectoral crest connected to thehumeral head by a thick ridge, do not resemble the situation in dinosaurs. Many of the joint surfaces at the shoulder and elbow are roughly textured and well-delineated. Theradius is thin and simple, similar in shape to that ofHerrerasaurus. However, it was also shorter than the humerus, only about 85% the length of that bone. The ulna is similar and has a largeolecranon process and anautapomorphic broad groove running down along its outer edge. The hand is poorly known, but the best preservedmetacarpal (probably asecond metacarpal) is small, indicating that the hand was likely much shorter than the foot.[2][3]
Asilisaurus's hip had a closedacetabulum, rather than the open acetabulum of dinosaurs. The postacetabular process of theilium possessed a brevis fossa (only visible from below) edged by subtle medial and lateral ridges, with the lateral ridge likely homologous to the brevis shelf of dinosaurs. The preacetabular process, on the other hand, was edged by a lateral ridge similar to that of other silesaurids. Theischium is generally similar toSacisaurus and other dinosauromorphs, with connected shafts separated by a groove on the top. A small expansion is present at the tip of the ischium. Thepubis is also similar to that ofSacisaurus, with a long shaft topped by a short crest. There is a small gap at the intersection of the ilium, ichium, and pubis.[2][3]
The upper portion of thefemur possesses many of the features typical for dinosauriforms. These include a groove on the top of the femoral head, afacies articularis antitrochanterica, three equally well-developed proximal tubera, and an anterior trochanter and trochanteric shelf. It lacks certain adaptations of more advanced silesaurids, such as a straight medial edge of the femoral head. The dorsolateral trochanter is poorly developed, in contrast to many dinosaurs. Much of the lower portion of the femur is also weakly developed, with the exception of a groove on the rear edge of the bone, which is extended up the shaft to the same extent as most other silesaurids. Thetibia is also par for the course for dinosauriforms, with a straightcnemial crest, two equally sizedproximal condyles, and a lateral groove in the distal portion. This is furthermore the case with thefibula, which closely resembles that ofTeleocrater andSaturnalia. The tibia and fibula are shorter than the femur, unlike basal dinosaurs.Asilisaurus retains a "primitive"crurotarsal ("crocodile-normal") ankle characterized by a convex-concave interaction between theastragalus andcalcaneum. The calcaneum is fairly similar to that ofTeleocrater. It has a convex fibular joint separated from a small but distinct calcaneal tuber. The astragalus is similar to that ofMarasuchus and has a low and ridge-like ascending process. In some respects the fourth distal tarsal resembles that ofLagerpeton, but in other respects (such as the large facet formetatarsal V) it is clearly different. The foot is shorter than that of other silesaurids but still rather long, with thethird metatarsal being the longest bone (at just under half the length of the tibia), followed by thesecond andfourth which are about the same length at each other. Overall the foot is similar to that ofSaturnalia, with some exceptions. Unlike many dinosauromorphs, the fifth metatarsal is not vestigial and instead is a fairly thick bone slightly longer than the thinfirst metatarsal, which is also not too short. All of the metatarsals had accompanyingphalanges, though complete toes are not known.Unguals were low and wide, a "hoof"-like appearance also seen inSilesaurus and the unrelatedshuvosaurids.[2][3]
Evidence for a beak at the tip of the snout and peg-like teeth further back support the idea thatAsilisaurus was anomnivore orherbivore. These traits are mirrored by dinosaurs which acquired a herbivorous diet, such asornithischians and advancedsauropodomorphs.[2][4] The conical shape of the teeth shares some similarities withpiscivorous reptiles such asspinosaurids andcrocodilians, leading to the possibility that fish were part of its diet.[6] The teeth are similar to those ofSilesaurus, which has been considered a primarily herbivorousbrowser based ondental microwear,[7] or aninsectivore based on referredcoprolites.[8] Regardless,Asilisaurus was very unlikely to be a specialized carnivore like the more basal silesauridLewisuchus, nor an obligate herbivore likeKwanasaurus.[9]
Femora assigned toAsilisaurus seem to exhibit both "slender" and "robust" morphologies. On average, "robust" femurs are larger and have more prominent bone scars. Many otherdinosauriforms are known to possess these different morphologies, such ascoelophysids,Masiakasaurus, andSilesaurus. This variation has traditionally (but perhaps incorrectly) been interpreted assexual dimorphism. In a 2016 study, a sample of 27Asilisaurus femurs were analyzed to determine how they developed. The study attempted to determine the sequence through which 11 femoral traits (mostly muscle scars) appeared in the bone. The study found that there was no universal sequence, and instead multiplepolymorphic trajectories, with many traits appearing earlier in the sequence in some bones and later in others. This large amount of variability is also observed in dinosaurs, but notDromomeron orSilesaurus (although these may be due to a small sample size). There are some general trends, such as the anterior trochanter andlinea intermuscularis cranialis appearing early, and thefourth trochanter transitioning to a wide and rounded shape late in development.[10]
There is only a weak relationship between skeletal maturity and size, as some small "robust" femora are more well-developed than some large "slender" ones. In most cases there is no correlation between the presence of absence of different traits. More consistency over the presence or absence of traits would be expected if the different morphologies were truly based on sexual dimorphism. However, this is not the case since nobimodality is observed. The size and developmental variation ofAsilisaurus suggests that "robust" morphologies are simply more mature individuals, while "slender" morphologies are less mature individuals. Large "slender" femora can be explained as coming from youngAsilisaurus which were able to increase in size by taking advantage of plentiful resources, but had not yet attained skeletal maturity. Small "robust" femora are a result of the opposite circumstance, belonging to matureAsilisaurus which grew up in more impoverished environments. This type of "developmental plasticity" has previously been proposed forPlateosaurus, some "pelycosaurs" (basalsynapsids), and observed in modernAlligator mississippiensis. The large amount of variability inAsilisaurus' sequence of muscle scar development is likely a matter of individual variation. Since dinosaurs experience the same variability, sexual dimorphism is unlikely to be responsible for their different femoral morphologies.[10]The study also involvedhistological analysis on the femur, humerus, tibia, and fibula. No LAGs (lines of arrested growth) are observed, even in bones from large individuals. The cross-sections are very similar to those ofSilesaurus and coelophysids, with a woven-fibered cortex full of longitudinal canals. The bone fibers are not well organized, but many are oriented parallel to the circumference of the bone. This is intermediate betweencrocodilians (which have discrete layers of slowly-growing bone), and dinosaurs (in which there is completely disorganized fast-growing bone).Osteocyte lacunae are abundant, as with other dinosauromorphs. Some of the longitudinal canals branch into irregular forms (in cross section) near the surface of the bone, to a greater extent thanSilesaurus but a lesser extent than coelophysids. Increasing size, abundance, and branching of canals is correlated with higher growth rates.Asilisaurus is close to, but not as developed as dinosaurs in these regards as well. It is hypothesized that a lack of LAGs is not indicative that all the specimens died within a year, as the estimated growth rate is not fast enough to achieve skeletal maturity within that time period. Instead,Asilisaurus may have had a constant and moderately high growth rate (though slower than dinosaurs), which was not impeded by any seasonal interruptions.[10]
