| Pisanosaurus | |
|---|---|
 | |
| Reconstruction of the skull, white bones are known, light grey bones were known from heavily eroded impressions | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia (?) |
| Genus: | †Pisanosaurus Casamiquela, 1967 |
| Species: | †P. mertii |
| Binomial name | |
| †Pisanosaurus mertii Casamiquela, 1967 | |
Pisanosaurus (/pɪˌsænəˈsɔːrəs/piss-AN-ə-SOR-əs) is anextinctgenus of earlydinosauriform, likely anornithischian orsilesaurid, from theLate Triassic ofArgentina. It was a small, lightly built, ground-dwellingherbivore, that could grow up to an estimated 1 m (3.3 ft) long. Only one species, thetype,Pisanosaurus mertii, is known, based on a single partial skeleton discovered in theIschigualasto Formation of theIschigualasto-Villa Unión Basin in northwestern Argentina. This part of the formation has been dated to the late Carnian, approximately 229million years ago.[1]
Pisanosaurus is known from a single fragmentedskeleton discovered in 1962 by Galileo Juan Scaglia at the Hoyada del Cerro Las Lajas locality[1] (also known as Agua de Las Catas)[2] in theIschigualasto Formation ofLa Rioja Province,Argentina.[3]
The genus isbased on a specimen given the designation PVL 2577, which consists of a partial skull including a fragmentary rightmaxilla with teeth, and incomplete right mandibular ramus (lower jaw), six incomplete cervical vertebrae, seven incomplete dorsal vertebrae, molds of five sacral vertebrae, a rib and several rib fragments, a fragmentary rightscapula, acoracoid, molds of a fragmentaryilium,ischium andpubic bone, an impression of three metacarpals, the completefemora, the righttibia, the rightfibula, with an articulatedastragalus andcalcaneum, atarsal element with a metatarsal,metatarsals III and IV, threephalanges from the third toe and five phalanges (including theungual) from the fourth toe, and an indeterminate long bone fragment.[4]
The genus namePisanosaurus means "Pisano's lizard" and combines "Pisano" in honor ofArgentinepaleontologist Juan Arnaldo Pisano ofLa Plata Museum, with aLatin "saurus" from theGreek (σαύρα) meaning "lizard".[5][6]Pisanosaurus was described and named by Argentine paleontologist Rodolfo Casamiquela in 1967. Thetype and only validspecies known today isPisanosaurus mertii. Thespecific name honors the late Araucanian naturalist Carlos Merti.
Based on the known fossil elements from a partial skeleton,Pisanosaurus was a small, lightly built dinosauriform, reaching 1–1.3 m (3.3–4.3 ft) in length and 2 kg (4.4 lb) in body mass.[7][8] These estimates vary due to the incompleteness of theholotype specimen PVL 2577. The orientation of the pubis is uncertain, with some skeletal reconstructions having it projecting down and forward (thepropubic condition) similar to that of the majority ofsaurischian dinosaurs.[9]
According to a redescription byJosé Bonaparte in 1976,Pisanosaurus has some distinctive characteristics. Theacetabulum (hip socket) is open. The peduncles of the ilium are short, resulting in a low and axially elongated acetabulum. The upper region of the ischium is wide, larger than that of the pubic bone. The metacarpals of the hand are apparently elongated, measuring about fifteen millimeters.[2]
| Cladogram of basal Ornithischia after Butleret al. (2008), showing the position ofPisanosaurus as the earliest example of an ornithischian.[3] |
Pisanosaurus is the type genus of thePisanosauridae, afamily erected by Casamiquela in the same paper which namedPisanosaurus.[4] The family Pisanosauridae has fallen into disuse; a 1976 study considered the group synonymous with the already namedHeterodontosauridae,[2] though this is not followed by more recent studies.
The exact classification ofPisanosaurus has been the topic of debate by scientists for over 40 years; until 2017, the consensus was thatPisanosaurus is the oldest knownornithischian, part of a diverse group of dinosaurs which lived during nearly the entire span of theMesozoic Era. More recently, some authors have begun to consider it a non-dinosauriansilesaurid,[10][11] though this hypothesis has not reached a consensus either.[1]
Pisanosaurus has traditionally been classified as very basal withinOrnithischia; thepostcrania seem to lack any good ornithischiansynapomorphy and it was even suggested byPaul Sereno in 1991 that the fossil is achimera.[12] However, recent studies suggest that the fossils belong to a single specimen.[3][13]
Over the years,Pisanosaurus has been classified as aheterodontosaurid, afabrosaurid, ahypsilophodont and has also been considered the earliest known ornithischian. A 2008 study placedPisanosaurus outside of (and more basal than)Heterodontosauridae. In this study,Pisanosaurus is the earliest and most primitive ornithischian.[3] This assignment is also supported by Normanet al. (2004), Langeret al. (2009) and the controversialOrnithoscelida hypothesis of Baron, Norman & Barrett (2017).[14][15][16] Other primitive ornithischians includeEocursor,Trimucrodon, and possiblyFabrosaurus.
The hypothesis of ornithischian affinities forPisanosaurus has not fallen out of favor despite competition from alternative hypothesis. Silesaurid-like traits, for example, may be dinosaurianplesiomorphies (ancestral conditions) rather than unique characteristics of silesaurids.[1]
A phylogenetic analysis informally conducted by Agnolin (2015) recoveredPisanosaurus as a possible non-dinosaurian member ofDinosauriformes related to thesilesaurids.[17] In 2017, two studies independently came to the conclusion thatPisanosaurus was a silesaurid: one was an expansive redescription by Agnolin and Rozadilla,[18] and the other was a re-analyzed Ornithoscelida matrix by Baron, Norman, & Barrett.[11]Pisanosaurus was also found as a silesaurid in a 2018 paper which combined the descriptive work of Agnolin and Rozadilla (2017) with the phylogenetic matrix of Baron, Norman, & Barrett (2017).[19]
The placement ofPisanosaurus is reliant on the placement of silesaurids as a whole, a situation which has invited much debate. While Silesauridae is often considered amonophyleticsister group of dinosaurs, some studies consider it aparaphyleticgrade ancestral to ornithischian dinosaurs in particular. One such study is Müller & Garcia (2020).[20] Although they regardedPisanosaurus as the basal-mostornithischian, taxa often considered members ofSilesauridae form a step-wise arrangement up toPisanosaurus. It acts a transitional form positioned on a rung between the "silesaurid" grade (Asilisaurus,Sacisaurus,Silesaurus, etc.) and traditional ornithischians (Eocursor,Scutellosaurus,Heterodontosaurus, etc.). This phylogenetic position may explain why some authors considerPisanosaurus a silesaur and others consider it an ornithischian, as following Müller & Garcia,Pisanosaurus has traits of both groups.[20][21]
The fossils ofPisanosaurus were discovered in the "Agua de las Catas" locality at theIschigualasto Formation in La Rioja,Argentina. Originally dated to theMiddle Triassic,[2] this formation is now believed to belong to theLate TriassicCarnianstage, deposited approximately 228 to 216.5 million years ago.[3] This specimen was collected byJosé Fernando Bonaparte, Rafael Herbst and the preparators Martín Vince and Scaglia in 1962, and is housed in the collection of the Laboratorio de Paleontologia de Vertebrados, Instituto "Miguel Lillo", inSan Miguel de Tucumán,Argentina.
TheIschigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate,[22] though subject to seasonal variations including strong rainfalls.[23] Vegetation consisted offerns,horsetails, and giantconifers, which formedhighland forests along the banks of rivers.[24]Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation.[25] Sereno (1993) noted thatPisanosaurus was found in "close association" withtherapsids,rauisuchians,archosaurs,Saurosuchus and the dinosaursHerrerasaurus andEoraptor, all of whom lived in its paleoenvironment. Bonaparte (1976) postulated thatPisanosaurus played a role in a fauna dominated by therapsids. The large carnivoreHerrerasaurus may have fed uponPisanosaurus. Herbivores were represented byrhynchosaurs such asHyperodapedon (a beaked reptile);aetosaurs (spiny armored reptiles);kannemeyeriiddicynodonts (stocky, front-heavy beaked quadrupedal animals) such asIschigualastia; andtraversodontids (somewhat similar in overall form to dicynodonts, but lacking beaks) such asExaeretodon. These non-dinosaurian herbivores were much more abundant than early dinosaurs.[26]
Media related toPisanosaurus at Wikimedia Commons
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