Pisanosaurus

Pisanosaurus (/pɪsænəˈsɔːrəs/) is an extinct genus of primitive dinosauriform that lived approximately 228 to 216 million years ago during the latter part of the Triassic Period in what is now South America. It was a small, lightly-built, ground-dwelling herbivore, that could grow up to an estimated 1 m (3.3 ft) long. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton discovered in the Ischigualasto Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina.

Pisanosaurus
Temporal range: Late Triassic
~228–216.5 Ma
Reconstruction of the skull, white bones are known, light grey bones were known from heavily eroded impressions
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauriformes
Clade: Dracohors
Genus: †Pisanosaurus
Casamiquela 1967
Species:
†P. mertii
Binomial name
†Pisanosaurus mertii
Casamiquela 1967

Discovery and naming
Reconstructed skeleton reflecting the traditional interpretation of Pisanosaurus as an ornithischian dinosaur, Royal Ontario Museum

Pisanosaurus is known from a single fragmented skeleton discovered in 1962 by Galileo Juan Scaglia at the Agua de Las Catas locality in the Ischigualasto Formation, in La Rioja, Argentina.[1] It is based on a specimen given the designation PVL 2577, which consists of a partial skull with a fragmentary right maxilla 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 right scapula, a coracoid, molds of a fragmentary ilium, ischium and pubic bone, an impression of three metacarpals, the complete femora, the right tibia, the right fibula, with an articulated astragalus and calcaneum, a tarsal element with a metatarsal, metatarsals III and IV, three phalanges from the third toe and five phalanges, among them the ungual, from the fourth toe, and an indeterminate long bone fragment.[2]

The genus name Pisanosaurus means "Pisano’s lizard" and combines "Pisano" in honor of Argentine paleontologist Juan Arnaldo Pisano of La Plata Museum, with a Latin "saurus" from the Greek (σαύρα) meaning "lizard".[3][4] Pisanosaurus was described and named by Argentine paleontologist Rodolfo Casamiquela in 1967. The type and only valid species known today is Pisanosaurus mertii. The specific name honors the late Araucanian naturalist Carlos Merti.

Description
Restoration of Pisanosaurus as a silesaurid

Based on the known fossil elements from a partial skeleton, Pisanosaurus was a small, lightly built dinosauriform approximately 1 m (3 ft 3 in) in length. Its weight was between 2.27–9.1 kg (5–20 lb).[5] These estimates vary due to the incompleteness of the holotype specimen PVL 2577. The orientation of the pubis is uncertain, with some skeletal reconstructions having it projecting down and forward (the propubic condition) similar to that of the majority of saurischian dinosaurs.[6]

According to a redescription by José Bonaparte in 1976, Pisanosaurus has some distinctive characteristics. The acetabulum, the hip-joint, is open. The pedicels of the ilium are short, resulting in a low axially long 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.[7]

Classification
   Ornithischia   
 

 Pisanosaurus

 
         
  Heterodontosauridae  
 

 Abrictosaurus  

 
 

 Heterodontosaurus  

 
 
  Genasauria  
 

Thyreophora

 
 

Neornithischia

 
 
 
 
Cladogram of basal Ornithischia after Butler et al. (2008), showing the position of Pisanosaurus as the earliest example of an ornithischian.[1]

Pisanosaurus is the type genus of the Pisanosauridae, a family erected by Casamiquela in the same paper which named Pisanosaurus.[2] The family Pisanosauridae has fallen into disuse, as a 1976 study considered the group synonymous with the already named Heterodontosauridae.[8]

Pisanosaurus has traditionally been classified as very basal within Ornithischia; the postcrania seem to lack any good ornithischian synapomorphy and it was even suggested by Paul Sereno in 1991 that the fossil is a chimera.[9] However, recent studies suggest that the fossils belong to a single specimen.[1][10]

Over the years, Pisanosaurus has been classified as a heterodontosaurid, a fabrosaurid, a hypsilophodont and has also been considered the earliest known ornithischian. A 2008 study placed Pisanosaurus outside of (and more basal than) Heterodontosauridae. In this study, Pisanosaurus is the earliest and most primitive ornithischian.[1] This assignment is also supported by Norman et al. (2004), Langer et al. (2009) and Baron, Norman & Barrett (2017).[11][12][13] Other primitive ornithischians include Eocursor, Trimucrodon, and possibly Fabrosaurus.

Contrastingly, a phylogenetic analysis conducted by Agnolin (2015) recovered Pisanosaurus as a possible non-dinosaurian member of Dinosauriformes related to the silesaurids.[14] In 2017, it was again suggested that Pisanosaurus was a silesaurid.[15][16]

The exact classification of Pisanosaurus has been the topic of debate by scientists for over 40 years; until 2017, the consensus was that Pisanosaurus is the oldest known ornithischian, part of a diverse group of dinosaurs which lived during nearly the entire span of the Mesozoic Era, but recently, some have begun to consider it a non-dinosaurian silesaurid.[17][16][18]

Paleoecology
Restoration reflecting the traditional interpretation of Pisanosaurus as an ornithischian dinosaur

The fossils of Pisanosaurus were discovered in the "Agua de las Catas" locality at the Ischigualasto Formation in La Rioja, Argentina. Originally dated to the Middle Triassic,[8] this formation is now believed to belong to the Late Triassic Carnian stage, deposited approximately 228 to 216.5 million years ago.[1] This specimen was collected by José 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", in San Miguel de Tucumán, Argentina.

The Ischigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate,[19] though subject to seasonal variations including strong rainfalls.[20] Vegetation consisted of ferns, horsetails, and giant conifers, which formed highland forests along the banks of rivers.[21] Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation.[22] Sereno (1993) noted that Pisanosaurus was found in "close association" with therapsids, rauisuchians, archosaurs, Saurosuchus and the dinosaurs Herrerasaurus and Eoraptor, all of whom lived in its paleoenvironment. Bonaparte (1976) postulated that Pisanosaurus played a role in a fauna dominated by therapsids. The large carnivore Herrerasaurus may have fed upon Pisanosaurus. Herbivores were represented by rhynchosaurs such as Hyperodapedon (a beaked reptile); aetosaurs (spiny armored reptiles); kannemeyeriid dicynodonts (stocky, front-heavy beaked quadrupedal animals) such as Ischigualastia; and traversodontids (somewhat similar in overall form to dicynodonts, but lacking beaks) such as Exaeretodon. These non-dinosaurian herbivores were much more abundant than early dinosaurs.[23]

References
  1. Butler, Richard J.; Upchurch, Paul and Norman; David, B. (2008). "The phylogeny of the ornithischian dinosaurs". Journal of Systematic Palaeontology. 6 (1): 1–40. doi:10.1017/S1477201907002271. Retrieved 2008-08-09.
  2. Casamiquela, R.M. (1967). "Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina". Ameghiniana. 4 (2): 47–64.
  3. Liddell, Henry George and Robert Scott (1980). A Greek-English Lexicon (Abridged Edition). United Kingdom: Oxford University Press. ISBN 978-0-19-910207-5.
  4. "Pisanosaurus". Retrieved 14 December 2013.
  5. Holtz, Thomas R. Jr. (2008) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages Supplementary Information
  6. Holtz, Thomas R. Jr. (2006). "The Dinosaur Family Tree: What is a dinosaur?". University of Maryland Department of Geology. Retrieved 2008-08-10.
  7. Bonaparte, J. F. (1976). "Pisanosaurus mertii Casamiquela and the origin of the Ornithischia". Journal of Paleontology. 50 (5): 808–820.
  8. Bonaparte, J.F. (1976). "Pisanosaurus mertii Casamiquela and the origin of the Ornithischia". Journal of Paleontology. 50 (5): 808–820. JSTOR 1303575.
  9. Sereno, P.C. (1991). "Lesothosaurus, "fabrosaurids", and the early evolution of Ornithischia". Journal of Vertebrate Paleontology. 11 (2): 168–197. doi:10.1080/02724634.1991.10011386.
  10. Irmis, R.B.; Parker, W.G.; Nesbitt, S.J.; Liu, J. (2007). "Early ornithischian dinosaurs: the Triassic record". Historical Biology. 19: 3–22. CiteSeerX 10.1.1.539.8311. doi:10.1080/08912960600719988.
  11. D. B. Norman, L. M. Witmer, and D. B. Weishampel. 2004. Basal Ornithischia. In D. B. Weishampel, H. Osmolska, and P. Dodson (eds.), The Dinosauria (2nd edition). University of California Press, Berkeley 325-334
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  13. Baron, M.G.; Norman, D.B.; Barrett, P.M. (2017). "A new hypothesis of dinosaur relationships and early dinosaur evolution". Nature. 543: 501–506. Bibcode:2017Natur.543..501B. doi:10.1038/nature21700. PMID 28332513.
  14. Federico L. Agnolin (2015). "Nuevas observaciones sobre Pisanosaurus mertii Casamiquela, 1967 (Dinosauriformes) y sus implicancias taxonómicas" (PDF). XXIX Jornadas Argentinas de Paleontología de Vertebrados. 27–29 de Mayo de 2015. Diamante, Entre Ríos. Libro de Resúmenes: 13–14. Archived from the original (PDF) on 2016-01-27. Retrieved 2015-08-07.
  15. Agnolín, Federico L.; Rozadilla, Sebastián (2017). "Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina". Journal of Systematic Palaeontology. 16 (10): 853–879. doi:10.1080/14772019.2017.1352623.
  16. Matthew G. Baron; David B. Norman; Paul M. Barrett (2017). "Baron et al. reply". Nature. 551 (7678): E4–E5. Bibcode:2017Natur.551E...4B. doi:10.1038/nature24012. PMID 29094705.
  17. Agnolín, Federico L.; Rozadilla, Sebastián (2017). "Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina". Journal of Systematic Palaeontology. 16 (10): 853–879. doi:10.1080/14772019.2017.1352623.
  18. Matthew G. Baron (2018). "Pisanosaurus mertii and the Triassic ornithischian crisis: could phylogeny offer a solution?". Historical Biology: An International Journal of Paleobiology. 31 (8): 1–15. doi:10.1080/08912963.2017.1410705.
  19. Tucker, Maurice E.; Benton, Michael J. (1982). "Triassic environments, climates, and reptile evolution" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. 40 (4): 361–379. Bibcode:1982PPP....40..361T. doi:10.1016/0031-0182(82)90034-7. Archived from the original (PDF) on 2009-01-26. Retrieved 2009-07-23.
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  21. Sereno, P.C.; Novas, F.E. (1992). "The complete skull and skeleton of an early dinosaur". Science. 258 (5085): 1137–1140. Bibcode:1992Sci...258.1137S. doi:10.1126/science.258.5085.1137. PMID 17789086.
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