Pisanosaurus

Pisanosaurus Temporal range: Late Triassic, 228–216.5 Ma PreЄ Є O S D C P T J K Pg N
Display at the Royal Ontario Museum
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	†Ornithischia
Family:	†Pisanosauridae Casamiquela, 1967
Genus:	†Pisanosaurus Casamiquela, 1967
Species:	† P. mertii
Binomial name
Pisanosaurus mertii Casamiquela, 1967

Pisanosaurus (pron.:"PIE-san-uh-SAWR-us") is an extinct genus of primitive ornithischian dinosaur 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, bipedal 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 Argentina's Ischigualasto Formation.

The exact classification of Pisanosaurus has been the topic of debate by scientists for over 40 years; the current consensus is 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.

Description

Restoration

Based on the known fossil elements from a partial skeleton, Pisanosaurus was a small, lightly built dinosaur approximately 1 m (3 ft 3 in) in length. Its weight was between 2.27–9.1 kg (5–20 lb).^[1] 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. The tail of Pisanosaurus has been reconstructed as being as long as the rest of the body, based on other early ornithischians,^[2] but as a tail has not been recovered, this is speculative. It was bipedal and, like all other known ornithischians, was probably herbivorous.

According to a redescription by 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.^[3]

Discovery and naming

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.^[4] 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.^[5]

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".^[6][7] 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.

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.[4]

Pisanosaurus is the type genus of the Pisanosauridae, a family erected by Casamiquela in the same paper which named Pisanosaurus.^[5] The Pisanosauridae family has fallen into disuse, as a 1976 study considered the group synonymous with the already named Heterodontosauridae.^[8]

Pisanosaurus is 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.^[4][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.^[4] This assignment is also supported by Norman et al. (2004) and Langer et al. (2009).^[11][12] Other primitive ornithischians include Eocursor, Trimucrodon, and possibly Fabrosaurus. On the other hand, a phylogenetic analysis conducted by Agnolin (2015) recovered Pisanosaurus as a non-dinosaurian member of Dinosauriformes related to the silesaurids.^[13]

Paleoecology

Provenance and occurrence

Front part of restored skeleton

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.^[4] 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.

Fauna and habitat

The Ischigualasto Formation was a volcanically active floodplain covered by forests, with a warm and humid climate,^[14] though subject to seasonal variations including strong rainfalls.^[15] Vegetation consisted of ferns, horsetails, and giant conifers, which formed highland forests along the banks of rivers.^[16] Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation.^[17] 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 it 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^[18]

References

1. Holtz, Thomas R. Jr. (2008) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages Supplementary Information
2. Holtz, Thomas R. Jr. (2006). "The Dinosaur Family Tree: What is a dinosaur?". University of Maryland Department of Geology. Retrieved 2008-08-10.
3. J. F. Bonaparte. 1976. "Pisanosaurus mertii Casamiquela and the origin of the Ornithischia". Journal of Paleontology 50(5): 808-820
4. 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.
5. Casamiquela, R.M. (1967). "Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina". Ameghiniana 4 (2): 47–64.
6. Liddell, Henry George and Robert Scott (1980). A Greek-English Lexicon (Abridged Edition). United Kingdom: Oxford University Press. ISBN 0-19-910207-4.
7. "Pisanosaurus". Retrieved 14 December 2013.
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. 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
12. Langer, M. C., Ezurra, M. D., Bittencourt, J. S., and Novas, F. E., 2009, The origin and early evolution of dinosaurs: Biological Review, v. 84, p. 1-56
13. 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.
14. Tucker, Maurice E.; Benton, Michael J. (1982). "Triassic environments, climates, and reptile evolution" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology 40 (4): 361–379. doi:10.1016/0031-0182(82)90034-7. Retrieved 2009-07-23.
15. Columbi, Carina E. (2008-10-05). Stable isotope analysis of fossil plants from the Upper Triassic Ischigualasto Formation in the northwest of Argentina. Houston, TX: The Geological Society of America. Retrieved 2009-07-23.
16. 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.
17. Rogers, R. R.; Swisher III, C.C.; Sereno, P.C.; Monetta, A.M.; Forster, C.A.; and Martinez, R.N. (1993). "The Ischigualasto tetrapod assemblage (Late Triassic, Argentina) and 40Ar/39Ar dating of dinosaur origins". Science 260 (5109): 794–797. Bibcode:1993Sci...260..794R. doi:10.1126/science.260.5109.794. PMID 17746113. Cite uses deprecated parameter |coauthors= (help)
18. Bonaparte, J.F. (1970). "Annotated list of the South American Triassic tetrapods". Gondwana Symposium Proceedings and Papers 2: 665–682.

External links

• Pisanosaurus from the PaleoBiology Database