Herrerasaurus

Taxobox
name = "Herrerasaurus"
fossil_range = Late Triassic

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image_width = 250px
image_caption = Mounted "Herrerasaurus" skeleton, at the Field Museum in Chicago
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Saurischia
subordo = ?Theropoda
infraordo = Herrerasauria
familia = Herrerasauridae
genus = "Herrerasaurus"
genus_authority = Reig, 1963
subdivision_ranks = Species
subdivision =
* "H. ischigualastensis" Reig, 1963 (type)
synonyms =
* "Ischisaurus" Reig, 1963
* "Frenguellisaurus" Novas, 1986

"Herrerasaurus" (meaning "Herrera's lizard," after the name of the rancher who discovered the first fossil of the animal) was one of the earliest dinosaurs. All known specimens of this carnivore have been discovered in northwest Patagonia, Argentina, in late Triassic Period rocks (early Carnian stage, around 228 million years ago). The type species, "Herrerasaurus ischigualastensis", was described by Osvaldo Reig in 1963 and is the only species assigned to the genus.

For many years, the classification of "Herrerasaurus" was unclear, as the animal was initially known from very fragmentary remains; it has been hypothesized to be a basal theropod, a basal sauropodomorph, a basal saurischian, or not a dinosaur at all. However, with the discovery of a mostly-complete skeleton and skull in 1988,cite journal|last=Sereno|first=P.C. |coauthors=Novas, F.E.; Arcucci, A.B.; and C. Yu |year=1988 |title=New evidence on dinosaur and mammal origins from the Ischigualasto Formation (Upper Triassic, Argentina)|journal=Journal of Vertebrate Paleontology |volume=8 |issue=3, supplement |pages=26A] cite journal |last=Sereno |first=P.C. |coauthors=and Novas, F.E. |title=The complete skull and skeleton of an early dinosaur |journal=Science |volume=258 |year=1992 |pages=1137–1140 |doi=10.1126/science.258.5085.1137 |pmid=17789086] "Herrerasaurus" has been classified as either an early theropod or an early saurischian in at least five recent surveys of theropod evolution. This medium-sized bipedal reptile is a member of the Herrerasauridae, a group of similar animals which were among the earliest of the dinosaurian radiation.

Description

"Herrerasaurus" was a lightly-built bipedal carnivore with a long tail and a relatively small head. Its length is estimated at 3 to 6 meters (10 to 20 ft), and its hip height at more than 1.1 meters (3.3 ft).cite book|last=Paul |first=G.S. |year=1988 |title=Predatory Dinosaurs of the World |location=New York |publisher=Simon and Schuster |pages=248–250 ] It may have weighed around 210–350 kilograms (463–772 lb). In a large specimen at first thought to belong to a separate genus, "Frenguellisaurus", the skull measured 56 centimeters (1.8 ft) in length.

kull

"Herrerasaurus" had a long, narrow skull that lacked nearly all the specializations that characterized later dinosaurs,cite journal|last=Sereno|first=P.C. |coauthors=and Novas, F.E. |year=1993 |title=The skull and neck of the basal theropod "Herrerasaurus ischigualastensis" |journal=Journal of Vertebrate Paleontology |volume=13 |pages=451–476] being not that different from those of more primitive archosaurs such as "Euparkeria". It had five pairs of fenestrae (skull openings) in its skull, two of which were for ocular and nasal openings. Between the eyes and the nostrils were two antorbital fenestrae and a pair of tiny, 1-centimeter-long (0.4 in) slit-like holes called promaxillary fenestrae.cite journal |last=Sereno |first=P.C. |authorlink= |title=The phylogenetic relationships of early dinosaurs: a comparative report |journal=Historical Biology |volume=19 |issue=1 |pages=145–155 |date=2007 |doi=10.1080/08912960601167435 |id=ISSN 0891-2963 ] Behind the eyes were large infratemporal fenestrae. These holes helped reduce the weight of the skull.

"Herrerasaurus" had a flexible joint in the lower jaw; this allowed the animal to slide its lower jaw back and forth and deliver a grasping bite. This cranial specialization is unusual among the dinosaurs but has evolved independently in some lizards.cite journal|last=McDowell|first=S.B., Jr.|coauthors=and C.M. Bogert|year=1954|title=The systematic position of "Lanthanotus" and the affinities of the anguinomorphan lizards|journal=Bulletin of the American Museum of Natural History |volume=105 |pages=1–142] The rear of the lower jaw also had fenestrae. The jaws were equipped with large serrated teeth for biting and eating flesh, and the neck was slender and flexible.cite journal|last=Novas|first=F. E.|year=1994|title=New information on the systematics and postcranial skeleton of "Herrerasaurus ischigualastensis" (Theropoda: Herrerasauridae) from the Ischigualasto Formation (Upper Triassic) of Argentina|journal=Journal of Vertebrate Paleontology|volume=13|issue=4|pages=400–423]

Limbs

"Herrerasaurus" had relatively short forelimbs, which were less than half the length of its hind limbs. The upper arm and forearm were rather short, while the manus was elongated. The first two fingers and the thumb bore curved, sharp claws for grasping prey. Its fourth and fifth digits were small stubs without claws.cite journal|last=Sereno|first=P.C.|year=1993|title=The pectoral girdle and forelimb of the basal theropod "Herrerasaurus ischigualastensis"|journal=Journal of Vertebrate Paleontology|volume=13 |pages=425–450] "Herrerasaurus" was bipedal. It had strong hind limbs with short thighs and rather long feet, indicating this animal was most likely a swift runner. The balancing tail, partially stiffened by overlapping vertebral processes, also indicates an adaptation for speed.cite book |last=Langer |first=Max C. |editor=Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.)|title=The Dinosauria |edition=2nd |year= 2004|publisher=University of California Press |location=Berkeley |isbn=0-520-24209-2 |pages=25–46 |chapter=Basal Saurischia ]

Derived and basal characteristics

This dinosaur is an enigmatic creature, showing traits that are found in different groups of dinosaurs. Although it shared most of the characteristics of dinosaurs, there were a few differences, particularly in regard to the shape of its hip and leg bones. Its pelvis was similar to that of saurischian dinosaurs, but it had a bony acetabulum (where the femur meets the pelvis) that was only partially open. The ilium, the main hip bone, was supported only by two sacrals, a basal trait, but the pubis pointed backwards, a derived trait that parallels what is seen in dromaeosaurids and birds. Additionally, the end of the pubis had a booted shape, similar to what is present in avetheropods, and the vertebral centra had an "Allosaurus"-like hourglass shape.

Classification

"Herrerasaurus" gives its name to its family, Herrerasauridae, of the mid- to late Triassic, though where it and its close relatives lie on the early dinosaur evolutionary tree is unclear. They are possibly basal theropods or basal saurischians but may in fact predate the saurischian-ornithischian split. Other members of the clade may include "Eoraptor" from the same Ischigualasto Formation of Argentina as "Herrerasaurus", "Staurikosaurus" from the Santa Maria Formation of southern Brazil, [cite journal|last=Colbert|first=E.H.|year=1970|title=A saurischian dinosaur from the Triassic of Brazil|journal=American Museum Novitates|volume=2405|pages=1–39] "Chindesaurus" from the Upper Petrified Forest (Chinle Formation) of Arizona, [cite journal|last=Long|first=R.A.|coauthors=and Murry, P.A.|year=1995|title=Late Triassic (Carnian and Norian) Tetrapods from the Southwestern United States|journal=New Mexico Museum of Natural History and Science, Bulletin 4|pages=1–254] and possibly "Caseosaurus" from the Dockum Formation of Texas, [cite journal|last=Hunt|first=A.P.|coauthors=Lucas, S.G.; Heckert, A.B.; Sullivan, R.M.; and Lockley, M.G.|year=1998|title=Late Triassic Dinosaurs from the Western United States|journal=Geobios|volume=31|issue=4|pages=511–531|doi=10.1016/S0016-6995(98)80123-X] although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods, "Alwalkeria" from the Late Triassic Maleri Formation of India, [cite journal|last=Chatterjee|first=S.|coauthors=and Creisler, B.S.|year=1994|title="Alwalkeria" (Theropoda) and "Morturneria" (Plesiosauria), new names for preoccupied "Walkeria" Chatterjee, 1987 and "Turneria" Chatterjee and Small, 1989|journal=Journal of Vertebrate Paleontology|issue=1|pages=142|unused_data=|volume14] and "Teyuwasu", known from very fragmentary remains from the Late Triassic of Brazil, might be related. [cite journal|last=Kischlat|first=E.-E.|year=1999|title=A new dinosaurian “rescued” from the Brazilian Triassic: "Teyuwasu barberenai", new taxon|journal=1st Simp. Brasil. Pal. Vert, Paleontologia em Destaque|volume=14|issue=26|pages=58] Novas (1992) defined the group as "Herrerasaurus", "Staurikosaurus", and their most common ancestor.cite journal|last=Novas|first=F. E.|year=1992|title=Phylogenetic relationships of the basal dinosaurs, the Herrerasauridae|journal=Palaeontology|volume=35|pages=51–62] Sereno (1998) defined the group as the most inclusive clade including "H. ischigualastensis" but not "Passer domesticus".cite journal |last=Sereno |first=P.C. |year=1998 |title=A rationale for phylogenetic definitions, with application to the higher-level taxonomy of Dinosauria |journal=Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen |volume=210 |issue=1 |pages=41–83 ] Langer (2004) created a higher level taxon, infraorder Herrerasauria.

History

"Herrerasaurus" was named by paleontologist Osvaldo Reig after Victorino Herrera, an Andean goatherd who first noticed its fossils in outcrops near the city of San Juan in 1959. These rocks, which later yielded "Eoraptor",cite journal|last=Sereno|first=P.C.|coauthors=Forster, C.A.; Rogers, R.R.; and Monetta, A.M.|year=1993|title=Primitive dinosaur skeleton from Argentina and the early evolution of Dinosaurs|journal=Nature|volume=361|issue=6407|pages=64–66|doi=10.1038/361064a0] are part of the Ischigualasto Formation and date from the late Ladinian - early Carnian stages of the Late Triassic period.cite journal|last=Rogers|first=R. R.|coauthors=Swisher III, C.C.; Sereno, P.C.; Monetta, A.M.; Forster, C.A.; and Martinez, R.N. |year=1993|title=The Ischigualasto tetrapod assemblage (Late Triassic, Argentina) and 40Ar/39Ar dating of dinosaur origins|journal=Science|volume=260|pages=794–797|doi=10.1126/science.260.5109.794|pmid=17746113] Reig named a second dinosaur from these rocks in the same publication as "Herrerasaurus"; this dinosaur, "Ischisaurus cattoi", is now considered a junior synonym and a juvenile of "Herrerasaurus". Two other partial skeletons, with skull material, were named "Frenguellisaurus ischigualastensis" by Fernando Novas in 1986, [cite journal|last=Novas|first=F. E.|year=1986|title=Un probable terópodo (Saurischia) de la Formación Ischigualasto (Triásico superior), San Juan, Argentina|journal=IV Congreso Argentino de Paleontología y Bioestratigrafía, Mendoza, november 23-27|volume=2|pages=1–6] but this species too is now thought to be a synonym.

Reig believed "Herrerasaurus" was an early example of a carnosaur,cite journal|last=Reig|first=O.A.|year=1963|title=La presencia de dinosaurios saurisquios en los "Estratos de Ischigualasto" (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina)|journal= Ameghiniana|volume=3|issue=1|pages=3–20] but this was the subject of much debate over the next 30 years, and the genus was variously classified during that time. In 1970, Steel classified "Herrerasaurus" as a prosauropod.cite journal|last=Steel|first=R.|year= 1970|title =Part 14. Saurischia. Handbuch der Paläoherpetologie/Encyclopedia of Paleoherpetology|journal= Gustav Fischer Verlag, Stuttgart|pages=1–87] In 1972, Peter M. Galton classified the genus as not diagnosable beyond Saurischia.cite journal|last=Galton|first=P.M.|year=1973|title=On the anatomy and relationships of "Efraasia diagnostica" (Huene) n.gen., a prosauropod dinosaur (Reptilia: Saurischia) from the Upper Triassic of Germany.|journal=Paläontologische Zeitschrift|volume=47|issue=3/4|pages=229–255] Later, using cladistic analysis, some researchers put "Herrerasaurus" and "Staurikosaurus" at the base of the dinosaur tree before the separation between ornithischians and saurischians.cite journal|last=Paul|first=G.S.|year=1984|title=The segnosaurian dinosaurs: relics of the prosauropod-ornithischian transition?|journal=Journal of Vertebrate Paleontology |volume=4|pages=507–515] cite journal|last=Novas|first=F. E.|year=1989|title=The tibia and tarsus in Herrerasauridae (Dinosauria, incertae sedis) and the evolution and origin of the dinosaurian tarsus|journal=Journal of Paleontology |volume=63|pages=677–690] cite journal|last=Brinkman|first=D.B.|coauthors=and Sues, H.-D.|year=1987|title=A staurikosaurid dinosaur from the Upper Triassic Ischigualasto Formation of Argentina and the relationships of the Staurikosauridae |journal=Palaeontology |volume=30 |pages=493–503] Several researchers classified the remains as non-dinosaurian.Citation| last =Gauthier| first =J.A., Cannatella, D., Queiroz, K., Kluge, A.G., and Rowe, T.| contribution =Tetrapod phylogeny| year =1989| title =The Hierarchy of Life| editor-last =Fernholm| editor-first =K. Bremer, & Jörnvall, H. | volume =| pages =337–353| place=Amsterdam| publisher =Elsevier Science Publishers B. V.| id = ] A complete "Herrerasaurus" skull was not found until 1988, by a team of paleontologists led by Paul Sereno. [cite episode|title=The Nature of the Beast |episodelink= |series=The Dinosaurs!|serieslink= |airdate=1990 |season= |number=3] Based on the new fossils, authors such as Thomas Holtzcite journal |last=Holtz |first= T.R., Jr.|coauthors= and Padian, K. |year=1995 |title=Definition and diagnosis of Theropoda and related taxa |journal=Journal of Vertebrate Paleontolology |volume=15 |issue=3, supplement |pages=35A ] and Jose Bonaparte [cite journal|last=Bonaparte |first=J.F. |coauthors=and Pumares, J.A. |year=1995 |title=Notas sobre el primer cráneo de "Riojasaurus incertus" (Dinosauria, Prosauropoda, Melanorosauridae) del Triásico superior de La Rioja, Argentina |journal=Ameghiniana |volume=32 |pages=341–349] classified "Herrerasaurus" at the base of the saurischian tree before the divergence between prosauropods and theropods. However, Sereno favored classifying "Herrerasaurus" (and the Herrerasauridae) as primitive theropods. These two classifications have become the most persistent, with Rauhut (2003)cite journal |last=Rauhut|first=O.W.M. |title=The interrelationships and evolution of basal theropod dinosaurs |journal=Special Papers in Palaeontology |volume=69 |pages=1–213 |publisher= |date=2003 ] and Bittencourt and Kellner (2004)cite journal| last =Bittencourt| first =J. |coauthors=and Kellner, W.A. |title=The phylogenetic position of "Staurikosaurus pricei" from the Triassic of Brazil |journal=Journal of Vertebrate Paleontology |volume=24 |issue=3, supplement |pages=39A |date=2004 |doi=] favoring the early theropod hypothesis, and Max Langer (2004), Langer and Benton (2006),cite journal |last=Langer |first =M.C| coauthors=and Benton, M.J. |title=Early dinosaurs: a phylogenetic study |journal=Journal of Systematic Palaeontology |volume=4 |issue=4 |pages=309–358 |date=2006 |doi=10.1017/S1477201906001970 ] and Randall Irmis and his coauthors (2007)cite journal |last=Irmis |first=Randall B. |coauthors=Nesbitt, Sterling J.; Padian, Kevin; Smith, Nathan D.; Turner, Alan H.; Woody, Daniel; and Downs, Alex |year=2007 |title=A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs |journal=Science |volume=317 |pages=358–361 |doi=10.1126/science.1143325 |pmid=17641198] favoring the basal saurischian hypothesis. If "Herrerasaurus" was indeed a theropod, it would indicate that theropods, sauropodomorphs, and ornithischians diverged even earlier than herrerasaurids, before the middle Carnian (age of the Ischigualasto Formation), and that "all three lineages independently evolved several dinosaurian features, such as a more advanced ankle joint or an open acetabulum".cite web |last=White |first=A.T. |coauthors=and Kazlev, M.A. |title=Theropoda: Basal Theropods |publisher=Palaeos.com |date=2003 |url=http://www.palaeos.com/Vertebrates/Units/340Theropoda/340.100.html |accessdate=2007-07-20 ] This view is further supported by ichnological records showing large tridactyl footprints that can be attributed only to a theropod dinosaur, dating from the Ladinian (Middle Triassic) of the Los Rastros Formation in Argentina and predating "Herrerasaurus" by 3 to 5 million years. [cite journal|last=Forster|first=C.A.|coauthors=Arcucci, A.B.; Marsicano, C.A.; Abdala, F.; and May, C.L. |year=1995 |title=New vertebrate material from the Los Rastros Formation (Middle Triassic), La Rioja province, northwestern Argentina|journal=Journal of Vertebrate Paleontology |volume=15 |issue=3, supplement |pages=29A] [cite journal|last=Arcucci|first=A. C.|coauthors=Forster, C.; May, Abdala, F.; and Marsicano, C. |year=1998 |title=Una nueva icnofauna de la Formación Los Rastros, Triásico Medio, en la Quebrada de Ischichuca Chica (Provincia de La Rioja, Argentina) |journal=Acta Geológica Lilloana |volume=18 |pages=152 ]

The importance of "Herrerasaurus" and "Eoraptor" lies in the fact that their remains allow for directly testing the idea of dinosaurs being a monophyletic group, i.e. all dinosaurs have a common ancestor. The monophyly of dinosaurs was explicitly proposed in the 1970s by Bakker,cite journal|last=Bakker|first=R.T.|coauthors=and Galton, P.M.|title=Dinosaur monophyly and a new class of vertebrates |journal=Nature |volume=248 |pages=168–172 |doi=10.1038/248168a0|year=1974 ] cite journal|last=Bonaparte|first=J. F.|year=1976|title="Pisanosaurus mertii" Casimiquela and the origin of the Ornithischia|journal=Journal of Paleontology|volume=50|pages=808–820.] and nine cranial and about fifty postcranial synapomorphies (common anatomical traits derived from the common ancestor) have been listed. However, an extensive study of "Herrerasaurus" by Sereno indicates that only one cranial and seven postcranial synapomorphies in Bakker's original list are actually supported while additional synapomorphies were discovered.

Paleoecology

Although "Herrerasaurus" shared the body shape of the large carnivorous dinosaurs, it lived about 230 million years ago, a time when dinosaurs were small and insignificant. It was the time of non-dinosaurian reptiles, not dinosaurs, and a major turning point in the Earth's ecology. The vertebrate fauna of the Ischigualasto Formation and the slightly later Los Colorados Formation consisted mainly of a variety of crurotarsal archosaurs and synapsids. [cite journal|last=Bonaparte|first=J. F.|year=1982|title=Faunal replacement in the Triassic of South America|journal=Journal of Vertebrate Paleontology |volume=2 |pages=362–371 ] cite journal|last=Bonaparte|first=J.F.|year=1979|title=Faunas y paleobiogeografía de los tetrápodos mesozoicos de América del Sur|journal=Ameghiniana, Revista de la Asociación Paleontológica Argentina|volume=16|issue=3–4|pages=217–238 |language=Spanish] For instance, in the Ischigualasto Formation, dinosaurs constituted only about 6% of the total number of fossils.Citation| first =Michael J.| last =Benton| author-link =| editor-last =Farlow| editor-first =James O.| editor2-last =Brett-Surman| editor2-first =M.K.| contribution =Origin and early evolution of dinosaurs| contribution-url =http://books.google.com/books?id=FOViD-lDPy0C&pg=PA212&lpg=PA212&dq=%22ischigualasto+formation%22+%226+percent%22&source=web&ots=N7LHhehm_c&sig=6QtMrZaVqjWpW8fNTG8X1rQ_aUc#PPA204,M1| title =The Complete Dinosaur| year =1999| pages =204–215| place =| publisher =Indiana University Press| url =| doi =| isbn =0-253-21313-4] By the end of the Triassic Period, dinosaurs were becoming the dominant large land animals, and the other archosaurs and synapsids lost diversity.Citation| first =J. Michael| last =Parrish| author-link =| editor-last =Farlow| editor-first =James O.| editor2-last =Brett-Surman| editor2-first =M.K.| contribution =Evolution of the archosaurs| contribution-url =http://books.google.com/books?id=FOViD-lDPy0C&pg=PA201&lpg=PA201&dq=archosaur+diversity+decline&source=web&ots=N7LHhejhUf&sig=uy-qcUtW1XVgym8X9s33l6juBLg#PPA201,M1| title =The Complete Dinosaur| year =1999| pages =191–203| place =| publisher =Indiana University Press| url =| doi =| isbn =0-253-21313-4]

Studies suggest that the paleoenvironment of the Ischigualasto Formation was a volcanically active floodplain covered by forests and subject to strong seasonal rainfalls. Vegetation consisted of ferns ("Cladophlebis"), sphenopsids (horsetails), and giant conifers ("Protojuniperoxylon"). The plants formed an upland riparian forest. "Herrerasaurus" remains appear to have been the most common among the carnivores of the Ischigualasto Formation. It lived in the jungles of Late Triassic South America alongside another early dinosaur, "Eoraptor", as well as "Saurosuchus", [cite journal|last = Sill|first=W.D.|year=1974 |title=The anatomy of "Saurosuchus galilei" and the relationships of the rauisuchid thecodonts |journal=Bulletin of the Museum of Comparative Zoology |volume=146 |pages=317–362 ] a giant land-living meat-eating rauisuchian; "Venaticosuchus", an ornithosuchid; and the predatory chiniquodontid cynodonts. Herbivores were much more abundant than carnivores and were represented by rhynchosaurs such as "Hyperodapedon" (formerly "Scaphonyx"); aetosaurs; kannemeyeriid dicynodonts such as "Ischigualastia", and traversodontids such as "Exaeretodon". These non-dinosaurian herbivores were much more abundant than early ornithischian dinosaurs like "Pisanosaurus" [cite journal |last=Bonaparte |first=J.F. |year=1970 |title=Annotated list of the South American Triassic tetrapods |journal=Gondwana Symposium Proceedings and Papers |volume=2 |pages=665–682] and therefore more likely prey for "Herrerasaurus" than were the early dinosaurs.

Paleobiology

The teeth of "Herrerasaurus" indicate it was a carnivore; its size indicates it would have preyed upon small and medium-sized animals. It may have fed on other dinosaurs, such as the herbivorous "Pisanosaurus". However, since "Herrerasaurus" lived during an era when other dinosaurs were uncommon, more plentiful prey would have included rhynchosaurs and aetosaurs. "Herrerasaurus" itself may have been preyed upon by giant rauisuchids like "Saurosuchus", as puncture wounds were found in one skull.

Coprolites (fossilized dung) containing small bones but no trace of plant fragments, discovered in the Ischigualasto Formation, have been assigned to "Herrerasaurus" based on fossil abundance. The mineralogical and chemical analysis of these coprolites indicate that the carnivorous animal had the ability to digest bones. [cite journal |last=Hollocher |first=K.T. |coauthors=Alcober, O.A.; Colombi, C.E.; and Hollocher, T.C. |title=Carnivore coprolites from the Upper Triassic Ischigualasto Formation, Argentina: chemistry, mineralogy, and evidence for rapid initial mineralization |journal=PALAIOS |year=2005 |volume=20 |pages=51–63 |doi=10.2110/palo.2003.p03-98 ]

References

External links

* [http://www.ucmp.berkeley.edu/diapsids/herrerasaurus.html Introduction to "Herrerasaurus"] , from the University of California Museum of Paleontology