Herrerasaurus

Herrerasaurus is a dinosaur genus which lived in the Late Triassic period. The meaning of the name Herrerasaurus is "Herrera's lizard"; this species got its name after the rancher who discovered its fossils in Northwestern Argentina in 1958. All known Herrerasaurus was found in the Ischigualasto Formation of Carnian age, Late Triassic, approximately 231 to 231 million years ago. Herrerasaurus ischigualastensis was described by Osvaldo Reig in 1963[1], and as of 2021 no further species belonging to the genus was described. Herrerasaurus was among the earliest taxa of the evolutionary radiation of dinosaurs[2]. Two similar dinosaur taxa, Frenguellisaurus ischigualastensis and Ischisaurus cattoi proved to be junior synonymies of H. ischigualastensis[2].

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Known occurrences of Herrerasaurus fossils.

Description and anatomy

Herrerasaurus was a bipedal predator dinosaur, having a long tail and a relatively small head (its skull was about 56 cm long), and a slender and flexible neck. Herrerasaurus had large and serrated teeth, referring to the carnivorous being of the animal. Unlike in other dinosaurs, joint in its lower jaw Herrerasaurus had a flexible that was capable of sliding back and forth, which resulted a kid of grasping bite[11]. The total lenth of the animal was about 6 meters, and its weight about 350 kg[12]. Its relatively short thigs and long feet indicates that this dinosaur was probably a quite fast runner. It had five toes on its feet, the middle three were larger and they bore the weight of the animal. The tail of Herrerasaurus was somewhat stiffened by vertebral projections which overlapped with each other, balancing the body and adjusting the speed of the animal during running. The forelimbs were smaller, with relatively shorter upper arm and forearm, and a longer hand. It had sharp and curved claws on its first and second fingers and on its thumb, while the fourth and fifth fingers were smaller and had no claws[13].

Classification

Herrerasaurus was known originally from very fragmentary fossils, so its classification was disputed for a long time. Some authors regarded it to be a basal theropod[1], others to a basal sauropodomorph[3], or a basal saurischian[4], or even to an other type of archosaurs. Based on cladistical analysis some researchers placed Herrerasaurus before the separation of ornithischian and saurischian dinosaurs[5]. In 1988 an almost complete skeleton and skull was explored, which helped to understand better the taxonomical position of this dinosaur[6]. Based on the new fossils, some researchers regard Herrerasaurus for an early saurischian, having a basal position on saurischian tree, before the divergence between prosauropods and theropods[6][7]. Others support the hypothesys that Herrerasaurus was an early theropod[8][9]. Accepting the first thesys would suggest that the divergence between sauropodomorphs, theropods and ornithischians and saurischians took place even earlier than it is currently accepted. This would also mean that "all three lineages independently evolved several dinosaurian features, such as a more advanced ankle joint or an open acetabulum"[10].

Herrerasaurus has a quite important role for understanding the concept of the monophyly of dinosaurs (meaning that they are a group which shares a common ancestor). It shows features that are characteristic of different dinosaur groups, and some further traits that are typical for non-dinosaur archosaurs. It differs from dinosaurs in its leg bones and in the shape of the hip. It has a saurischian-like pelvis, but has a partially open bony acetabulum. Its main hip bone, the ilium, was supported by only two sacrals, which is a plesiomorphic character[14]. In contrast with this, Herrerasaurus also posses some apomorphic characters, its pubis is directed backwards, just like in dromeaosaurids and in modern birds. Furthermore, similarly to avetheropods, the end of its pubis shows a booted shape[12]. In addition, the skull of Herrerasaurus lacked most of the specializations that were characteristic for dinosaurs[11], and it is more similar to the skull of primitive archosauria (e.g. Euparkeria).

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Herrerasaurus ischigualastensis. By Fred Wierum - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49270112

Paleobiology

As mentioned above, Herrerasaurus was probably a carnivore. Concluding from its size, it prayed most likely upon smaller herbivors and insectivors, and maybe also medium-sized herbivors. Preys could have belong to other dinosaurs, synapsids and rhynchosaurs[15]. However, puncture-like wounds on a fossil Herrerasaurus skull suggest that this dinosaur was also preyed upon by larger predators, like Saurosuchus and other huge rauisuchids[13]. Coprolites assigned to Herrerasaurus containing small bones indicate that this dinosaur was able to digest the bones of its preys[16]. Herrerasaurus was probably a cathemeral animal, which means that it was active for short certain intervals throughout the day[17].

Ischigualasto Formation, which was actually the paleoenvironment of Herrerasaurus, was probably a foodplain covered by forests and with considerable volcanic activity. It seems that the paleolcimate of the area was mostly warm and moist, with strong seasonal rainfalls, and having different seasons[18]. The vegetation of this environment was probably dominated by horsetails, ferns, and giant conifers[19].

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Herrerasaurus ischigualastensis skeleton. By Jonathan Chen - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=72960592

Scientific references

[1] Reig, O.A. (1963): La presencia de dinosaurios saurisquios en los "Estratos de Ischigualasto" (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana (in Spanish). 3 (1): 3-20.

[2] Sereno, P.C.; Novas, F.E. (1992): The complete skull and skeleton of an early dinosaur. Science. 258 (5085): 1137-1140. doi:10.1126/science.258.5085.1137.

[3] Steel, R. (1970): Part 14. Saurischia. Handbuch der Paläoherpetologie/Encyclopedia of Paleoherpetology. Gustav Fischer Verlag, Stuttgart: 1-87.

[4] Galton, P.M. (1973): On the anatomy and relationships of Efraasia diagnostica (Huene) n.gen., a prosauropod dinosaur (Reptilia: Saurischia) from the Upper Triassic of Germany. Paläontologische Zeitschrift. 47 (3/4): 229-255. doi:10.1007/BF02985709.

[5] Novas, F.E. (1992): Phylogenetic relationships of the basal dinosaurs, the Herrerasauridae. Palaeontology. 35 (1): 51-62.

[6] Brinkman, D.B.; Sues, H.D. (1987): A staurikosaurid dinosaur from the Upper Triassic Ischigualasto Formation of Argentina and the relationships of the Staurikosauridae. Palaeontology. 30 (3): 493-503.

[7] Holtz, Thomas R. Jr. (2011): Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2010 Appendix.

[8] Rauhut, O.W.M. (2003): The interrelationships and evolution of basal theropod dinosaurs. Special Papers in Palaeontology. 69: 1-213.

[9] Bittencourt, J.; Kellner, W.A. (2004): The phylogenetic position of Staurikosaurus pricei from the Triassic of Brazil. Journal of Vertebrate Paleontology. 24 (3, supplement): 39A. doi:10.1080/02724634.2004.10010643.

[10] White, A.T.; Kazlev, M.A. (2003): Theropoda: Basal Theropods. Science. 326 (5959): 1530-1533. doi:10.1126/science.1180350.

[11] Sereno, P.C.; Novas, F.E. (1993): The skull and neck of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology. 13 (4): 451-476. doi:10.1080/02724634.1994.10011525.

[12] Paul, G.S. (1988): Predatory Dinosaurs of the World. New York: Simon and Schuster. pp. 248-250. ISBN 978-0-671-68733-5.

[13] Sereno, P.C. (1993): The pectoral girdle and forelimb of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology. 13 (4): 425-450. doi:10.1080/02724634.1994.10011524.

[14] Langer, M.C. (2004): Basal Saurischia. In Weishampel, David B.; Dodson, Peter; Osmólska Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 25-46. ISBN 978-0-520-24209-8.

[15] Sues, H.D. (1990): Staurikosaurus and Herrerasauridae. In Weishampel, D.B.; Dodson, P.; Osmólska, H. (eds.). The Dinosauria. University of California Press. pp. 143-47. ISBN 978-0-520-06726-4.

[16] Hollocher, K.T.; Alcober, O.A.; Colombi, C.E.; Hollocher, T.C. (2005): Carnivore coprolites from the Upper Triassic Ischigualasto Formation, Argentina: chemistry, mineralogy, and evidence for rapid initial mineralization. PALAIOS. 20 (1): 51-63. doi:10.2110/palo.2003.p03-98.

[17] Schmitz, L.; Motani, R. (2011): Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology. Science. 332 (6030): 705-8. doi:10.1126/science.1200043.

[18] Columbi, C.E. (2008): Stable isotope analysis of fossil plants from the Upper Triassic Ischigualasto Formation in the northwest of Argentina. Houston, Texas: The Geological Society of America.

[19] Bonaparte, J.F. (1979): Faunas y paleobiogeografía de los tetrápodos mesozoicos de América del Sur. Ameghiniana, Revista de la Asociación Paleontológica Argentina (in Spanish). 16 (3-4): 217-238.