The Chañares Formation and the origin of dinosaurs.

The Chañares Formation (© 2012 Idean)

The Chañares Formation (© 2012 Idean)

The Chañares Formation crops out in the Ischigualasto-Villa Unión Basin, formed along the western margin of South America  during the  breakup  of  Gondwana. It represents one of the most continuous continental Triassic succesions in South America. These beds were explored by Alfred Romer and Jensen (1966) in their report on the geology of the Rio Chañares and Rio Gualo region.

Located in Talampaya National Park (La Rioja Province), the Chañares Formation is characterized at its base by a sandstone–siltstone fluvial facies with distinct lower and upper levels. The lower levels are composed of light olive grey fine-grained sandstones with abundant small brown carbonate concretions. The upper levels include fine-grained sandstones and siltstones that preserve vertebrate remains (Mancuso et al., 2014).

Geological map of the Chañares–Gualo area in Talampaya National Park (From Marsicano et al., 2015)

Geological map of the Chañares–Gualo area in Talampaya National Park (From Marsicano et al., 2015)

Volcanism played an important role in the generation and preservation of the Chañares Formation’s exceptional tetrapod fossil record. The diverse and well-preserved tetrapod assemblage includes proterochampsids, pseudosuchians, ornithodirans, large dicynodonts and smaller cynodonts. Almost all dinosauromorphs are preserved in diagenetic concretions that erode out of a thick siltstone interval 15–20 m above the base of the formation, and include Lagosuchus talampayensis, Marasuchus lilloensis Lewisuchus admixtus and Pseudolagosuchus major.

Analysing the ratio of U–Pb inside the zircon crystals found in the rocks assigns the Chañares Formation to the Late Triassic, specifically the early Carnian (236–234 Ma), between 5 to 10 million years younger than previous estimate. This also suggests a similarly age for the lower Santa Maria Formation in southern Brazil, because it shares with the Chañares assemblage a variety of tetrapod genera and species unknown from anywhere else. The new results provide the basis to construct a robust framework for calibrating the timing of macro-evolutionary patterns related to the origin and early diversification of dinosaurs in Gondwana (Marsicano et al., 2015). It also suggests there was little compositional difference between the Chañares assemblage and the earliest dinosaur assemblage from the lower part of the Ischigualasto succession, where dinosauromorphs (including dinosaurs) are a minority, with synapsids still dominant. Only 15 million years later dinosaurs begin to dominate the ecosystem.

Artist’s reconstruction of the Chanares environment during the Middle Triassic. (From Mancusso et al., 2014. Art by Jorge Fernando Herrman.)

Artist’s reconstruction of the Chanares environment during the Middle Triassic. (From Mancusso et al., 2014. Art by Jorge Fernando Herrman.)

 

References:

Marsicano, C. A., Irmis, R. B., Mancuso, A. C., Mundil, R. & Chemale, F., The precise temporal calibration of dinosaur origins, Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1512541112 (2015).

Brusatte SL, et al. (2010) The origin and early radiation of dinosaurs. Earth Sci Rev 101:68100.

Mancuso AC, Gaetano LC, Leardi JM, Abdala F, Arcucci AB (2014) The ChañaresFormation: A window to a Middle Triassic tetrapod community. Lethaia 47:244265.

Romer AS, Jensen J (1966) The Chañares (Argentina) Triassic reptile fauna. II. Sketch of the geology of the Rio Chañares, Rio Gualo region. Breviora 252:1–20.

 

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A brief introduction to the T. rex Family Tree.

“Sue” specimen, Field Museum of Natural History, Chicago

Tyrannosaurus rex is the most iconic dinosaur of all timeIt was discovered in the Hell Creek Formation by Barnum Brown in 1902, and later described  by Henry Fairfield Osborn in 1905. Osborn actually named two large Hell Creek tyrannosaurids, T. rex and Dynamosaurus imperiosus. He later realized that Dynamosaurus imperiosus and Tyrannosaurus rex were synonymous, but Tyrannosaurus has priority, as it preceded Dynamosaurus in the description (Osborn, 1906).

T. rex and its closest relatives, known as tyrannosaurids, comprise the clade Tyrannosauroidea, a relatively derived group of theropod dinosaurs, more closely related to birds than to other large theropods such as allosauroids and spinosaurids, that originated in the Middle Jurassic, approximately 165 million years ago. All tyrannosaurs were bipedal predators characterized by premaxillary teeth with a D-shaped cross section, fused nasals, extreme pneumaticity in the skull roof and lower jaws, a pronounced muscle attachment ridge on the ilium, and an elevated femoral head (Brusatte et al., 2010). For most of their evolutionary history, tyrannosauroids were mostly small-bodied animals and only reached gigantic size during the final 20 million years of the Cretaceous.

Skulls of the basal tyrannosauroids Guanlong (A), Dilong (B); Skulls of juvenile (C) and adult (D)Tyrannosaurus. (Adapted from Brusatte et. al., 2010)

Skulls of the basal tyrannosauroids Guanlong (A), Dilong (B); Skulls of juvenile (C) and adult (D)Tyrannosaurus. (Adapted from Brusatte et. al., 2010)

During the past 15 years, new discoveries from Russia, Mongolia and China helped to build the Tyranosaurs family tree. The oldest and most basal tyrannosaurs comprise a subclade, Proceratosauridae, which includes Kilesksus, Gualong, and Proceratosaurus. They were small-bodied animals no larger than a human, with elaborate cranial crests, extremely elongated external naris, a short ventral margin of the premaxilla, and depth of the antorbital fossa ventral to the antorbital fenestra is greater than the depth of the maxilla below the ventral margin of the antorbital fossa (Rauhut et al., 2010; Averianov et al., 2010).

Kileskus artistotocus, from the Middle Jurassic (167 mya), was discovered in 2010 in Western Siberia, by Alexander Averianov on the basis of an associated maxilla and premaxilla, a mandible fragment, and some possible associated postcranial elements. The cranial crest is currently unknown for Kileskus.

Pedal ungual phalanx of Kileskus aristotocus. Abbreviations: ft – flexor tubercle; lgr – lateral groove. Scale bar = 1 cm (From Averianov et. al.; 2010)

Pedal ungual phalanx of Kileskus aristotocus. Abbreviations: ft – flexor tubercle; lgr – lateral groove. Scale bar = 1 cm (From Averianov et. al.; 2010)

Guanlong wucaii, from the Late Jurassic of China, was first described in 2006. The generic name is derived from the Chinese Guan (crown) and long (dragon). The specific epithet wucaii (five colours) referred to the colours of rock of the Wucaiwan area where the fossil was found. The most striking trait of Guanlong is the complex nasal crest consisting of a highly pneumatic median crest that is about 1.5 mm thick for most of its length, and four supporting lateral laminae (Xu et al., 2006).

Proceratosaurus bradleyi, discovered in Gloucestershire, England in 1910 and described by Arthur Smith Woodward, it was originally thought to be an ancestor of Ceratosaurus. Some of the characters uniting Proceratosaurus with Guanlong are the  strongly  enlarged  nares, and a midline cranial crest or horn  on  the  nasals (Rauhut et al., 2010).

Guanlong wucaii. (Image adapted from Xu et al., 2006)

Guanlong wucaii. (Image adapted from Xu et al., 2006)

The giant, feathered tyrannosaur Yutyrannus huali, lived during the early Cretaceous period in what is now Northeastern  China. It was discovered in 2012 by Chinese palaeontologist Xing Xu. Yutyrannus weighed about 1,400 kilograms and  was at least 8 metres in length, and shares some features, particularly of the cranium, with derived tyrannosauroids, but is similar to other basal tyrannosauroids in possessing a three-fingered manus and a typical theropod pes.

Dilong paradoxus, also described by Xu, was discovered in 2004. This small tyrannosauroid shows a mosaic of characters, including a derived cranial structure resembling that of derived tyrannosauroids and a primitive postcranial skeleton similar to basal coelurosaurians. And at least one specimen was preserved with remnants of protofeathers.

Yutyrannus skeleton (From Wikimedia Commons)

Yutyrannus skeleton (From Wikimedia Commons)

Eotyrannus lengi, from the Early Cretaceous of the Isle of Wight, United Kingdom, was described in 2001. The holotype of Eotyrannus are estimated to have measured about 4 m (13 ft) long. However, as it is believed to have been juvenile, an adult specimen might have been somewhat larger.

Qianzhousaurus sinensis, was discovered in 2014 in China. Nicknamed “Pinocchio rex”, this long-snouted tyrannosaurids along with Alioramus, shows that these type of tyrannosaurids were widely distributed in Asia.

Nanuqsaurus hoglund, was a small dinosaur discovered in Alaska in 2014. The name is the combination ofnanuqthe Iñupiaq word for polar bear and the Greek ‘sauros’ (lizard). The specific name, hoglundi, honors the Texas philanthropist Forrest Hoglund.

Skull of Qianzhousaurus sinensis (Image credit: Junchang Lü et al.)

Skull of Qianzhousaurus sinensis (Image credit: Junchang Lü et al.)

Until recently, all tyrannosaurs fossils were limited to Asia and North America, but the latest discoveries suggest a more  cosmopolitan distribution during their early evolution.  Tyrannosaurs more derived than Eotyrannus, exhibit a purely Asian or North American distribution, which indicates an increasing Laurasian-Gondwanan provincialism during the final stages of the Age of Dinosaurs (Brusatte et al., 2010).

References:

Averianov, A., Krasnolutskii, S., Ivantsov, S. 2010. A new basal coelurosaur (Dinosauria: Theropoda) from the Middle Jurassic of Siberia. Proceedings of the Zoological Institute RAS 314, 1: 42–57.

Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, et al. (2010) Tyrannosaur paleobiology: new research on ancient exemplar organisms. Science 329: 1481–1485. doi: 10.1126/science.1193304

Fiorillo AR, Tykoski RS (2014) A Diminutive New Tyrannosaur from the Top of the World. PLoS ONE 9(3): e91287. doi:10.1371/journal.pone.0091287

Loewen MA, Irmis RB, Sertich JJW, Currie PJ, Sampson SD (2013) Tyrant Dinosaur Evolution Tracks the Rise and Fall of Late Cretaceous Oceans. PLoS ONE 8(11): e79420. doi:10.1371/journal.pone.0079420

RAUHUT, O. W. M., MILNER, A. C. and MOORE-FAY, S. (2010), Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi (Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society, 158: 155–195. doi: 10.1111/j.1096-3642.2009.00591.x

Xu X., Clark, J.M., Forster, C. A., Norell, M.A., Erickson, G.M., Eberth, D.A., Jia, C., and Zhao, Q. (2006). “A basal tyrannosauroid dinosaur from the Late Jurassic of China”, Nature 439 (7077): 715–718. doi:10.1038/nature04511

Darwin, Owen and the ‘London specimen’.

Portrait of Charles Darwin painted by George Richmond (1840)

Portrait of Charles Darwin painted by George Richmond (1840)

The Archaeopteryx story began in  the summer of 1861, two years after the publication of the first edition of Darwin’s Origin of Species, when workers in a limestone quarry in Germany discovered the impression of a single 145-million-year-old feather. On August 15, 1861, German paleontologist Hermann von Meyer wrote a letter to the editor of the journal Neues Jahrbuch für Mineralogie, Geologie und Palaeontologie, where he made the first description of the fossil. Later, on September 30, 1861, he wrote a new letter:  “I have inspected the feather from Solenhofen closely from all directions, and that I have come to the conclusion that this is a veritable fossilisation in the lithographic stone that fully corresponds with a birds’ feather. I heard from Mr. Obergerichtsrath Witte, that the almost complete skeleton of a feather-clad animals had been found in the lithographic stone. It is reported to show many differences with living birds. I will publish a report of the feather I inspected, along with a detailed illustration. As a denomination for the animal I consider Archaeopteryx lithographica to be a fitting name”. 

The near complete fossil skeleton found in a Langenaltheim quarry near Solnhofen – with clear impressions of wing and tail feathers –  was examined by Andreas Wagner, director of the Paleontology Collection of the State of Bavaria in Germany. He reached the conclusion that the fossil was a reptile, and gave it the name Griphosaurus. He wrote: “Darwin and his adherents will probably employ the new discovery as an exceedingly welcome occurrence for the justification of their strange views upon the transformations of animals.”

Archaeopteryx lithographica, Archaeopterygidae, Replica of the London specimen; Staatliches Museum für Naturkunde Karlsruhe, Germany. From Wikimedia Commons

Archaeopteryx lithographica, Archaeopterygidae, Replica of the London specimen; Staatliches Museum für Naturkunde Karlsruhe, Germany. From Wikimedia Commons

The fossil was later bought by the British Museum of Natural History in London. Richard Owen, head of the Museum, was the first to describe the fossil and named it Archaeopteryx macrura, arguing that its identity with Meyer’s specimen could not be satisfactorily established (Owen 1862a, p. 33 n.). This fossil is also know as the London specimen. Owen, a fervent opponent of the evolutionary theory of Charles Darwin, was convinced that all animals within each larger systematic group were only variations of a single theme, the ‘ideal archetype’.

Hugh Falconer, a Scottish geologist and paleontologist, saw the Archaeopteryx as a valid “transitional” fossil. At that time, he was in  a dispute with Owen, and pointed out that Owen’s description of the Archaeopteryx had missed some essential elements. On January 3, 1863, he wrote a letter to Darwin about the significance of this fossil:  “It is a much more astounding creature—than has entered into the the conception of the describer—who compares it with the Raptores & Passeres & Gallinaceæ, as a round winged (like the last) `Bird of flight.’ It actually had at least two long free digits to the fore limb—and those digits bearing claws as long and strong as those on the hind leg. Couple this with the long tail—and other odd things,—which I reserve for a jaw—and you will have the sort of misbegotten-bird-creature—the dawn of an oncoming conception `a la Darwin.”

Darwin answered that letter on January 20, 1863, and commented about Owen’s mistake: “Has God demented Owen, as a punishment for his crimes, that he should overlook such a point?. “

Richard Owen stands next to the largest of all moa, Dinornis maximus (now D. novaezealandiae). From Wikimedia Commons.

Richard Owen stands next to the largest of all moa, Dinornis maximus (now D. novaezealandiae). From Wikimedia Commons.

In later editions of The Origin of Species, Darwin mention the Archaeopteryx: “That strange bird, Archaeopteryx, with a long lizardlike tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws . . . Hardly any recent discovery shows more forcibly than this, how little we as yet know of the former inhabitants of the world.”

 

References:

MEYER v., H. (1861): Archaeopterix lithographica (Vogel-Feder) und Pterodactylus von Solenhofen. Neues Jahrbuch fur Mineralogie, Geognosie, Geologie und Petrefakten-Kunde. 6: 678-679

Falconer, H. letter of January 3, 1863 to Charles Darwin; The Correspondence of Charles Darwin Vol. 11, edited by F. Furkhardt, DM Porter, S. A Dean, J. R Tophan, and S. Wilmot.  Cambridge University Press, Cambridge, 1999

OWEN, R. (1863): On the Archaeopteryx of von Meyer, with a description of the fossil remains of a long-tailed species, from the lithographic stone of Solenhofen. Philosophical Transactions of the Royal Society of London 153: 33-47

Prothero, D. R.  Evolution: What the Fossils Say and Why it Matters. Columbia University Press, New York, 2007.

Peter Wellnhofer, A short history of research on Archaeopteryx and its relationship with dinosaurs, Geological Society, London, Special Publications, 343:237-250, doi:10.1144/SP343.14, 2010

 

Links:

Darwin Correspondence Project http://www.darwinproject.ac.uk/entry-3899

 

A brief introduction to the origin of Birds.

Archaeopteryx lithographica, specimen displayed at the Museum für Naturkunde in Berlin. (From Wikimedia Commons)

Archaeopteryx lithographica, specimen displayed at the Museum für Naturkunde in Berlin. (From Wikimedia Commons)

Birds originated from a theropod lineage more than 150 million years ago. Their evolutionary history is one of the most enduring and fascinating debates in paleontology. In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. The discovered fossils demonstrate that distinctive bird characteristics such as feathers, flight, endothermic physiology, unique strategies for reproduction and growth, and a novel pulmonary system have a sequential and stepwise transformational pattern, with many arising early in dinosaur evolution, like the unusually crouched hindlimb for bipedal locomotion,the furcula and the “semilunate” carpal that appeared early in the theropod lineage (Allen et al., 2013; Xu et al., 2014).  Also, the discovery of Mahakala – a basal dromaeosaurid dinosaur named for one of the eight protector deities in Tibetan Buddhism – suggests that extreme miniaturization and laterally movable arms necessary for flapping flight are ancestral for paravian theropods. In contrast, a number of basal birds resemble theropods in many features.

Sin título

Sciurumimus (A); the basal coelurosaur Sinosauropteryx (B) with filamentous feathers; the deinonychosaurs Anchiornis (C) and Microraptor (D). Adapted from Xu et al., 2014.

Anatomical features like aspects of egg shape, ornamentation, microstructure, and porosity of living birds trace their origin to the maniraptoran theropods, such as oviraptorosaurs and troodontids. In addition, some preserving brooding postures, are known for four oviraptorosaurs, two troodontids, a dromaeosaur, and one basal bird providing clear evidence for parental care of eggs.

In birds, particularly their forebrains, are expanded relative to body size. The volumetric expansion of the avian endocranium began relatively early in theropod evolution. Archaeopteryx lithographica is volumetrically intermediate between those of more basal theropods and crown birds (Balanoff et al., 2013). The digital brain cast of Archaeopteryx also present an indentation that could be from the wulst, a neurological structure present in living birds used in information processing and motor control with two primary inputs: somatosensory and visual. Birds also exhibit the most advanced vertebrate visual system, with a highly developed ability to distinguish colors over a wide range of wavelengths.

Reconstruction of pulmonary components [cervical air-sac system (green), lung (orange), and abdominal air-sac system (blue)] in the theropod Majungatholus (From Xu et al., 2014)

Reconstruction of pulmonary components [cervical air-sac system (green), lung (orange), and abdominal air-sac system (blue)] in the theropod Majungatholus (From Xu et al., 2014)

Feathers were once considered to be unique avialan structures. The megalosaurus Sciurumimus, the compsognathus Sinosauropteryx, and a few other dinosaurs, document the appearance of primitive feathers. More recent studies indicated that non avian dinosaurs, as part of Archosauria, possessed the entirety of the known non keratin protein-coding toolkit for making feathers (Lowe et al., 2015)

The evolution of flight involved a series of adaptive changes at the morphological and molecular levels,like the fusion and elimination of some bones and the pneumatization of the remaining ones. The extensive skeletal pneumaticity in theropods such as Majungasaurus demonstrates that a complex air-sac system and birdlike respiration evolved in birds’ theropod ancestors. The increased metabolism associated with homeothermy and powered flight requires an efficient gas exchange process during pulmonary ventilation. Moreover, recent anatomical and physiological studies show that alligators, and monitor lizards exhibit respiratory systems and unidirectional breathing akin to those of birds, which indicate that unidirectional breathing is a primitive characteristic of archosaurs or an even more inclusive group with the complex air-sac system evolving later within Archosauria.

The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period and after the mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary, the Neoaves, the most diverse avian clade, suffered a rapid global expansion and radiation. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates.

 

References:

Xing Xu, Zhonghe Zhou, Robert Dudley, Susan Mackem, Cheng-Ming Chuong, Gregory M. Erickson, David J. Varricchio, An integrative approach to understanding bird origins, Science, Vol. 346 no. 6215, DOI: 10.1126/science.1253293.

Puttick, M. N., Thomas, G. H. and Benton, M. J. (2014), HIGH RATES OF EVOLUTION PRECEDED THE ORIGIN OF BIRDS. Evolution, 68: 1497–1510. doi: 10.1111/evo.12363 A.

H. Turner, D. Pol, J. A. Clarke, G. M. Erickson, M. A. Norell, A basal dromaeosaurid and size evolution preceding avian flight. Science 317, 1378–1381 (2007).pmid: 17823350.

V. Allen, K. T. Bates, Z. Li, J. R. Hutchinson, Linking the evolution of body shape and locomotor biomechanics in bird-line archosaurs. Nature 497, 104–107 (2013). doi: 10.1038/nature12059; pmid: 23615616

A. M. Balanoff, G. S. Bever, T. B. Rowe, M. A. Norell, Evolutionary origins of the avian brain. Nature 501, 93–96 (2013). doi: 10.1038/nature12424; pmid: 23903660

M. S. Y. Lee, A. Cau, D. Naish, G. J. Dyke, Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds. Science 345, 562–566 (2014). doi: 10.1126/science.1252243; pmid: 25082702

Craig B. Lowe, Julia A. Clarke, Allan J. Baker, David Haussler and Scott V. Edwards, Feather Development Genes and Associated Regulatory Innovation Predate the Origin of Dinosauria, Mol Biol Evol (2015) 32 (1): 23-28. doi: 10.1093/molbev/msu309

Early studies of South American Fossils.

 

Megatherium americanum, MACN.

Megatherium americanum on display at the MACN.

The first notices of South American fossils were reported by early Spanish explorers. These fossils were interpreted as the remains of an ancestral race of giant humans erased from the face of the Earth by a divine intervention. In the second half of the sixteenth century, Fray Reginaldo de Lizarraga (1540-1609), referred in his writings to those “graves of giants” found in Córdoba, Argentina. In 1760, the English Jesuit Thomas Falkner, discovered the first remains of a glyptodon. He wrote: “I myself found the shell of an animal, composed of little hexagonal bones, each bone an inch in diameter at least; and the shell was near three yards over. It seemed in all respects, except it’s size, to be the upper part of the shell of the armadillo; which, in these times, is not above a span in breadth.” (1774, p. 54-55).  However, the first formal description of a gliptodonte was performed in 1838, by English naturalist Sir Richard Owen.

In 1766, by order of Juan de Lezica y Torrezuri (1709-1783), Mayor of Buenos Aires, fossil remains recovered in Arrecifes, were sent to Spain. Previously to the trip, three surgeons, Matías Grimau, Juan Parán and Ángel Casteli, analyzed the bones to determine if these were humans. In Spain, scholars of the Real Academia de la Historia, stated that the remains were not human, conjecturing that those bones resembled those of a quadruped, and perhaps an Elephant. The scholars were right, the remains in question belonged to mastodons, extinct relatives of elephants.

Portrait of  Manuel Torres by Francisco Fortuny.

Portrait of Manuel Torres by Francisco Fortuny.

In 1787, Fray Manuel de Torres found near the banks of the Lujan River,  the skeletal remains of a gigantic mammal. He carefully documented this extraordinary finding. On April 29, 1787, he sent a letter to the Viceroy Francisco Nicolás Cristóbal del Campo, Marqués de Loreto, with details of his work. In 1789, the specimen was sent to the Cabinet of Natural History in Madrid where was illustrated by Juan Bautista Brú de Ramón (1740-1799). This is the real starting point of paleontological studies in the Rio de la Plata.

In 1795, Philippe-Rose Roume (1724-1804), a French officer, sent Bru’s illustrations to the Institut de France, with a little description of the skeleton. A year later, George Cuvier (1769-1832) published the first scientific work on a South American fossil. He assigned the fossil the scientific name Megatherium americanum. Cuvier also studied fossils from Bolivia, Chile, Colombia, and Ecuador, among which he recognized three morphotypes, designated informally as “mastodonte a dents étroites”, “mastodonte Cordillierès” and “mastodonte humboldien”. Cuvier (1823) later formally named them Mastodon angustidens, Mastodon andium and Mastodon humboldti, respectively (Fernicola et al, 2009).

References:

PASQUALI, Ricardo C  y  TONNI, Eduardo P. Los hallazgos de mamíferos fósiles durante el período colonial en el actual territorio de la Argentina. Ser. correl. geol.[online]. 2008, n.24 [citado  2014-12-08], pp. 35-43 . Disponible en: . ISSN 1666-9479.

Fernicola, J. C., Vizcaino, F, and de Iuliis, G. (2009), ‘The Fossil Mammals collected by Charles Darwin in South America during his travels on board the HMS Beagle’, Revista de la Asociatión Geológica Argentina. 64 (1), 147-59.

Fariña, Richard A.; Vizcaíno, Sergio F.; De Iuliis, Gerry (2013). Megafauna. Giant Beasts of Pleistocene South America. Indiana University Press.

A brief introduction to Dinosaur Herbivory.

 

Artist’s impression of the eastern flank of the Antarctic Peninsula during theMaastrichtian (Artist: James McKay, University of Leeds.)

Artist’s impression of the eastern flank of the Antarctic Peninsula during the Maastrichtian (Artist: James McKay, University of Leeds.)

Dinosaurs had diverse feeding mechanisms that strongly influenced their ecology and evolution. Herbivory probably evolved independently in derived silesaurids and various dinosaur groups. Although in the early Late Triassic, dinosaur herbivores were rare, by the early Middle Jurassic until the end of the Cretaceous, they became the dominant vertebrate herbivores.

Herbivory  requires numerous physiological, anatomical, and behavioral adaptations,  including cranial modifications and specialization of the gastrointestinal tract. On the contrary, plants have developed certain features to dissuade herbivores, and evolved to compensate the effects of herbivory by extending growing periods, delaying leaf senescence, and improving nutrient and water availability to surviving leaves (Barret, 2014). Notwithstanding, some plants attract herbivores to enable seed dispersal or pollination, usually by producing fleshy fruits or nectar. The sum of these factors lead to diverse mutualistic interactions between plants and vertebrate herbivores.

Possible interactions between anatomical and physiological traits in herbivorous dinosaurs. From Barret, 2014

Possible interactions between anatomical and physiological traits in herbivorous dinosaurs. From Barret, 2014

The evolution of sauropod herbivory was intimately associated with increased body size and quadrupedal locomotion. In Gondwanan faunas, titanosaurian sauropods were the principal herbivorous dinosaurs.

Several anatomical features enabled sauropods to ingest and digest massive quantities of vegetation (approximately up to 40 kg per day), much of it probably low in nutritional quality. Their long necks helped them to reach vegetation inaccessible to other herbivores, and their large bodies enabled the slower passage of plants through the gut with longer periods of gut fermentation, which allowed that enzymes chemically degrade very hard plants or large amounts of foliage, without employ other mechanical methods for breaking down food (although, is very common found sauropod skeletons with gastroliths).

Differences in body size, skull morphology, neck length, mobility, and dental features probably allowed coexisting sauropods to target different food sources and feed in distinct ways. Bonitasaura, a small sauropod from the Late Cretaceous of Argentina, may have been adapted for feeding on harder vegetation close to the ground. This is very different to the usual image of sauropods browsing high in the treetops and may have been a common feeding strategy among sauropods (Brusatte, 2012).

Triassic cycadophytes from Argentina: A) Pseudoctenis spatulata Du Toit; B) Taeniopteris Brongniart . From Cúneo et al, 2010

Triassic cycadophytes from Argentina: A) Pseudoctenis spatulata Du Toit; B) Taeniopteris Brongniart . From Cúneo et al, 2010

There were profound changes in floral composition and structure during the Mesozoic, including the rise of ferns, cycadophytes, and conifers during the Triassic and Jurassic, followed by the sharp decline of cycadophyte abundance and richness in the Early Cretaceous, and the origin and subsequent diversification of angiosperms. All these floral events have been linked to changes in dinosaur ecology, but currently the evidence for coevolutionary interactions between plants and dinosaurs is weak.

Reference:

Paul Barret, Paleobiology of Herbivorous Dinosaurs, Annu. Rev. Earth Planet. Sci. 2014. 42:207–30, DOI: 10.1146/annurev-earth-042711-105515

Brusatte SL, Benton MJ, Ruta M, Lloyd GT. 2008. The first 50 Myr of dinosaur evolution: macroevolutionary pattern and morphological disparity. Biol. Lett. 4:733–36

Langer MC, Ezcurra MD, Bittencourt JS, Novas FE. 2010. The origin and early evolution of dinosaurs. Biol. Rev. 85:55–110

Martínez RN, Sereno PC, Alcober OA, Colombi CE, Renne PR, et al. 2011. A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea. Science 311:206–10

Tiffney BH. 1992. The role of vertebrate herbivory in the evolution of land plants. Palaeobotanist 41:87–97

Brief introduction to Paleobiology of South American titanosaurs.

 

Argentinosaurus huinculensis reconstruction at Museo Municipal Carmen Funes, Plaza Huincul, Neuquén, Argentina. PLoS ONE. From Wikimedia Commons.

Argentinosaurus huinculensis reconstruction at Museo Municipal Carmen Funes, Plaza Huincul, Neuquén, Argentina. PLoS ONE. From Wikimedia Commons.

Titanosaurus were a diverse group of sauropod dinosaurs represented by more than 30 genera, which included all descendants of the more recent common ancestor of Andesaurus  and Saltasaurus (Wilson and Upchurch, 2003). They were important terrestrial herbivores during the Jurassic and the Cretaceous periods. The group exhibits a worldwide distribution and  some of them, were the largest animals to ever walk the Earth: Argentinosaurus, Futalognkosaurus, and Puertasaurus surpassed lengths of 30m and masses of 70 tons.

The discoveries in Patagonia of embryos, eggs (Chiappe et al., 1998, 2001; Salgado et al., 2005; García et al., 2010) and exceptionally articulated specimens show the importance of the South American record for understanding the phylogeny and paleobiology of titanosaurs.

Paleoenvironmental reconstruction of the egg-bearing lower section of the Anacleto Formation at Auca Mahuevo and Los Barreales localities. From Garrido 2010

Paleoenvironmental reconstruction of the egg-bearing lower section of the Anacleto Formation at Auca Mahuevo and Los Barreales localities. From Garrido 2010

The hundreds of eggs containing embryos found in the outcrops of the Anacleto Formation at Auca Mahuevo and Los Barreales corroborated the hypothesis that sauropods were oviparous. The eggs were relatively small (10–25 cm of diameter) and were found  in excavated nests. The embryos from Auca Mahuevo present an ‘egg-tooth’-like structure which is more frequent in altricial birds (García, 2007a, 2008). If we assume that titanosaurs followed a sequence of ontogenetic stages similar to modern birds, these embryos would correspond with the stage 36-37, within the 42 prenatal stages established for birds.

The titanosaur embryos discovered in Auca Mahuevo are exclusively represented by cranial material. Comparing the skull of adults titanosaurs with the embryos from Auca Mahuevo, it seem evident that the Patagonian dinosaurs experienced a deep ontogenetic modification in this part of the skeleton.

Ontogenetic variation in titanosaurian skull morphology. From García et al, 2014.

Ontogenetic variation in titanosaurian skull morphology: aof, antorbital fenestra; en, external nares; f, frontal; j, jugal; l, lacrimal; mx, maxilla; o, orbit; paof, preantorbital fenestra; pmx, premaxilla; qj, quadratojugal; vn, ventral notch. From García et al, 2014.

The rostral portion of the embryonic skull never surpasses 50% of the total skull length while adult sauropods possess a relatively elongated skull. The premaxillae of  the embryos have extremely short nasal processes contrary to those of adult titanosaurs. It’s possible that the remodeling of the premaxillae in the ontogeny has implicated the elongation of the nasal process as well, which in turn would be related to the ontogenetic retraction of the external nares (García et al, 2014). The type of teeth is basically similar in the embryos and those of the few known adult titanosaur skulls, which may be indicative of the same basic diet.

The brain morphology shows a tendency to the reduction of the midbrain and the olfactory tract and bulbs. Titanosaurs also exhibit a reduction of the anterior semicircular canal of the inner ear and a robustness of the labyrinth in comparison with other sauropods.

Shoulder and pelvic girdle architecture of titanosaurs suggests a broader posture than that other sauropods, which is related to a shift in the specific muscular attachments that would counteract the wide posture of the limbs (García et al, 2014).

The ichnological record offers valuable information about different strategies of titanosaur locomotion and behavior. Most of the trackways are parallel and show the same direction of travel which is indication that titanosaurs moved in social groups.

References:

GARCÍA, Rodolfo A. et al. PALEOBIOLOGY OF TITANOSAURS: REPRODUCTION, DEVELOPMENT, HISTOLOGY, PNEUMATICITY, LOCOMOTION AND NEUROANATOMY FROM THE SOUTH AMERICAN FOSSIL RECORD, doi:10.5710/AMGH.16.07.2014.829. Ameghiniana, [S.l.], jul. 2014. ISSN 1851-8044

Sellers WI, Margetts L, Coria RA, Manning PL (2013) March of the Titans: The Locomotor Capabilities of Sauropod Dinosaurs. PLoS ONE 8(10): e78733. doi:10.1371/journal.pone.0078733

The Megaraptor mystery.

 

A. Cranial reconstruction of Megaraptor namunhuaiquii. B. skull of Dilong paradoxus. Scale bars equal 2 cm. From Porfiri et al. 2014.

A. Cranial reconstruction of Megaraptor namunhuaiquii. B. skull of Dilong paradoxus. Scale bars equal 2 cm. From Porfiri et al. 2014.

The Cretaceous beds of Patagonia posses the most comprehensive record of  non-avian theropods  from Southern Hemisphere. Megaraptora  is a clade represented by Megaraptor, Orkoraptor and Aerosteon, and characterized by the formidable development of their manual claws on digits I and II and the transversely compressed and ventrally sharp ungual of the first manual digit (Novas et al, 2013).

For years, Megaraptor has been alternatively interpreted as belonging to different theropod lineages: as basal coelurosaurians (Novas,1998), basal tetanurans (Calvo et al.,2004; Smith et al., 2008), and allosauroids closely related with carcharodontosaurids (Smith et al., 2007; Benson et al., 2010; Carrano et al., 2012).

The main reason for so many different interpretations is the incomplete nature of most available megaraptorid skeletons and the little information about their cranial anatomy. But the partially preserved skeleton of a juvenile specimen of Megaraptor namunhuaiquii allows to make for the first time a reconstruction of the skull and body of megaraptorids.

Right maxilla of Megaraptor namunhuaiquii in medial view. Scale bar 3 cm. From Porfiri et al. 2014.

Right maxilla of Megaraptor namunhuaiquii in medial view. Scale bar equal 3 cm. From Porfiri et al. 2014.

The data gathered from the specimen indicates that Megaraptorids had an elongated skull, with a gracile snout bearing small teeth, a gracile S-shaped neck, and a very wide and deep thorax, with gastralia similar in size to dorsal ribs. The pectoral girdle supported elongate and robust forelimbs, with large and sharp unguals on digits I and II, and the hindlimbs were gracile and slender.

Based on that information, the researchers found that Megaraptor and related taxa  are nested within Coelurosauria and Tyrannosauroidea.  They  found 14 synapomorphies between megaraptorans and  tyrannosauroids like several foramina on the premaxillary body, extremely long and straight prenarial process of the premaxilla, incisiviform premaxillary teeth with a D-shaped cross-section, and supratemporal fossae separated by a sharp sagittal median crest on frontals.

The study also shows that tyrannosaurs followed two distinct trajectories in the northern and southern continents. While in megaraptorids the forelimbs became powerful and with large-clawed hands (Calvo et al., 2004), in tyrannosaurids the overall trend was towards forelimb reduction (Brusatte et al., 2010b). However, both evolutionary trends present a common pattern which is the reduction of the third manual digit (Porfiri et al. 2014)

 

References:

Porfiri, J. D., Novas, F. E., Calvo, J. O., Agnolín, F. L., Ezcurra, M. D. & Cerda, I. A. 2014. Juvenile specimen of Megaraptor (Dinosauria, Theropoda) sheds light about tyrannosauroid radiation. Cretaceous Research 51: 35-55.

Benson, R.B.J., Carrano, M.T., Brusatte, S.L., 2010. A new clade of archaic large-bodied predatory dinosaurs (Theropoda: Allosauroidea) that survived to the latest Mesozoic. Naturwissenschaften 97, 71-78.

Novas, F.E., 2009. The Age of Dinosaurs in South America. Indiana University Press, Bloomington, pp. 1-536

Novas, F.E., 1998. Megaraptor namunhuaiquii gen. et. sp. nov., a large-clawed, Late Cretaceous Theropod from Argentina. Journal of Vertebrate Paleontology 18, 4-9.