Pisanosaurus and the Triassic ornithischian crisis

Pisanosaurus mertii holotype. Dorsal vertebrae in left lateral (A) and right lateral (B) views. Scale bar: 5 cm. From Agnolín and Rozadilla, 2017.

In 1887, Harry Govier Seeley was the first to subdivide dinosaurs into Saurischians and the Ornithischians based on the nature of their pelvic bones and joints. While the clade Saurischia is well represented in the Late Triassic, the record of the Ornithischia is certainly more problematic. Only a single Triassic ornithischian taxon was generally considered to still be valid: Pisanosaurus mertii, originally described by Argentinian paleontologist Rodolfo Casamiquela in 1967, based on a poorly preserved but articulated skeleton from the upper levels of the Ischigualasto Formation (Late Triassic).

The holotype and only known specimen (PVL 2577) is a fragmentary skeleton including partial upper and lower jaws, seven articulated dorsal vertebrae, four fragmentary vertebrae of uncertain position in the column; the impression of the central portion of the pelvis and sacrum; an articulated partial hind limb including the right tibia; fibula; proximal tarsals and pedal digits III and IV; the distal ends of the right and left femora; a left scapular blade (currently lost); a probable metacarpal III;  and the impressions of some metacarpals (currently lost).

Reconstructed skeleton reflecting the traditional interpretation of Pisanosaurus (Royal Ontario Museum)

But Pisanosaurus shows some derived traits that resulted as unambiguous synapomorphies of the Silesauridae clade, and include: reduced to absent denticles on maxillary and dentary teeth; sacral ribs shared between two sacral vertebrae; lateral side of proximal tibia with a fibular flange; dorsoventrally flattened ungual phalanges; and ankylothecodonty, teeth partially fused to maxilla and dentary bone. The inclusion of Pisanosaurus within Silesauridae implies that this taxon does not constitute the oldest ornithischian. This is consistent with previous interpretations proposing that ornithischian radiation occurred after the Triassic–Jurassic boundary.

To explain the relatively low diversity exhibited by Ornithischia in the Late Triassic-Early Jurassic, several hypotheses have been proposed. One, suggests that Ornithischia is the sister-taxon of Neotheropoda (the least inclusive clade that includes Coelophysis and modern birds) within the clade of ‘traditional theropod taxa’. In this model, a ‘transitional’ ornithischian may possess some anatomical features of theropods that appear to be more like those in more derived than Eodromaeus murphi and Tawa hallae.

Hypothesis 4, in which Ornithischia forms the sister-taxon of Averostra (From Baron 2017)

In a second hypothesis, Ornithischia is positioned as the sister-taxon to the coelophysids. In this model, Neotheropoda and Ornithoscelida would encompass the same set of taxa, but Ornithoscelida would, theoretically, take priority of Neotheropoda as it is the older name. In a third hypothesis, Ornithischia is positioned as the sister-taxon to the ‘other neotheropods’ not contained in the coelophysid clade.

Another hypothesis proposes that Ornithischia forms the sister-taxon of Averostra. Like Ornithischia, Averostra is only known from the Jurassic and Cretaceous Periods, and both share a number of anatomical features, such as fusion of the sacral neural. Another anatomical traits that could unite such a group include the possession of six or more sacral vertebrae; and the fusion of the sacral neural spines into a broad and continuous sheet, as in early ornithischians like Lesothosaurus diagnosticus and tetanuran theropods like Megalosaurus bucklandii. It’s worth mentioning the fact the earliest known unambiguous members of both Ornithischia and Averostra, are found in the same formation in South America: Laquintasaura venezuelae and Tachiraptor admirabilis.

Laquintasaura venezuelae gen. et sp. nov (From Barret et al., 2014)

 

It was suggested (Baron and Barrett 2017) that Chilesaurus diegosaurezi from the Late Jurassic, might represent the earliest diverging member of Ornithischia. Chilesaurus shows several characters typical of ornithischians. The features include a premaxilla with an edentulous anterior region;  loss of recurvature in maxillary and dentary teeth; a postacetabular process that is 25–35% of the total anteroposterior length of the ilium; possession of a retroverted pubis; a pubis with a rod-like pubic shaft; a pubic symphysis that is restricted to the distal end of the pubis; and a femur that is straightened in anterior view. The unique combination of ‘primitive’ and ‘derived’ characters for Chilesaurus has the potential to illuminate the order in which traditional ornithischian synapomorphies were acquired.

The Phytodinosauria hypothesis suggest that Ornithischia is nested among the taxa traditionally termed as sauropodomorphs could also offer a solution to the problem of the lack of unambiguous ornithischians in the Carnian and Late Triassic in general.

 

References:

Baron, M. G. (2017): Pisanosaurus mertii and the Triassic ornithischian crisis: could phylogeny offer a solution?, Historical Biology, DOI: 10.1080/08912963.2017.1410705

Agnolín FL, Rozadilla S. (2017): Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina, Journal of Systematic Palaeontology DOI: 10.1080/14772019.2017.1352623

Baron M. G, Barrett P. M. 2017, A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biol. Lett. 13: 20170220. http://dx.doi.org/10.1098/rsbl.2017.0220

Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution.  Nature 543, 501–506  (2017).  doi:10.1038/nature21700

Barrett, Paul M.; Butler, Richard J.; Mundil, Roland; Scheyer, Torsten M.; Irmis, Randall B.; Sánchez-Villagra, Marcelo R. (2014). A palaeoequatorial ornithischian and new constraints on early dinosaur diversification, Proceedings of the Royal Society B, DOI: 10.1098/rspb.2014.1147

Max C. Langer, Martín D. Ezcurra, Oliver W. M. Rauhut, Michael J. Benton, Fabien Knoll, Blair W. McPhee, Fernando E. Novas, Diego Pol & Stephen L. Brusatte, Untangling the dinosaur family tree, Nature 551 (2017) doi; oi:10.1038/nature24012

Padian K. Dividing the dinosaurs. Nature 543, 494–495 (2017) doi:10.1038/543494a

 

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Historical perspective on the dinosaur family tree

Megalosaurus at Crystal Palace Park, London. From Wikimedia Commons.

In the 19th century, the famous Victorian anatomist Richard Owen diagnosed Dinosauria using three taxa: Megalosaurus, Iguanodon and Hylaeosaurus, on the basis of three main features: large size and terrestrial habits, upright posture and sacrum with five vertebrae (because the specimens were from all Late Jurassic and Cretaceous, he didn’t know that the first dinosaurs had three or fewer sacrals). These characteristics were more mammalian than reptilian. But new fossil findings from Europe and particularly North America forced to a new interpretation about those gigantic animals.

In 1887, Harry Govier Seeley summarised the works of Cope, Huxley and Marsh who already subdivided the group Dinosauria into various orders and suborders. However, he was the first to subdivide dinosaurs into Saurischians and the Ornithischians, based on the nature of their pelvic bones and joints. He wrote: The characters on which these animals should be classified are, I submit, those which pervade the several parts of the skeleton, and exhibit some diversity among the associated animal types. The pelvis is perhaps more typical of these animals than any other part of the skeleton and should be a prime element in classification. The presence or absence of the pneumatic condition of the vertebrae is an important structural difference…” Based on these features, Seeley denied the monophyly of dinosaurs.

Seeley’s (1901) diagram of the relationships of Archosauria. From Padian 2013

At the mid 20th century, the consensual views about Dinosauria were: first, the group was not monophyletic; second almost no Triassic ornithischians were recognised, so they were considered derived morphologically, which leads to the third point, the problem of the ‘‘origin of dinosaurs’’ usually was reduced to the problem of the ‘‘origin of Saurischia,’’ because theropods were regarded as the most primitive saurischians. A great influence on the views about the dinosaur origins was Alan Charig. He was Curator of Amphibians, Reptiles and Birds at the British Museum (Natural History), now the Natural History Museum, in London for almost thirty years. Charig thought that the first dinosaurs were quadrupedal, not bipedal. He based this on the kinds of animals that he and his colleagues found in the early Triassic localities of eastern and South Africa. He thought that forms such as ‘‘Mandasuchus’’ were related to dinosaurs, but that they had a posture intermediate between a sprawling and upright gait that he called ‘‘semi-improved” or ‘‘semi-erect’’.

Herrerasaurus skull. From Wikimedia Commons.

The discovery of Lagosuchus and Lagerpeton from the Middle Triassic of Argentina induced a change in the views of dinosaurs origins. Also from South America came Herrerasaurus from the Ischigualasto Formation, the basal sauropodomorphs Saturnalia, Panphagia, Chromogisaurus, and the theropods Guibasaurus and Zupaysaurus, but no ornithischians except a possible heterodontosaurid jaw fragment from Patagonia. The 70s marked the beginning of a profound shift in thinking on nearly all aspects of dinosaur evolution, biology and ecology. Robert Bakker and Peter Galton, based on John Ostrom’s vision about Dinosauria, proposed, for perhaps the first time since 1842, that Dinosauria was indeed a monophyletic group and that it should be separated (along with birds) from other reptiles as a distinct ‘‘Class”. In 1986, the palaeontologist Jacques Gauthier showed that dinosaurs form a single group, which collectively has specific diagnostic traits that set them apart from all other animals.

From Baron et al., 2017.

Phylogenetic analyses of early dinosaurs have  supported the traditional scheme. But back in March of this year, a paper, authored by Matthew Baron, David Norman and Paul Barrett, challenged this paradigm with a new phylogenetic analysis that places theropods and ornithischians together in a group called Ornithoscelida. The team analysed a wide range of dinosaurs and dinosauromorphs (74 taxa were scored for 457 characters), and they arrived at a dinosaur evolutionary tree containing one main branch that subdivides into the groupings of Ornithischia and Theropoda, and a second main branch that contains the Sauropoda and Herrerasauridae (usually positioned as either basal theropods or basal Saurischia, or outside Dinosauria but close to it). The term Ornithoscelida was coined in 1870 by Thomas Huxley for a group containing the historically recognized groupings of Compsognatha, Iguanodontidae, Megalosauridae and Scelidosauridae. The synapomorphies that support the formation of the clade Ornithoscelida includes: an anterior premaxillary foramen located on the inside of the narial fossa; a sharp longitudinal ridge on the lateral surface of the maxilla; short and deep paroccipital processes; a post-temporal foramen enclosed within the paroccipital process; a straight femur, without a sigmoidal profile; absence of a medioventral acetabular flange; a straight femur, without a sigmoidal profile; and fusion of the distal tarsals to the proximal ends of the metatarsals.

Of course, those results have great implications for the very origin of dinosaurs. Ornithischia don’t begin to diversify substantially until the Early Jurassic. By contrast, the other dinosaurian groups already existed by at least the early Late Triassic. If the impoverished Triassic record of ornithischians reflects a true absence, ornithischians might have evolved from theropods in the Late Triassic (Padian, 2017). The study also suggest that dinosaurs might have originated in the Northern Hemisphere, because most of their basal members, as well as their close relatives, are found there. Furthermore, their analyses places the origin of dinosaurs at the boundary of the Olenekian and Anisian stages (around 247 Ma), slightly earlier than has been suggested previously.

 

The dinosaur family tree Credit: Max Langer

More recently, an international team of early dinosaur evolution specialists, led by Max Langer, highlighted that the lack of some important taxa (for example, the early thyreophoran Scutellosaurus, the possible theropod Daemonosaurus, and the newly described Ixalerpeton and Buriolestes) may have a substantial effect on character optimizations near the base of the dinosaur tree, and thus on the interrelationships of early dinosaurs. The study did not find strong evidence to discard the traditional Ornithischia–Saurischia division. But they reintroduced a third possibility that was articulated in the 1980s but rarely discussed since: that sauropodomorphs and ornithischians may form their own herbivorous group, separate from the ancestrally meat-eating theropods. The Phytodinosauria hypothesis was coined by Robert T. Bakker in his book The Dinosaur Heresies: “Therefore all the plant-eating dinosaurs of every sort really constitute one, single natural group branching out from one ancestor, a primitive anchisaurlike dinosaur. And a new name is required for this grand family of vegetarians. So I hereby christen them the Phytodinosauria, the “plant dinosaurs”‘

References:

Max C. Langer, Martín D. Ezcurra, Oliver W. M. Rauhut, Michael J. Benton, Fabien Knoll, Blair W. McPhee, Fernando E. Novas, Diego Pol & Stephen L. Brusatte, Untangling the dinosaur family tree, Nature 551 (2017) doi; oi:10.1038/nature24012

Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution.  Nature 543, 501–506  (2017).  doi:10.1038/nature21700

Padian K. Dividing the dinosaurs. Nature 543, 494–495 (2017) doi:10.1038/543494a

Padian K. The problem of dinosaur origins: integrating three approaches to the rise of Dinosauria. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, Available on CJO 2013 doi:10.1017/S1755691013000431 (2013).

Seeley, H. G. On the classification of the fossil animals commonly named DinosauriaProc. R. Soc. Lond. 43165171 (1887).

Huxley, T. H. On the classification of the Dinosauria, with observations on the Dinosauria of the Trias. Quarterly Journal of the Geological Society, London 26, 32-51. (1870).

The Enigmatic Chilesaurus and the evolution of ornithischian dinosaurs

Chilesaurus diegosuarezi (MACN)

Chilesaurus diegosuarezi is a bizarre dinosaur from the Upper Jurassic of southern Chile. Holotype specimen (SNGM-1935) consists of a nearly complete, articulated skeleton, approximately 1.6 m long. Four other partial skeletons (specimens SNGM-1936, SNGM-1937, SNGM-1938, SNGM-1888) were collected in the lower beds of Toqui Formation. All the preserved specimens of Chilesaurus show ventrally flexed arms with the hands oriented backwards, an arrangement that closely resembles the resting posture similar described in Mei long, Sinornithoides youngi, and Albinykus baatar. 

Chilesaurus possesses a number of surprisingly plesiomorphic traits on the hindlimbs, especially in the ankle and foot, which resemble basal sauropodomorphs; but the pubis closely resembles that of basal ornithischians. The bizarre anatomy of Chilesaurus raises interesting questions about its phylogenetic relationships. The features supporting the basal position of Chilesaurus within Tetanurae are: scapular blade elongate and strap-like; distal carpal semilunate; and manual digit III reduced.

Chilesaurus holotype cast (MACN)

But the position of Chilesaurus within within Tetanurae conflicts with the presence of several highly derived coelurosaurian features (e.g., opisthopubic pelvis, large supratrochanteric process on ilium, reduced supracetabular crest) which are present in combination with a number of surprisingly plesiomorphic traits present in basal sauropodomorphs.

Ornithischian features of Chilesaurus (From Baron and Barret, 2017)

Chilesaurus also shows several characters typical of ornithischians. The features include a premaxilla with an edentulous anterior region;  loss of recurvature in maxillary and dentary teeth; a postacetabular process that is 25–35% of the total anteroposterior length of the ilium; possession of a retroverted pubis; a pubis with a rod-like pubic shaft; a pubic symphysis that is restricted to the distal end of the pubis; and a femur that is straightened in anterior view.

The unique combination of ‘primitive’ and ‘derived’ characters for Chilesaurus has the potential to illuminate the order in which traditional ornithischian synapomorphies were acquired. For instance, Chilesaurus lacks a predentary bone, one of the features previously regarded as a fundamental ornithischian feature, although it possesses a retroverted pubis, suggesting that opisthopuby preceded the evolution of some craniodental modifications. Opisthopuby has also been related to herbivory, as it has been suggested that pubic retroversion might be related to the evolution of a more complex, longer digestive tract (Baron and Barret, 2017).

References:

Baron MG, Barrett PM. 2017, A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biol. Lett. 13: 20170220. http://dx.doi.org/10.1098/rsbl.2017.0220

Nicolás R. Chimento, Federico L. Agnolin, Fernando E. Novas, Martín D. Ezcurra, Leonardo Salgado, Marcelo P. Isasi, Manuel Suárez, Rita De La Cruz, David Rubilar-Rogers & Alexander O. Vargas (2017) Forelimb posture in Chilesaurus diegosuarezi (Dinosauria, Theropoda) and its behavioral and phylogenetic implications. Ameghiniana doi: 10.5710/AMGH.11.06.2017.3088

Novas, F.E., Salgado, L., Suarez, M., Agnolín, F.L., Ezcurra, M.D., Chimento, N.R., de la Cruz, R., Isasi, M.P., Vargas, A.O., and Rubilar-Rogers, D. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature 522: 331-334. doi:10.1038/nature14307

Patagotitan and the problem of body mass estimation

Image: A. Otero.

Since the discovery of dinosaur remains in the Neuquen basin in 1882, Argentina has gained the title of Land of the Giants. The tittle was reinforced by recent discoveries of more remains of giant titanosaurs like Argentinosaurus, Dreadnoughtus, Notocolossus, Puertasaurus.

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. The group includes the smallest (e.g. Rinconsaurus, Saltasaurus; with estimated body masses of approximately 6 tonnes) and largest sauropods known to date. They had their major radiation during the middle Early Cretaceous. The evolution of body mass in this clade is key element to understand sauropod evolution.

Patagotitan reconstruction (Image: Diego Pol)

Patagotitan mayorum, originally discovered in 2010 by the rural farmer Aurelio Hernandez  is the largest and the most complete titanosaur taxa recovered to date. The generic name Patagotitan is derived from Patago (in reference to the geographic origin of the fossils, Patagonia), and titan (symbolic of its large size). The species name honours the Mayo family (owner of La Flecha Farm, the place where the fossils were found). The holotype (MPEF-PV 3400), includes an anterior and two middle cervical vertebrae, three anterior, two middle and two posterior dorsal vertebrae, six anterior caudal vertebrae, three chevrons, dorsal ribs, both sternal plates, right scapulocoracoid, both pubes and both femora. Six individuals were found in the same quarry, distributed in three distinct but closely spaced horizons, corresponding to  three different burial events. The first estimations of Patagotitan body mass suggest that it would weigh around 70 tons. The dorsal vertebrae preserved in Patagotitan, Argentinosaurus and Puertasaurus allows distinguishing the new taxon from previously known giant titanosaurs from the ‘mid-Cretaceous’ of Patagonia.

(a) Middle cervical vertebra in right lateral view; (b) anterior dorsal vertebra in anterior view (From Carballido et al., 2017)

During the last decades Argentinosaurus hiunculensis has been considered the largest dinosaur that ever walked the Earth. But because of the fragmentary nature of the type specimen, quantitative methods for body mass estimation cannot be directly applied. Two previous studies (Mazzetta et al., 2004; Benson et al., 2014) estimated the body mass of Argentinosaurus by applying scaling equations and measurements taken from two isolated femoral shafts found in deposits of the Huincul Formation. Calculations based in one of these fragmentary femora, housed at the Museo de La Plata collection and at the Museo Municipal “Carmen Funes”, estimates a body mass of 73 tons, but for the moment none of the femora can be confidentially referred to Argentinosaurus given the complete absence of femoral remains in the type material.

The team lead by Dr. José Luis Carballido from the Egidio Feruglio Paleontology Museum (Mef), used the anterior dorsal vertebrae (preserved in Argentinosaurus, Puertasaurus, Notocolossus) for a size comparison between Patagotitan and other giant titanosaurs from Patagonia. The direct comparison of these elements indicate that the dorsal vertebrae of Patagotitan are 8%–18% larger than that of Argentinosaurus and Puertasaurus, and even larger when compared to Notocolossus. Unfortunatelly, as the team remarks, this cannot be extrapolate to determine the body mass for Argentinosaurus and Puertasaurus and the only way to obtain a reliable body mass estimation is contingent on finding new associated material that can be referred to these taxa.

 

References:

Carballido JL, Pol D, Otero A, Cerda IA, Salgado L, Garrido AC, Ramezani J, Cúneo NR, Krause JM. 2017 A new giant titanosaur sheds light on body mass evolution among sauropod dinosaurs. Proc. R. Soc. B 284: 20171219.
DOI: 10.1098/rspb.2017.1219

Mazzetta, G. V., Christiansen, P., & Fariña, R. a. (2004). Giants and Bizarres: Body Size of Some Southern South American Cretaceous Dinosaurs. Historical Biology: A Journal of Paleobiology, 16(2–4), 71–83. http://dx.doi.org/10.1080/08912960410001715132

Benson, R. B. J., Campione, N. E., Carrano, M. T., Mannion, P. D., Sullivan, C., Upchurch, P., & Evans, D. C. (2014). Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage. PLoS Biology, 12(5), http://doi.org/10.1371/journal.pbio.1001853.

 

Pisanosaurus revisited

Reconstructed skeleton of Pisanosaurus (Royal Ontario Museum)

Pisanosaurus mertii was originally described by Argentinian paleontologist Rodolfo Casamiquela in 1967, based on a poorly preserved but articulated skeleton from the upper levels of the Ischigualasto Formation (Late Triassic). The holotype and only known specimen (PVL 2577) is a fragmentary skeleton including partial upper and lower jaws, seven articulated dorsal vertebrae, four fragmentary vertebrae of uncertain position in the column, the impression of the central portion of the pelvis and sacrum, an articulated partial hind limb including the right tibia, fibula, proximal tarsals and pedal digits III and IV, the distal ends of the right and left femora, a left scapular blade (currently lost), a probable metacarpal III, and the impressions of some metacarpals (currently lost).

Pisanosaurus mertii holotype. Right lower mandible in medial (A) and lateral (B) views. Scale bar: 5 cm. From Agnolín and Rozadilla, 2017.

In the original description, Casamiquela considered that Pisanosaurus was a very distinct ornithischian, and even proposed a family: Pisanosauridae. The dentition and tooth-bearing bones of Pisanosaurus possess a large number of ornithischian traits, like its barricade-like dentition. But Pisanosaurus shows some features that strongly differ from those of ornithischians. For instance, vertebral centra are very elongated and transversely compressed, differing from the short and stout dorsal vertebrae of known ornithischians, including heterodontosaurids. The pelvis is another portion of the skeleton of Pisanosaurus strongly different from that of ornithischians.

Pisanosaurus mertii holotype. Dorsal vertebrae in left lateral (A) and right lateral (B) views. Scale bar: 5 cm. From Agnolín and Rozadilla, 2017.

On the other hand, Pisanosaurus shows some derived traits that resulted as unambiguous synapomorphies of the Silesauridae clade, and include: reduced to absent denticles on maxillary and dentary teeth; sacral ribs shared between two sacral vertebrae; lateral side of proximal tibia with a fibular flange (present also in heterodontosaurids and several saurischians); dorsoventrally flattened ungual phalanges; and ankylothecodonty, teeth partially fused to maxilla and dentary bone. The first and last characters are lacking in ornithischians. Of course, the inclusion of Pisanosaurus within Silesauridae implies that this taxon does not constitute the oldest ornithischian. This also suggests a significant gap between Pisanosaurus and the oldest unambiguous records of ornithischians: Laquintasaura and Lesothosaurus, which may be dated as Hettangian in age. This is consistent with previous interpretations proposing that ornithischian radiation occurred after the Triassic–Jurassic boundary.

References:

Federico L. Agnolín & Sebastián Rozadilla (2017): Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina, Journal of Systematic Palaeontology DOI: 10.1080/14772019.2017.1352623

Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution.  Nature 543, 501–506  (2017).  doi:10.1038/nature21700

Padian K. The problem of dinosaur origins: integrating three approaches to the rise of Dinosauria. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, Available on CJO 2013 doi:10.1017/S1755691013000431 (2013).

Meet Borealopelta markmitchelli

Holotype of Borealopelta markmitchelli (From Brown et al., 2017)

The Ankylosauria is a group of herbivorous, quadrupedal, armoured dinosaurs subdivided in two major clades, the Ankylosauridae and the Nodosauridae. The most derived members of this clade are characterized by shortened skulls, pyramidal squamosal horns, and tail clubs, among other features. Nodosauridae have a kinked ischium and more massive osteoderms, but lack a tail club. Ankylosaurs were present primarily in Asia and North America,  but the early origins of this clade are ambiguous. A three-dimensionally preserved ankylosaurian discovered in the Suncor Millennium Mine in northeastern Alberta, Canada, offers new evidence for understanding the anatomy of this group.

The new specimen, Borealopelta markmitchelli, from the Early Cretaceous of Alberta, preserves integumentary structures as organic layers, including continuous fields of epidermal scales and intact horn sheaths capping the body armor. The generic name Borealopelta is derived from “borealis” (Latin, “northern”) and “pelta” (Greek, “shield”). The specific epithet markmitchelli honors Mark Mitchell for his preparation of the holotype.

Schematic drawing of TMP 2011.033.0001 in dorsal view (From Brown et al., 2017)

The holotype (TMP 2011.033.0001), with an estimated living mass of 1,300 kg, is an articulated specimen preserving the head, neck, most of the trunk and sacrum, a complete right and a partial left forelimb and manus, and partial pes. The skull is covered in dermal plates, which are overlain by their associated epidermal scales. Cervical and thoracic osteoderms form continuous transverse rows completely separated by transverse rows of polygonal basement scale. Osteoderms are covered by a thick, dark gray to black organic layer, representing the original, diagenetically altered, keratinous epidermal scales. The distribution of the film correlates well to the expected distribution of melanin, a pigment present in some vertebrate integumentary structures. The keratinized tissues in this nodosaur are heavily pigmented. The possible presence of eumelanin and pheomelanin, suggested it had reddish-brown camouflage. The evidence of countershading in a large, heavily armored herbivorous dinosaur also provides a unique insight into the predator-prey dynamic of the Cretaceous Period.

 

References:

Brown, C.M.; Henderson, D.M.; Vinther, J.; Fletcher, I.; Sistiaga, A.; Herrera, J.; Summons, R.E. “An Exceptionally Preserved Three-Dimensional Armored Dinosaur Reveals Insights into Coloration and Cretaceous Predator-Prey Dynamics”. Current Biology. doi:10.1016/j.cub.2017.06.071

Arbour, V. M.; Currie, P. J. (2015). “Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs”. Journal of Systematic Palaeontology: 1–60. doi: 10.1080/14772019.2015.1059985

Zuul, the Gatekeeper

Skull of Zuul (Photograph: Brian Boyle/Royal Ontario Museum)

The Ankylosauria is a group of herbivorous, quadrupedal, armoured dinosaurs subdivided in two major clades, the Ankylosauridae and the Nodosauridae. Zuul crurivastator, from the Coal Ridge Member of the Judith River Formation of northern Montana, is the most complete ankylosaurid ever found in North America. The generic name refers to Zuul the Gatekeeper of Gozer (from the 1984 film Ghostbusters), and the species epithet combines crus (Latin) for shin or shank, and vastator (Latin) for destroyer, in reference to the sledgehammer-like tail club. The extraordinary preservation of abundant soft tissue in the skeleton, including in situ osteoderms and skin impressions make this specimen an important reference for understanding the evolution of dermal and epidermal structures in this clade. Until the discovery of Zuul, Laramidian ankylosaurin specimens were primarily assigned to three taxa: Euoplocephalus tutus and Ankylosaurus magniventris from northern Laramidia, and Nodocephalosaurus kirtlandensis from southern Laramidia.

The holotype (ROM 75860)  is a partial skeleton consisting of a nearly complete cranium, and a partially articulated postcranium. It is estimated to be over 6 metres long, and it would have weighed approximately 2500 kg. It has been dated to approximately 75 million years ago, and it was discovered accidentally on 16 May 2014 during overburden removal for a scattered tyrannosaurid skeleton, when a skid-steer loader encountered the tail club knob.

Overview of the tail of Zuul crurivastator in dorsal view, with insets of detailed anatomy (From Arbour and Evans, 2017)

The skull is almost complete, missing only the tip of the right quadratojugal horn and the ventral edge of the vomers, and is the largest ankylosaurine skull recovered from Laramidia. The skull is relatively flat dorsally, and had an elaborate ornamentation across the snout. The squamosal horns are robust and pyramid-shaped, and the quadratojugal horns had a sharp, posteriorly offset apex.

The tail club (including the 13 caudal vertebrae in the handle and the knob) is at least 210 cm long. Osteoderms are preserved not only in the anterior, flexible portion of the tail but also along the tail club handle. The first three pairs of caudal osteoderms are covered with a black film, that probably represent preserved keratin, and is similar to the texture observed at the base of bovid horn sheaths.

The discovery of Zuul fills a gap in the ankylosaurine record and further highlights that Laramidian ankylosaurines were undergoing rapid evolutionary rates and stratigraphic turnover as observed for Laramidian ceratopsids, hadrosaurids, pachycephalosaurids and tyrannosaurids.

References:

Arbour V. M., Evans D. C., (2017), A new ankylosaurine dinosaur from the Judith River Formation of Montana, USA, based on an exceptional skeleton with soft tissue preservation , Royal Society Open Science, rsos.royalsocietypublishing.org/lookup/doi/10.1098/rsos.161086

Arbour, V. M.; Currie, P. J. (2015). “Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs”. Journal of Systematic Palaeontology: 1–60. doi: 10.1080/14772019.2015.1059985

Jianianhualong and the evolution of feathers.

Jianianhualong tengi holotype (From Xu, X. et al., 2017)

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. Most of these fossils are from the Jehol Biota of northeastern China, dated between approximately 130.7 and 120 million years ago. Among them are many fossils of troodontids, which are considered as the closest relatives of birds. Previous reported troodontid species include Mei long, Sinovenator changii, Sinusonasus magnodens and Jinfengopteryx elegans. Now a new troodontid, Jianianhualong tengi gen. et sp. nov., has anatomical features that shed light on troodontid character evolution.

The holotype (DLXH 1218) is a nearly complete skeleton with associated feathers, and is inferred to be an adult. It is estimated to be 112 cm in total skeletal body length with a fully reconstructed tail, and its body mass is estimated to be 2.4 kg, similar to most other Jehol troodontids, such as Sinovenator. The skull and mandible are in general well preserved, and  has a relatively short snout and highly expanded skull roof. There are probably 21 maxillary teeth and 25 dentary teeth on each side of the jaw. The vertebral column is nearly completely represented and  the tail is 54 cm long. The furcula is poorly preserved, and the humerus is 70% of femoral length. The manus is typical of maniraptoran theropods, and measures 112 mm in length. The pelvis is in general similar to those of basal troodontids, with a proportionally small ilium, a posteroventrally oriented pubis, and a short ischium. A phylogenetic analysis places Jianianhualong in an intermediate position together with several species between the basalmost and derived troodontids.

Plumage of Jianianhualong tengi (Adapted from Xu, X.  et al, 2017)

The tail frond of Jianianhualong preserves an asymmetrical feather, the first example of feather asymmetry in troodontids. Feathers were once considered to be unique avialan structures. Since the discovery of the feathered Sinosauropteryx in 1996, numerous specimens of most theropod groups and even three ornithischian groups preserving feathers have been recovered from the Jurassic and Cretaceous beds of China, Russia, Germany, and Canada. These feathers fall into several major morphotypes, ranging from monofilamentous feathers to highly complex flight feathers.

Evidence indicates that the earliest feathers evolved in non-flying dinosaurs for display or thermoregulation, and later were co-opted into flight structures with the evolution of asymmetrical pennaceous feathers in Paraves, therefore, the discovery of tail feathers with asymmetrical vanes in a troodontid theropod indicates that feather asymmetry was ancestral to Paraves.

 

 

References:

Xu, X. et al. Mosaic evolution in an asymmetrically feathered troodontid dinosaur with transitional features. Nat. Commun. 8, 14972 doi: 10.1038/ncomms14972 (2017).

Xu, X. et al. An integrative approach to understanding bird origins, Science, Vol. 346 no. 6215 (2014). DOI: 10.1126/science.1253293

Re-examining the dinosaur evolutionary tree.

Close up of “Sue” at the Field Museum of Natural History in Chicago, IL (From Wikimedia Commons)

In the nineteen century, the famous Victorian anatomist Richard Owen diagnosed Dinosauria using three taxa: Megalosaurus, Iguanodon and Hylaeosaurus, on the basis of three main features: large size and terrestrial habits, upright posture and sacrum with five vertebrae (because the specimens were from all Late Jurassic and Cretaceous, he didn’t know that the first dinosaurs had three or fewer sacrals). Later, in 1887, Harry Govier Seeley summarised the works of Edward Drinker Cope, Thomas Huxley and Othniel Charles Marsh, and subdivide dinosaurs into Saurischians and the Ornithischians. He wrote: The characters on which these animals should be classified are, I submit, those which pervade the several parts of the skeleton, and exhibit some diversity among the associated animal types. The pelvis is perhaps more typical of these animals than any other part of the skeleton and should be a prime element in classification. The presence or absence of the pneumatic condition of the vertebrae is an important structural difference…” Based on these features, Seeley denied the monophyly of dinosaurs.

Seeley’s (1901) diagram of the relationships of Archosauria. From Padian 2013

At the mid 20th century, the consensual views about Dinosauria were: first, the group was not monophyletic; second almost no Triassic ornithischians were recognised, so they were considered derived morphologically, which leads to the third point, the problem of the ‘‘origin of dinosaurs’’ usually was reduced to the problem of the ‘‘origin of Saurischia,’’ because theropods were regarded as the most primitive saurischians. But the discovery of Lagosuchus and Lagerpeton from the Middle Triassic of Argentina induced a change in the views of dinosaurs origins. Also from South America came Herrerasaurus from the Ischigualasto Formation, the basal sauropodomorphs Saturnalia, Panphagia, Chromogisaurus, and the theropods Guibasaurus and Zupaysaurus, but no ornithischians except a possible heterodontosaurid jaw fragment from Patagonia. The 70s marked the beginning of a profound shift in thinking on nearly all aspects of dinosaur evolution, biology and ecology. Robert Bakker and Peter Galton, based on John Ostrom’s vision about Dinosauria, proposed, for perhaps the first time since 1842, that Dinosauria was indeed a monophyletic group and that it should be separated (along with birds) from other reptiles as a distinct ‘‘Class”. In 1986, the palaeontologist Jacques Gauthier showed that dinosaurs form a single group, which collectively has specific diagnostic traits that set them apart from all other animals.

The dinosaur evolutionary tree (From Padian, 2017.

Phylogenetic analyses of early dinosaurs have  supported the traditional scheme. But a new study authored by Matthew Baron, David Norman and Paul Barrett, reach different conclusions from those of previous studies by incorporating some different traits and reframing others. Baron and colleagues, analysed a wide range of dinosaurs and dinosauromorphs, including representatives of all known dinosauromorph clades. 74 taxa were scored for 457 characters. The team  arrived at a dinosaur evolutionary tree containing one main branch that subdivides into the groupings of Ornithischia and Theropoda, and a second main branch that contains the Sauropoda and Herrerasauridae (usually positioned as either basal theropods or basal Saurischia, or outside Dinosauria but close to it). The union between ornithischians and theropods is called Ornithoscelida. The term was coined in 1870 by Thomas Huxley for a group containing the historically recognized groupings of Compsognatha, Iguanodontidae, Megalosauridae and Scelidosauridae.

From Baron et al., 2017.

The synapomorphies that support the formation of the clade Ornithoscelida includes: an anterior premaxillary foramen located on the inside of the narial fossa; a sharp longitudinal ridge on the lateral surface of the maxilla; short and deep paroccipital processes; a post-temporal foramen enclosed within the paroccipital process; a straight femur, without a sigmoidal profile; absence of a medioventral acetabular flange; a straight femur, without a sigmoidal profile; and fusion of the distal tarsals to the proximal ends of the metatarsals.

Of course, those results have great implications for the very origin of dinosaurs. Ornithischia don’t begin to diversify substantially until the Early Jurassic. By contrast, the other dinosaurian groups already existed by at least the early Late Triassic. If the impoverished Triassic record of ornithischians reflects a true absence, ornithischians might have evolved from theropods in the Late Triassic (Padian, 2017). The study also suggest that dinosaurs might have originated in the Northern Hemisphere, because most of their basal members, as well as their close relatives, are found there. Furthermore, their analyses places the origin of dinosaurs at the boundary of the Olenekian and Anisian stages (around 247 Ma), slightly earlier than has been suggested previously.

 

References:

Baron, M. G., Norman, D. B. & Barrett, P. M. A new hypothesis of dinosaur relationships and early dinosaur evolution.  Nature 543, 501–506  (2017).  doi:10.1038/nature21700

Padian K. Dividing the dinosaurs. Nature 543, 494–495 (2017) doi:10.1038/543494a

Padian K. The problem of dinosaur origins: integrating three approaches to the rise of Dinosauria. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, Available on CJO 2013 doi:10.1017/S1755691013000431 (2013).

Seeley, H. G. On the classification of the fossil animals commonly named Dinosauria. Proc. R. Soc. Lond. 43, 165171 (1887).

Huxley, T. H. On the classification of the Dinosauria, with observations on the Dinosauria of the Trias. Quarterly Journal of the Geological Society, London 26, 32-51. (1870).

 

A brief introduction to the Early dinosaurs from Argentina.

eoraptor-skeleton

Articulated skeleton of Eoraptor lunensi (From Sereno 2013)

The oldest record of Argentinean dinosaurs comes from the Ischigualasto Formation, NW Argentina, dated from 231.4 Ma to 225.9 Ma. Adolf Stelzner in 1889 published the first data on the geology of Ischigualasto, but it was not until 1911 that Guillermo Bodenbender briefly refers to the fossils of the site. In the early 40′s, Joaquin Frenguelli, initiates a geological survey in the western margin of the basin. Later, in 1943, Angel Cabrera described fragmentary therapsid fossils. However, intensive paleontological study of the Ischigualasto and Chañares Formations, began only in the late 1950s.

The Ischigualasto Formation has 300–700 m of mudstone, sandstone, conglomerate, and basalt, and consists of four lithostratigraphic members which in ascending order include the La Peña Member, the Cancha de Bochas Member, the Valle de la Luna Member, and the Quebrada de la Sal Member. Eight valid species of dinosaurs are known from the Ischigualasto Formation: Pisanosaurus mertii, Herrerasaurus ischigualastensis, Sanjuansaurus gordilloi, Eodromaeus murphi, Eoraptor lunensis, Panphagia protos, and Chromogisaurus novasi.

 

Skull of Herrerasaurus ischigualastensis (Sereno, 2013)

Skull of Herrerasaurus ischigualastensis (Sereno, 2013)

Pisanosaurus mertii is a small specimen, know by an incomplete maxilla and lower jaw fragments bearing teeth, vertebrae, incomplete hind limb, and the impression of the pelvis. Described in 1967 by Rodolfo CasamiquelaPisanosaurus is considered as the oldest known ornithischian.

Herrerasaurus ischigualastensis was described by Osvaldo Reig in 1963. The taxon is one of the best known Triassic dinosaurs and the largest dinosaur of the Ischigualasto Formation. Herrerasaurus was fully bipedal, with strong hind limbs, short thighs and long feet. The skull has a rectangular profile and a transversely narrow snout (Sereno and Novas, 1992). The presence of two sacral vertebrae and lack of brevis fossa made Herrerasaurus, and other herrerasaurids, a controversial group.

Sanjuansaurus gordilloi is similar to Herrerasaurus ischigualastensis, although more gracile and possessing short and straight pubis among other differences (Alcober & Martínez, 2010). It’s known from one specimen that preserves left maxilla, partial axial column, scapulae, left ulna, ungual of manual digit III, partial left ilium and pubis, both femora and tibiae, right fibula, right astragalus and calcaneum, and left metatarsal.

Skull and skeleton of Eodromaeus murphi (PVSJ 560). Scale bar equals 10 cm.

Skull and skeleton of Eodromaeus murphi
(PVSJ 560). Scale bar equals 10 cm.

Eodromaeus murphi is a small species with a total length of about 1.2 metres, known from five specimens. The trunk was long and slender, and forelimbs were shorter than the hindlimbs. The skull is relatively low and lightly built with a relatively spacious antorbital fenestra.  A phylogenetic analysis places Eodromaeus within Theropoda as the sister taxon to Neotheropoda

Eoraptor lunensis is known from eight specimens, including the holotype that preserves most of the skeleton. Eoraptor had a slender body with an estimated weight of about 10 kilograms. The lightly built skull has a slightly enlarged external naris and the premaxilla is observed to have a slender posterolateral process. The long bones of the hind limb have more robust shafts than those of Eodromaeus, although in both genera the tibia remains slightly longer than the femur (Sereno et al., 2013). Initially considered a basal theropod, the sauropodomorph affinity of Eoraptor has been strengthened after the publication of its anatomy in 2013.

Panphagia protos is a small species, known from one partial skeleton including several skull bones, lower jaw, and partial axial skeleton. The specimen is an immature individual with an estimated body length of approximately 1.30 m. It was originally proposed as the most basal sauropodomorph (Martinez and Alcober, 2009)

Chromogisaurus novasi is also similar in size to Eoraptor lunensis. It’s known from a partial skeleton lacking the skull. It includes elements of the front and hind limbs, the pelvis and two caudal vertebrae.

References:

Martín D. EZCURRA & Ricardo N. MARTÍNEZ (2016), Dinosaur precursors and early dinosaurs from Argentina., In book: Historia Evolutiva y Paleobiogeografía de los Vertebrados de América del Sur, Publisher: Contribuciones del MACN, Editors: F. Agnolíin, G.L. Lio, F. Brissón Egli, N.R. Chimento, F. Novas, pp.97-107

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.

Sereno, P.C.; Novas, F.E. (1992). “The complete skull and skeleton of an early dinosaur”. Science. 258 (5085): 1137–1140.

Ricardo N. Martinez; Paul C. Sereno; Oscar A. Alcober; Carina E. Colombi; Paul R. Renne; Isabel P. Montañez; Brian S. Currie (2011). “A Basal Dinosaur from the Dawn of the Dinosaur Era in Southwestern Pangaea”. Science. 331 (6014): 206–210. doi:10.1126/science.1198467

Martinez RN, Alcober OA (2009) A Basal Sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the Early Evolution of Sauropodomorpha. PLoS ONE 4(2): e4397. doi:10.1371/journal.pone.0004397

Ezcurra, M. D. 2010. “A new early dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: a reassessment of dinosaur origin and phylogeny.” Journal of Systematic Palaeontology 8: 371-425.