Ichthyornis and the evolution of the avian skull.

 

Ichthyornis skull

Birds originated from a theropod lineage more than 150 million years ago. By the Early Cretaceous, they diversified, evolving into a number of groups of varying anatomy and ecology. Much of birds anatomical variety is related to their skulls and in particulary with their beaks.

Discovered in 1870 by Benjamin Franklin Mudge, a professor from Kansas State Agricultural College and good friend of Othniel Charles Marsh, Ichthyornis, which means‭ ‘‬fish bird‭’‭, was a small early ornithuromorph from the Late Cretaceous of North America. Ornithuromorphs, include Gansus, Patagopteryx, Yixianornis, and Apsaravis, which form a grade on the line to Ornithurae, a derived subgroup that includes modern birds and their closest fossil relatives.

 

3D reconstruction of the skull of I. dispar (From Field et al., 2018)

The skull of I. dispar shows a transitional point in the evolutionary history of birds. The upper margin of the beak is concave in profile, a derived condition shared with living birds. The fused, toothless premaxillae have a terminal hook, and occupy the anterior quarter of the rostrum. Neurovascular foramina indicate the presence of a highly keratinized region of rhamphotheca called the premaxillary nail. The maxilla is plesiomorphically long. The dentition is extensive in both upper and lower jaws. A sulcus on the rostral half of the maxilla suggests a broad naso-maxillary contact and a correspondingly broad postnarial bar. The palatine is narrow and elongate, unlike that of Archaeopteryx and more stemward theropods. The quadrate exhibits two rounded capitular condyles that fit into cotyles on the prootic and squamosal bones to form a mobile joint with the cranium. The arrangement of the rostrum, jugal, and quadratojugal, the mobile suspensorium and the narrow, linear palatine all indicate that I. dispar possessed a fully functional avian cranial kinetic system.

The endocranial cavity appears essentially modern in sagittal section. The forebrain was enlarged and posteroventrally rotated while the optic lobes were inflated and laterally shifted, as in living birds. The squamosal exhibits an archaic, deinonychosaur-like morphology. The zygomatic process is deep and triangular in lateral view. The nuchal crest extends from the midline of the skull onto the zygomatic process, forming the upper edge of the squamosal bone, as in non-avialan theropods.

Darwin’s letter to Marsh (Yale Peabody Museum Archives)

Since its discovery, Ichthyornis has been viewed as a classical example of evolution, due to the combination of an advanced postcranial morphology and retention of toothed jaws. In a letter, dated August 31, 1880, Charles Darwin thanks Marsh for a copy of his monograph Odontornithes, which reported two contrasting bird genera: Hesperornis, which was about 1.8 metres tall, and Ichthyornis, which had an average wingspan of about 60 centimetres. In his letter, Darwin wrote: “I received some time ago your very kind note of July 28th, & yesterday the magnificent volume. I have looked with renewed admiration at the plates, & will soon read the text. Your work on these old birds & on the many fossil animals of N. America has afforded the best support to the theory of evolution, which has appeared within the last 20 years.”

 

References:

Daniel J. Field, Michael Hanson, David Burnham, Laura E. Wilson, Kristopher Super, Dana Ehret, Jun A. Ebersole & Bhart-Anjan S. Bhullar, Complete Ichthyornis skull illuminates mosaic assembly of the avian head, Nature (2018). nature.com/articles/doi:10.1038/s41586-018-0053-y
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.

 

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A brief introduction to the Carnian Pluvial Episode.

Early-late Carnian (Late Triassic) palaeogeographic reconstruction showing some of the main vertebrate-bearing units (From Bernardi et al. 2018)

Dinosaurs likely originated in the Early to Middle Triassic. The Manda beds of Tanzania yielded the remains of the possible oldest dinosaur, Nyasasaurus parringtoni, and Asilisaurus, a silesaurid (the immediate sister-group to Dinosauria). However the oldest well-dated identified dinosaurs are from the late Carnian of the lower Ischigualasto Formation in northwestern Argentina, dated from 231.4 Ma to 225.9 Ma. The presence of dinosaurs, such as Panphagia, Eoraptor, and Herrerasaurus support the argument that Dinosauria originated during the Ladinian or earlier and that they were already well diversified in the early Carnian. Similarly, the Santa Maria and Caturrita formations in southern Brazil preserve basal dinosauromorphs, basal saurischians, and early sauropodomorphs. In North America, the oldest dated occurrences of vertebrate assemblages with dinosaurs are from the Chinle Formation. Two further early dinosaur-bearing formations, are the lower (and upper) Maleri Formation of India and the Pebbly Arkose Formation of Zimbabwe. These skeletal records of early dinosaurs document a time when they were not numerically abundant, and they were still of modest body size (Eoraptor had a slender body with an estimated weight of about 10 kilograms).

Trace fossil evidence suggests that the first dinosaur dispersal in the eastern Pangaea is synchronous with an important climate-change event named the “Carnian Pluvial Episode” (CPE) or “Wet Intermezzo”, dated to 234–232 Ma.

Skeleton of Eoraptor lunensis (PVSJ 512) in left lateral view. Scale bar equals 10 cm. From Sereno et al., 2013.

The Late Triassic is marked by a return to the hothouse condition of the Early Triassic, with two greenhouse crisis that may also have played a role in mass extinctions. Isotopic  records suggest  a global carbon cycle perturbation during the Carnian that was coincident with complex environmental changes and biotic turnover.

The CPE is often described as a shift from arid to more humid conditions (global warming, ocean acidification, mega-monsoonal conditions, and a generalised increase in rainfall). In the marine sedimentary basins of the Tethys realm, an abrupt change of carbonate factories and the establishment of anoxic conditions mark the beginning of the climate change. The CPE also marks the first massive appearance of calcareous nannoplankton, while groups, like bryozoans and crinoids, show a sharp decline during this event.

Palynological association from the Heiligkreuz Formation provide information on palynostratigraphy and palaeoclimate during the last part of the Carnian Pluvial Event (CPE). From Roghi et al., 2014

On land, palaeobotanical evidence shows a shift of floral associations of towards elements more adapted to humid conditions (the palynological record across the CPE suggest at least 3–4 discrete humid pulses). Several families and orders make their first appearance during the Carnian: bennettitaleans, modern ferns, and conifer families (Pinaceae, Araucariaceae, Cheirolepidaceae). The oldest biological inclusions found preserved in amber also come from the Carnian; and key herbivorous groups such as dicynodonts and rhynchosaurs, which had represented 50% or more of faunas, disappeared, and their places were taken by dinosaurs.

Despite the global significance of the CPE,  the trigger of the environmental change  is still disputed. Volcanic emissions from the Wrangellia igneous province and the dissociation of methane clathrates could be linked to the CPE. It seems that the combination of that events  would be the most likely explanation for the substantial shift of the C isotope excursion observed at the CPE.

 

References:

Massimo Bernardi et al. Dinosaur diversification linked with the Carnian Pluvial Episode, Nature Communications (2018). DOI: 10.1038/s41467-018-03996-1

Miller et al., Astronomical age constraints and extinction mechanisms of the Late Triassic Carnian crisis, Scientific Reports | 7: 2557 | (2017) DOI:10.1038/s41598-017-02817-7

Rogui et al. Field trip to Permo-Triassic Palaeobotanical and Palynological sites of the Southern Alps, Geo.Alp. 11. 29-84. (2014)

Paul C. Sereno, Ricardo N. Martínez & Oscar A. Alcober (2013) Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha),Journal of Vertebrate Paleontology, 32:sup1, 83-179, DOI: 10.1080/02724634.2013.820113

The oldest Archaeopteryx

Overview of the skeleton of the new Archaeopteryx specimen (From Rauhut et al., 2018)

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. On 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.” In later editions of The Origin of Species, Darwin indeed 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.”

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

Over the years, eleven Archaeopteryx specimens has being recovered. The new specimen from the village of Schamhaupten, east-central Bavaria is the oldest representative of the genus (earliest Tithonian). The new specimen was discovered by a private collector. Although it was registered as German national cultural heritage, which guarantees its permanent availability, the specimen remains in private hands (Datenbank National Wertvollen Kulturgutes number DNWK 02924).

The new specimen is preserved as a largely articulated skeleton. However, the shoulder girdles and arms, as well as the skull have been slightly dislocated from their original positions, but the forelimbs remain in articulation. The skull is triangular in lateral outline and has approximately 56 mm long. The orbit is the largest cranial opening (approximately 16 mm long), and the lateral temporal fenestra is collapsed. There are probably four tooth positions in the premaxilla, nine in the maxilla and 13 in the dentary.

Articulated dorsal vertebral column of the new Archaeopteryx, including dorsal ribs and gastralia. Scale bar is 10 mm. (From Rauhut et al., 2018)

The postcranial skeleton was affected by breakage and loss of elements prior to or at the time of discovery. The sacral region, and the caudal vertebrae are very poorly preserved. Several dorsal ribs are preserved, and gastralia ribs are present in the thoracic region and the abdominal region. The shoulder girdle comprises the scapula, coracoid and furcula. Both humeri are poorly preserved. Parts of the phalanges are, largely poorly, preserved, and they do not show much detail. The pubic shafts are slender and very slightly flexed posteroventrally, an as in all specimens of Archaeopteryx, the distal end of the ischium is bifurcated. The femora are also poorly preserved and largely collapsed. Both tibiae are preserved in articulation with the fibulae and the proximal tarsals. The digits of the feet are complete on both sides, but partially overlapping.

Until recently, the referral of new specimens from the Solnhofen Archipelago to the genus Archaeopteryx, seems unproblematic, but the re-examination of the fourth (Haarlem) specimen of Archaeopteryx, and the discovery in the last years of specimens from the Late Jurassic of China that are similar to Archaeopteryx, raises the question if the specimens referred to Archaeopteryx represent a monophyletic taxon.

 

References:

Rauhut OWM, Foth C, Tischlinger H. (2018The oldest Archaeopteryx (Theropoda: Avialiae): a new specimen from the Kimmeridgian/Tithonian boundary of Schamhaupten, BavariaPeerJ 6:e4191 https://doi.org/10.7717/peerj.4191

Foth C, Rauhut OWM. 2017. Re-evaluation of the Haarlem Archaeopteryx and the radiation of maniraptoran theropod dinosaurs. BMC Evolutionary Biology 17:236 https://doi.org/10.1186/s12862-017-1076-y

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

Wellnhofer Peter, 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

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

 

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