Introducing Dynamoterror dynastes, the powerful terror ruler.

Frontals of Dynamoterror dynastes in rostral view. From McDonald et al., 2018. (Scale bars = 5 cm)

Tyrannosauroidea is a relatively derived group of theropod dinosaurs more closely related to birds than to other large theropods such as allosauroids and spinosaurids. The clade originated in the Middle Jurassic, approximately 165 million years ago, and for most of their evolutionary history, tyrannosauroids were mostly small-bodied animals that only reached gigantic size during the final 20 million years of the Cretaceous. Until recently, all tyrannosaurs fossils were limited to Asia and North America, but the latest discoveries suggest a more cosmopolitan distribution during their early evolution.

All tyrannosaurs were bipedal predators characterized by premaxillary teeth with a D-shaped cross section, fused nasals, extreme pneumaticity in the skull roof and lower jaws, a pronounced muscle attachment ridge on the ilium, and an elevated femoral head. The clade was a dominant component of the dinosaur faunas of the American West shortly after the emplacement of the Western Interior Seaway (about 99.5 Mya).

Paleogeography of North America during the late Campanian Stage of the Late Cretaceous (∼75 Ma). From Sampson et al., 2010

Dynamoterror dynastes, the most recent taxon described from the lower Campanian of northwestern New Mexico, provides additional data on the morphology and diversity of early tyrannosaurines in Laramidia. The new specimen lived during the Late Cretaceous period, approximately 78 million years ago. The name derived from Greek word dynamis (“power”) and the Latin word terror. The specific name is a Latin word meaning “ruler. Dynamoterror was collected in San Juan County, New Mexico, and is the first associated tyrannosaurid skeleton reported from the Menefee Formation.

The holotype (UMNH VP 28348) is an incomplete associated skeleton including the left and right frontals, four fragmentary vertebral centra, fragments of dorsal ribs, right metacarpal II, supraacetabular crest of the right ilium, unidentifiable fragments of long bones, phalanx 2 of left pedal digit IV, and phalanx 4 of left pedal digit IV. The right and left frontals both are incomplete; the dimensions of the right frontal are similar to a subadult specimen of Tyrannosaurus rex, suggesting that UMNH VP 28348 represents a subadult or adult individual. The reconstructed skull roof of Dynamoterror present several tyrannosaurine features, such as large supratemporal fossae and a tall sagittal crest on the frontals, providing an expanded attachment area for enormous jaw-closing muscles.

 

References:

McDonald AT, Wolfe DG, Dooley AC Jr. (2018) A new tyrannosaurid (Dinosauria: Theropoda) from the Upper Cretaceous Menefee Formation of New Mexico. PeerJ 6:e5749 https://doi.org/10.7717/peerj.5749

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

Sampson SD, Loewen MA, Farke AA, Roberts EM, Forster CA, Smith JA, et al. (2010) New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism. PLoS ONE 5(9): e12292. https://doi.org/10.1371/journal.pone.0012292

 

 

 

 

 

 

 

 

 

 

 

 

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The Tyrannosauroids from the Southern Hemisphere.

Santanaraptor lived in South America during the Early Cretaceous about 112 million years ago (From Wikimedia Commons).

Tyrannosauroidea, the superfamily of carnivorous dinosaurs that includes the iconic Tyrannosaurus rex, was mainly distributed in the Northern Hemisphere. However, a few specimens from Australia (Timimus hermani and the articulated pubes NMV P186046) and Brazil (Santanaraptor placidus), have been referred to this clade.

Santanaraptor was unearthed in 1996 in the Romualdo Group (Santana Formation). The holotype is a juvenile partial skeleton that may have reached 1.25 metres (4.1 ft) in length, and it was presumed to be similar to Dilong and Guanlong. It was first described as a coelurosaurian theropod by Alexander Kellner in 1999, but in 2014 Thomas Holtz classified Santanaraptor as the first tyrannosauroid known from Gondwana. Delcourt and Grillo (2018), also placed Santanaraptor within Tyrannosauroidea based on the following features: the absence of an accessory ridge on the lateral surface of the cnemial crest; the absence of a horizontal groove across the astragalar condyles anteriorly; a deep fossa on the medial surface of the femoral head, lateral to the trochanteric fossa; an ischial medial apron positioned along the anterior margin of its shaft in medial view; the lesser trochanter and the greater trochanter extending to approximately the same level proximally; the proximal margin of the femur is concave in posterior view due to a greater trochanter that is elevated substantially relative to the lateral portion of the proximal surface of the head; and a shallow femoral extensor groove on the anterior surface of the distal end that is expressed as a broad concave anterior margin in distal view but present as an extensive depression on the anterior surface of the femur.

Holotypic left femur of Timimus hermani (From Wikimedia Commons)

Timimus was unearthed in 1994 from Eumeralla Formation and shares similar features with Tyrannosauroidea, but due to the incompleteness of the Timimus holotype, is difficult to properly evaluate its phylogenetic position. The same applies to NMV P186046. It was suggested (Benson et al., 2012) that NMV P186046 and the Timimus holotype may represent a single taxon given their similar phylogenetic positions and congruent sizes, although they did not come from the same site.

Time-calibrated phylogeny of Tyrannosauroidea. From Delcourt and Grillo, 2018.

Tyrannosauroidea had a Eurasian distribution, but basal lineages of the newly proposed clade, Pantyrannosauria (the most inclusive clade, containing Tyrannosaurus rex and Dilong paradoxus, but not Proceratosaurus bradleyi), were distributed across Europe (Juratyrant, Eotyrannus and Aviatyrannis), North America (Stokesosaurus), South America (Santanaraptor), Australia (Timimus), and Asia (Dilong and Xiongguanlong). It was hypothesized that all basal lineages of Pantyrannosauria were already established in the Late Jurassic before the complete separation of Gondwana and Laurasia.

The fact that Santanaraptor and Timimus were relatively small suggests that Gondwanan tyrannosauroids remained small in comparison to northern species. The presence of Santanaraptor in a semi-arid environment with marine incursion also suggests that tyrannosauroids were not only found in humid paleoenvironments.

References:

Rafael Delcourt, Orlando Nelson Grillo , Tyrannosauroids from the Southern Hemisphere: Implications for biogeography, evolution, and taxonomy. Palaeo (2018), doi: 10.1016/j.palaeo.2018.09.003

Apesteguía, S., Smith, N.D., Juárez Valieri, R., Makovicky, P.J., 2016. An Unusual New Theropod with a Didactyl Manus from the Upper Cretaceous of Patagonia, Argentina. PLoS One 11, 1–41. doi:10.1371/journal.pone.0157793

Benson, R.B.J., Rich, T.H., Vickers-Rich, P., Hall, M., 2012. Theropod fauna from southern Australia indicates high polar diversity and climate-driven dinosaur provinciality. PLoS One 7, e37122. doi:10.1371/journal.pone.0037122

Porfiri, J. D., Novas, F. E., Calvo, J. O., Agnolín, F. L., Ezcurra, M. D. & Cerda, I. A. 2014. Juvenile specimen of Megaraptor (Dinosauria, Theropoda) sheds light about tyrannosauroid radiation. Cretaceous Research 51: 35-55. https://doi.org/10.1016/j.cretres.2014.04.007

Holtz Jr, T.R., 2004. Tyrannosauroidea, in: Weishampel, D.B., Dodson, P., Osm (Eds.), The Dinosauria. University of California Press, Berkeley, pp. 111–136.

A very short history of Dinosaurs.

Evolutionary relationships of dinosaurs. From Benton 2018.

On 20 February 1824, William Buckland published the first report of a large carnivore animal: the Megalosaurus. The description was based on specimens in the Ashmolean Museum, in the collection of Gideon Algernon Mantell of Lewes in Sussex, and a sacrum donated by Henry Warburton (1784–1858). One year later, the Iguanodon entered in the books of History followed by the description of Hylaeosaurus in 1833. After examined the anatomy of these three genera, Richard Owen erected the clade Dinosauria in 1842.

Dinosaurs likely originated in the Early to Middle Triassic. The closest evolutionary relatives of dinosaurs include flying pterosaurs and herbivorous silesaurids. Early ecological divergences in dinosaur evolution are signaled by disparity in dental morphology, which indicates carnivory in early theropods, herbivory in ornithischians, and omnivory in sauropodomorph (subsequently sauropodomorphs underwent a transition to herbivory).

Eoraptor lunensis, outcropping from the soil. Valle de la Luna (Moon Valley), Parque Provincial Ischigualasto, Provincia de San Juan, Argentina.

The oldest dinosaurs remains are from the late Carnian (230 Ma) of the lower Ischigualasto Formation in northwestern Argentina. 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 size.

During the Late Triassic period numerous extinctions, diversifications and faunal radiations changed the ecosystems dynamics throughout the world. Nevertheless, dinosaurs exhibited high rates of survival. According to the competitive model, the success of dinosaurs was explained in terms of their upright posture, predatory skills, or warm-bloodedness. In the opportunistic model, dinosaurs emerged in the late Carnian or early Norian, and then diversified explosively. The current model contains some aspects of both the classic competition model and the opportunistic model. In this model, the crurotarsan-dominated faunas were replaced by a gradual process probably accelerated by the ecological perturbation of the CPE (Carnian Pluvial Episode).

Ingentia prima outcropping from the soil.

In the Jurassic and Cretaceous dinosaurs achieved enormous disparity. Sauropodomorphs achieved a worldwide distribution and become more graviportal and increased their body size. Gigantism in this group has been proposed as the result of a complex interplay of anatomical, physiological and reproductive intrinsic traits. For example, the upright position of the limbs has been highlighted as a major feature of the sauropodomorph bauplan considered an adaptation to gigantism. However, the discovery of Ingentia prima, from the Late Triassic of Argentina, indicates that this feature was not strictly necessary for the acquisition of gigantic body size.

Ornithischian were primitively bipedal, but reverted to quadrupedality on at least three occasions: in Ceratopsia, Thyreophora and Hadrosauriformes. The presence of early armored dinosaurs (thyreophorans) in North America, Asia, and Europe, but their absent from the southern African record, suggests some degree of provinciality in early ornithischian faunas.

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

Theropod dinosaurs also increased their diversity and exhibit a greater range of morphological disparity. The group underwent multiple parallel increases in brain size. The volumetric expansion of the avian endocranium began relatively early in theropod evolution. For instance, the endocranium of Archaeopteryx lithographica is volumetrically intermediate between those of more basal theropods and crown birds. The digital brain cast of Archaeopteryx also present an indentation that could be from the wulst, a neurological structure present in living birds used in information processing and motor control with two primary inputs: somatosensory and visual. The extensive skeletal pneumaticity in theropods such as Majungasaurus demonstrates that a complex air-sac system and birdlike respiration evolved in birds’ theropod ancestors. Anatomical features like aspects of egg shape, ornamentation, microstructure, and porosity of living birds trace their origin to the maniraptoran theropods, such as oviraptorosaurs and troodontids. In addition, some preserving brooding postures, are known for four oviraptorosaurs, two troodontids, a dromaeosaur, and one basal bird providing clear evidence for parental care of eggs.

Nonavian dinosaurs disappeared more or less abruptly at the end of the Cretaceous (66 mya). Birds, the only living dinosaurs, with more than 10,500 living species are the most species-rich class of tetrapod vertebrates.

 

References:

Benson, R. B. J. (2018). Dinosaur Macroevolution and Macroecology. Annual Review of Ecology, Evolution, and Systematics, 49(1).  doi:10.1146/annurev-ecolsys-110617-062231

Michael J. Benton et al. The Carnian Pluvial Episode and the origin of dinosaurs, Journal of the Geological Society (2018). DOI: 10.1144/jgs2018-049

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.

 

Lingwulong shenqi, the “Amazing Dragon”, and the dispersal of Sauropods.

Skeletal reconstruction and exemplar skeletal remains of Lingwulong shenqi. Scale bars = 100 cm for a and 5 cm for b–o. From Xu et al., 2018

Sauropods were the largest terrestrial vertebrates. Their morphology is easy recognizable: a long, slender neck and a tail at the end of a large body supported by four columnar limbs. Sauropods dominated many Jurassic and Cretaceous terrestrial faunas. Although they were globally distributed, the absence of Diplodocoidea from East Asia has been interpreted as a biogeographic pattern caused by the Mesozoic fragmentation of Pangea. However, a newly discovered dinosaur from the Middle Jurassic of northern China suggests that Sauropods dispersed and diversified earlier than previously thought.

Lingwulong shenqi — literally the “amazing dragon from Lingwu” — is the first well-preserved confirmed diplodocoid from East Asia (23 synapomorphies support the placement of Lingwulong within Diplodocoidea with 10 of these being unequivocal). The holotype, (LM) V001a, is a partial skull comprising the braincase, skull roof, and occiput, and an associated set of dentary teeth. The paratype, (LGP) V001b, comprises a semi-articulated partial skeleton including a series of posterior dorsal vertebrae, complete sacrum, the first caudal vertebra, partial pelvis, and incomplete right hind limb.

An artist’s interpretation of what Lingwulong shenqi (Image: Zhang Zongda)

The Lingwulong specimens were found in the Yanan Formation at Ciyaopu, in northwest China. This formation has been divided in four or five members. Although, no radiometric constraints have been obtained for the Yanan Formation, its age has been estimated on the basis of biostratigraphy. The presence of a conchostracans assemblage (including Palaeoleptoestheria, Triglypta, and Euestheria) is indicative of a Middle Jurassic age.

The East Asian Isolation Hypothesis (EAIH) has become a well-established explanation of profound differences between Jurassic (and sometimes Early Cretaceous) Asian terrestrial faunas, that resulted in the evolution of endemic groups such as mamenchisaurid sauropods, and the early diverging lineage of tetanurans, oviraptorosaurs, therizinosaurs. In this model, the isolation ended in the Early Cretaceous when marine regressions allowed the invasion of groups from elsewhere in Pangaea, and the dispersal of Asian endemics (e.g., oviraptorosaurs, marginocephalians) into Europe and North America. However, it was claimed that diplodocoids never took part in these dispersals because the end-Jurassic extinction that greatly reduced their diversity and geographic range in the Early Cretaceous. The discovery of Lingwulong undermines the EAIH, forcing a significant revision of hypotheses concerning the origins and early radiation of Neosauropoda.

 

References:

Xing Xu, Paul Upchurch, Philip D. Mannion, Paul M. Barrett, Omar R. Regalado-Fernandez, Jinyou Mo, Jinfu Ma and Hongan Liu. 2018. A New Middle Jurassic Diplodocoid Suggests An Earlier Dispersal and Diversification of Sauropod Dinosaurs. Nature Communications.9, 2700.  DOI:  10.1038/s41467-018-05128-1 

 

 

 

Introducing Akainacephalus johnsoni

Skeletal reconstructions of Akainacephalus johnsoni. From Wiersma and Irmis, 2018

The Ankylosauria is a group of herbivorous, quadrupedal, armoured dinosaurs subdivided in two major clades, the Ankylosauridae and the Nodosauridae. The group is predominantly recorded from the Late Cretaceous (Turonian—late Maastrichtian) of Asia and the last Cretaceous (early Campanian—late Maastrichtian) of western North America (Laramidia). Ankylosauridae were present primarily in Asia and North America, and the most derived members of this clade are characterized by shortened skulls, pyramidal squamosal horns, and tail clubs.

Akainacephalus johnsoni, a new genus and species of an ankylosaurid dinosaur from the upper Campanian Kaiparowits Formation of southern Utah, represents the most complete ankylosaurid specimen from southern Laramidia to date, and reveals new details about the diversity and evolution of this clade. The genus name is derived from the Greek akaina, meaning “thorn” or “spine,” referring to the thorn-like cranial caputegulae of the holotype; and “cephalus,” the Greek meaning for head. The specific epithet honors Randy Johnson, volunteer preparator at the Natural History Museum of Utah.

Skull of Akainacephalus johnsoni. From Wiersma and Irmis, 2018

The holotype (UMNH VP 20202) is a partial skeleton comprising a complete skull, both mandibles, predentary, four dorsal, four dorsosacral, three sacral, one caudosacral, and eight caudal vertebrae, dorsal ribs, a complete tail club, both scapulae, left coracoid, right humerus, right ulna, partial left ilium, left femur, left tibia, left fibula, phalanx, two partial cervical osteoderm half rings, and 17 dorsal and lateral osteoderms of various sizes and morphologies.

The most striking feature of Akainacephalus johnsoni is the skull ornamentation comprising several symmetrical rows of small pyramidal and conical caputegulae along the dorsolateral surface of the skull. The postorbital horns are dorsoventrally tall, backswept, and project laterally in dorsal view. The quadratojugal horns display an  asymmetrical triangular morphology with a vertically positioned apex. Only a partial squamosal horn is preserved, but is largely broken.

Life reconstruction of Akainacephalus johnsoni (Image credit: Andrey Atuchin and the Denver Museum of Nature & Science)

The unique anatomical features of Akainacephalus johnsoni indicate a close taxonomic relationship with Nodocephalosaurus kirtlandensis, that clearly distinguish them from other Late Cretaceous Laramidian (although both taxa are temporally separated by nearly three million years). Because both taxa a more closely related to Asian ankylosaurids, the geographic distribution of Late Cretaceous ankylosaurids throughout the Western Interior could be the result of several geologically brief intervals of lowered sea level that allowed Asian ankylosaurid dinosaurs to immigrate to North America several times during the Late Cretaceous. The dispersal of ankylosaurids into Laramidia is coeval with the dispersal of other dinosaur clades, like tyrannosaurids and ceratopsians. The climate gradients and the fluctuations in sea level, may have helped reinforced Campanian provincialism.

 

References:

Wiersma JP, Irmis RB. (2018) A new southern Laramidian ankylosaurid, Akainacephalus johnsoni gen. et sp. nov., from the upper Campanian Kaiparowits Formation of southern Utah, USA. PeerJ 6:e5016 https://doi.org/10.7717/peerj.5016

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

Ingentia prima, the first giant

Skeletal anatomy of Ingentia prima (From Apaldetti et al., 2018)

During the Late Triassic period numerous extinctions, diversifications and faunal radiations changed the ecosystem dynamics throughout the world. Followed the extinction of rhynchosaurs in most, or all, parts of the world, there was a burst of dinosaurian diversity in the late Carnian, represented by the upper Ischigualasto Formation and coeval units, with mostly carnivorous small- to medium-sized dinosaurs. Then, the long span of the early Norian, from 228.5–218 Ma, during which dicynodonts and sauropodomorph dinosaurs were the major herbivores.

Sauropods evolved from small, gracile, bipedal forms, and it was long thought that acquisition of giant body size in this clade occurred during the Jurassic and was linked to several skeletal modifications. Ingentia prima — literally the “first giant” in Latin — from the Late Triassic of Argentina shed new lights on the origin of gigantism in this group. The holotype, PVSJ 1086, composed of six articulated posterior cervical vertebrae, glenoid region of right scapula and right forelimb lacking all phalanges, has been recovered from the southern outcrops of the Quebrada del Barro Formation, northwestern Argentina. Discovered in 2015 by Diego Abelín and a team led by Cecilia Apaldetti of CONICET-Universidad Nacional de San Juan, Argentina, this new fossil weighed up to 11 tons and measured up to 32 feet (10 meters) long.

Bones of Ingentia prima (Image credit: Cecilia Apaldetti, CONICET-Universidad Nacional de San Juan, Argentina)

Ingentia was unearthed with three new specimens of Lessemsaurus sauropoides. The four dinosaurs belongs to the clade Lessemsauridae, that differs from all other Sauropodomorpha dinosaurs in possessing robust scapulae with dorsal and ventral ends equally expanded; slit-shaped neural canal of posterior dorsal vertebrae; anterior dorsal neural spines transversely expanded towards the dorsal end; a minimum transverse shaft width of the first metacarpal greater than twice the minimum transverse shaft of the second metacarpal; and bone growth characterized by the presence of thick zones of highly vascularized fibrolamellar bone, within a cyclical growth pattern.

The age of the oldest lessemsaurid (mid-Norian) indicates the appearance of an early trend towards large body size at least 15 Myr earlier than previously thought. For a long time, gigantism in eusauropods has been proposed as the result of a complex interplay of anatomical, physiological and reproductive intrinsic traits. For example, the upright position of the limbs has been highlighted as a major feature of the sauropodomorph bauplan considered an adaptation to gigantism. However Lessemsaurids lacked the purported adaptations related to a fully erect forelimb and the marked modifications of the hindlimb lever arms in eusauropods, showing that these features were not strictly necessary for the acquisition of gigantic body size. Another feature interpreted as a key acquisition was the elongated neck. However, lessemsaurids also lacked an elongated neck as they had proportionately short cervical vertebrae, indicating that the neck elongation was not a prerequisite for achieving body sizes comparable to those of basal eusauropods or gravisaurians.

 

References:

Cecilia Apaldetti, Ricardo N. Martínez, Ignacio A. Cerda, Diego Pol and Oscar Alcober (2018). An early trend towards gigantism in Triassic sauropodomorph dinosaurs. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-018-0599-y

Triassic World: Rise of the Kingdom

Eoraptor lunensis, outcropping from the soil. Valle de la Luna (Moon Valley), Parque Provincial Ischigualasto, Provincia de San Juan, Argentina.

“Jurassic World: Fallen Kingdom” has finally been released, but don’t worry, this post is spoiler free. I just use the hype to the tell the story of the rise of dinosaurs to ecological dominance.

There have been many opinions about the origin of the dinosaurs. In the competitive model the success of dinosaurs was explained in terms of their upright posture, predatory skills, or warm-bloodedness. In the opportunistic model, dinosaurs emerged in the late Carnian or early Norian, and then diversified explosively. The current view contains some aspects of both the classic competition model and the opportunistic model. In this model, the crurotarsan-dominated faunas were replaced by a gradual process probably accelerated by the ecological perturbation of the CPE (Carnian Pluvial Episode).

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

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). The widespread extinction caused by the CPE was followed by the first substantial diversification of dinosaurs. That diversification can in fact be
divided into three phases: (1) the possible origin in the Olenekian-Anisian (248–245 Ma) related to the turmoil of recovering life in the aftermath of the devastating Permian-Triassic mass extinction (PTME), (2) a rapid diversification of saurischians, primarily sauropodomorphs and possible theropods, termed the dinosaur diversification event (DDE), at 232 Ma, and (3) a further diversification of theropods and especially ornithischians after the end-Triassic mass extinction, 201 Ma.

In Tanzania, the Manda Beds 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. 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).

 

Mounted skeleton of Plateosaurus engelhardti (almost complete specimen AMNH FARB 6810 from Trossingen, Germany)

The CPE is one of the most severe biotic crises in the history of life. 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.

The DDE likely occurred in steps. Followed the extinction of rhynchosaurs in most, or all, parts of the world, there was a burst of dinosaurian diversity in the late Carnian, represented by the upper Ischigualasto Formation and coeval units, with mostly carnivorous small- to medium-sized dinosaurs. Then, the long span of the early Norian, from 228.5–218 Ma, during which dicynodonts and sauropodomorph dinosaurs were the major herbivores. Finally, with the disappearance of dicynodonts, sauropodomorph dinosaurs became truly large in the middle and late Norian, from 218 Ma. This was followed by the extinction of basal archosaur groups during the end-Triassic mass extinction, 201 Ma, and the diversification of sauropods, larger theropods, ornithopods, and armoured dinosaurs subsequently, in the Jurassic.

 

References:

Michael J. Benton et al. The Carnian Pluvial Episode and the origin of dinosaurs, Journal of the Geological Society (2018). DOI: 10.1144/jgs2018-049

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

Jessica H. Whiteside, Sofie Lindström, Randall B. Irmis, Ian J. Glasspool, Morgan F. Schaller, Maria Dunlavey, Sterling J. Nesbitt, Nathan D. Smith, and Alan H. Turner. 2015. Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years. PNAS: doi:10.1073/pnas.1505252112

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.

 

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