Sapeornis and the flight modes of birds

Sapeornis chaoyangensis (DNHM-3078) showing well-preserved primary (P) and secondary (S) feathers. From Serrano and Chiappe, 2017

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. Most of these fossils, like Sapeornis chaoyangensis (125 to 120 Ma), are from the Jehol Biota of northeastern China. Sapeornis shows a combination of derived and primitive features, like a short, robust non-strut-like coracoid and a fibula reaching the distal end of the tarsal joint, a pygostyle, reduced manual digits, and a well-fused carpometacarpus. All of these features indicates a mosaic pattern in the early evolution of birds and confirm the basal position of Sapeornis near Archaeopteryx and Jeholornis in the phylogeny of early birds.

The evolution of flight involved a series of adaptive changes at the morphological and molecular levels, that included the fusion and elimination of some bones and the pneumatization of the remaining ones. Archaeopteryx lacked a bony sternum and a compensatory specialized gastral basket for anchoring large flight muscles, while Jelohornis had several derived flight-related features of modern birds like fused sacral vertebrae, an elongated coracoid with a procoracoid process, a complex sternum, a narrow furcula, and curved scapula. In Enantiornithines, their robust pygostyle appears to have been unable to support the muscles that control the flight feathers on the tail in modern birds.

Morphofunctional fitness of the wing shape for soaring as depicted by the relation between the lift surface and the wingspan in modern soaring birds and Sapeornis (From Serrano and Chiappe, 2017)

The flight modes of modern birds are a reflection of their different strategies to reduce the energetic costs of a highly demanding style of locomotion. Among these features are wing shape, and the use of thermals and tail winds. Flapping flight is energetically more costly  than gliding and soaring flight, consequently, large birds have either elongated wingspans that allow them to gain height through air currents and to glide for long distances with much lower transit costs than flapping.

Fossil evidence suggests that S. chaoyangensis was a specialized flier that used continental soaring as its main flight mode. Computational models of S. chaoyangensis are also congruent with other morphological similarities between S. chaoyangensis and modern soaring birds including the shape of the furcula and the proportions of the forelimbs. Modern soaring birds include dynamic soarers that exploit air velocity gradients over sea waves, and thermal soarers that use ascending air currents mainly generated in continental areas. Because, exceptionally well preserved fossils of S. chaoyangensis have revealed seeds and/or fruits in its intestinal tract, this interpretation of the flight capabilities of S. chaoyangensis is consistent with the energetic disadvantages from a herbivorous diet, because soaring is a less demanding flight mode than continuous flapping.

References:

Serrano FJ, Chiappe LM. 2017 Aerodynamic modelling of a Cretaceous bird reveals thermal soaring capabilities during early avian evolution. J. R. Soc. Interface 14: 20170182. http://dx.doi.org/10.1098/rsif.2017.0182

Butler PJ. 2016 The physiological basis of bird flight. Phil. Trans. R. Soc. B 371, 20150384 doi:10. 1098/rstb.2015.0384

Zhou, Zhonghe & Zhang, Fucheng (2003): Anatomy of the primitive bird Sapeornis chaoyangensis from the Early Cretaceous of Liaoning, China. Canadian Journal of Earth Sciences 40(5): 731–747. doi 10.1139/E03-011

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Osteohistological analysis of Vegavis iaai

Vegavis iaai by Gabriel Lio. / Photo: CONICET

The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates. Vegavis iaai is the first unquestionable neornithine bird from the Cretaceous and is known by the holotype and specimen MACN-PV 19.748. The holotype specimen MLP 93-I-3-1 (Museo de La Plata, Argentina) from Vega Island, western Antarctica, was discovered in 1992 by a team from the Argentine Antarctic Institute, but was only described as a new species in 2005 (Clarke et al., 2005). Polarornis gregrorii, from the López de Bertodano Formation of Seymour Island, Antarctica, and Vegavis form a monophyletic basal clade of foot-propelled anseriform birds (Agnolín 2016), a group that includes ducks, geese and swans.

Osteohistological analysis of the femur and humerus of V. iaai. shows a highly vascularized fibrolamellar matrix lacking lines of arrested growths, features widespread among modern birds. The femur has some secondary osteons, and shows several porosities, one especially large, posterior to the medullar cavity. The humerus exhibits a predominant fibrolamellar matrix, but in a portion of the anterior and medial sides of the shaft there are a few secondary osteons, some of them connected with Volkman’s canals, and near to these canals, there are a compact coarse cancellous bone (CCCB) with trabeculae. This tissue disposition and morphology suggests that Vegavis had remarkably high growth rates.

Detail of the humerus of Vegavis iaai (MACN-PV 19.748) in polarised light. Scale = 1 mm. (From G. Marsà et al., 2017)

Many studies on avian microanatomy have established a relationship between high bone compactness (i.e., considerable degree of osteosclerosis) and diving behavior. Differences in the degree of osteosclerosis could be tentatively related to variations in diving behaviour. Vegavis was a diver, characterised by a medium level of limb osteosclerosis. Polarornis, with more massive bones, was possibly adapted to deeper and more prolonged diving than Vegavis, as occurs in modern penguins.

The value of Relative Bone Thickness (RBT) in Vegavis is comparable with two genera of extant foot-propelled diving ducks. A high RBT is related with increased stiffening the forelimb, regardless of body mass or depth of diving. Flightless Pan-Alcidae and penguins, have a very rigid, flipper-like wings suggesting that decreased wing flexion and increased cortical thickness of forelimbs are somehow correlated. Based on  the values of RBT present in both Vegavis and Polarornis is possible to infer that these taxa were foot-propelled birds.

References:

Jordi Alexis Garcia Marsà, Federico L. Agnolín & Fernando Novas (2017): Bone microstructure of Vegavis iaai (Aves, Anseriformes) from the Upper Cretaceous of Vega Island, Antarctic Peninsula, Historical Biology, DOI: 10.1080/08912963.2017.1348503

Agnolín FL. 2016. A brief history of South American birds. Contribuciones del MACN 6:157–172

Clarke, J. A., C. P. Tambussi, J. I. Noriega, G. M. Erickson, and R. A. Ketcham. 2005. Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433:305-308. DOI: 10.1038/nature03150

The sixth mass extinction

Painting of the Dodo by Roelandt Savery executed in ca. 1626 and held at the NHMUK, London.

Mass extinctions had shaped the global diversity of our planet several times during the geological ages. The fossil record indicates that more than 95% of all species that ever lived are now extinct. During times of normal background extinction, the taxa that suffer extinction most frequently are characterized by small geographic ranges and low population abundance. Occasionally extinction events reach a global scale, with many species of all ecological types dying out in a near geological instant. In a conservative palaeontological sense, a mass extinction occurs when extinction rates accelerate relative to origination rates such that over 75% of species disappear within a geologically short interval (typically less than 2 million years).

Over the past 500 years, humans have triggered a wave of extinction, threat, and local population declines that may be comparable with the five previous mass extinctions of Earth’s history. Although anthropogenic climate change is playing a growing role, the primary drivers of modern extinctions seem to be habitat loss, human predation, and introduced species. The term defaunation was created to designate the declining of top predators and herbivores triggered by human activity, that results in a lack of agents that control the components of the ecosystems vegetation.

The percentage of species of land mammals from five major continents/
subcontinents in the period ∼1900–2015 (From Ceballos et al., 2017)

The most recent Living Planet Index (LPI) has estimated that wildlife abundance on the planet decreased by as much as 58% between 1970 and 2012. Several species of mammals that were relatively safe one or two decades ago are now endangered. The highest percentage of decreasing species is concentrated in tropical regions, mostly in the Neotropics and Southeast Asia. In 2016, there were only 7,000 cheetahs in existence, and less than 5,000 Borneo and Sumatran orangutans. Populations of African lion has dropped 43% since 1993, and populations of giraffes dropped from around 115,000 individuals in 1985 to around 97,000 representing what is now recognized to be four species (Giraffa giraffa, G. tippelskirchi, G. reticulata, and G. camelopardalis) in 2015.

Amphibians offer an important signal to the health of biodiversity; when they are stressed and struggling, biodiversity may be under pressure. Decreasing amphibians are prominent in Mexico, Central America, the northern Andes, Brazil, West Africa, Madagascar, India, Indonesia and Philippine. In the case of reptiles, the proportional decline concentrates almost exclusively in Madagascar; and decreasing species of birds are found over large regions of all continents. Other studies document that invertebrates and plants are suffering massive losses of populations and species. Long-term distribution data on moths and four other insect orders in the UK show that a substantial proportion of species have experienced severe range declines in the past several decades. Therefore, the acceleration of extinctions over the past decades, in which humans have played an increasingly important role, has left a number of hard questions about how the Anthropocene should be defined and whether or not extinctions should contribute to this definition.

 

References:

Gerardo CeballosPaul R. Ehrlichand Rodolfo Dirzo; Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines, Proceedings of the National Academy of Sciences (2017) doi: 10.1073/pnas.1704949114 

Rodolfo Dirzo et al., Defaunation in the Anthropocene, Science 345, 401 (2014); DOI: 10.1126/science.1251817

 

Molecular signatures of fossil leaves

Leaves of Ptilophyllum mueller, from Emmaville, New South Wales. Scale bars=10 mm (From McLoughlin et al., 2011)

The first plants colonized land approximately 450 million years ago. The transition from an exclusively aquatic to a terrestrial life style implied the evolution of a new set of morphological and physiological features. The most critical adaptive trait for survival during terrestrialization was the ability to retain water in increasingly dehydrating habitats. Consequently, the capacity to maintain a hydrophobic surface layer, or cuticle, over the surfaces of aerial organs was arguably one of the most important innovations in the history of plant evolution.

Spores, pollen and leaf cuticles, are among the most resilient organic structures in the geological record. These components may retain some phylogenetically unique signals, not only in well-preserved fossils, but also in remains with a high level of diagenetic maturity.

Ginkgo biloba, Eocene fossil leaf from the Tranquille Shale of MacAbee, British Columbia, Canada (From Wikipedia Commons)

Generally, the cuticle is divided into two major structural constituents: cutin and cutan. The fatty acid polyesters which constitute cutin, gives the cuticle considerable resistance to biodegradation. Cutan is a non-ester and non-hydrolyzable matrix of aliphatic compounds linked by ether bonds, which remain after cutin hydrolysis. Additionally, the surface of the cuticle may be covered by various long-chain hydrophobic waxes. All these components  favours the survival of the cuticle in many fossil plants, and can be used to resolve the stratigraphic ranges and relationships of extinct plants.

Data from infrared spectroscopy of modern plant cuticles, have been used successfully to support and clarify the species-level taxonomy of extant plants, for example, in Camellia and angiosperm pollen. Using infrared spectroscopy and statistical analysis, researchers at Lund University, the Swedish Museum of Natural History in Stockholm, and Vilnius University, analysed a selection of fossil Cycadales, Ginkgoales and conifers. The team obtained two major groups in the dendrogram of infrared spectra. One branch unites podocarpacean and araucariacean conifers (excluding the Jurassic Allocladus). A relationship consistent with all modern phylogenetic analyses of gymnosperm. The second branch unites a broad range of gymnosperms. Within this branch, Bennettitales (Otozamites and Pterophyllum) form a well-defined group in association with Ptilozamites and Nilssoniales. This cluster is linked to a group incorporating Cycadales on one sub-branch, and Leptostrobales, Ginkgoales and the putative araucariacean Jurassic conifer Allocladus on a second sub-branch.

 

Dendrogram based on HCA of infrared absorption spectra of an expanded group of 13 fossil gymnosperm taxa (From Vajda et al., 2017)

Early palaeobotanical studies generally linked Bennettitales to Cycadales, but more recent anatomical studies and cladistic analyses have indicated that Bennettitales are not closely related to Cycadales. By contrast, Bennettitales, Nilssoniales and Ptilozamites are grouped closely. Additionally,  the systematic position of Allocladus within Araucariaceae should be reassessed based on its close association with Ginkgo in the cluster analysis of infrared spectra.

References:

Vivi Vajda, Milda Pucetaite, Stephen McLoughlin, Anders Engdahl, Jimmy Heimdal, Per Uvdal. Molecular signatures of fossil leaves provide unexpected new evidence for extinct plant relationships. Nature Ecology & Evolution, 2017; DOI: 10.1038/s41559-017-0224-5

Stephen McLoughlinRaymond J. Carpenter, and Christian Pott, Ptilophyllum muelleri (Ettingsh.) comb. nov. from the Oligocene of Australia: Last of the Bennettitales?, International Journal of Plant Sciences 2011 172:4574-585, DOI: 10.1086/658920

Solving a Darwinian mystery

Macrauchenia patachonica by Robert Bruce Horsfall.

During the first two years of his voyage aboard HMS Beagle, Charles Darwin collected a considerable number of fossil mammals from various South American localities. Darwin sent all the specimens to the Reverend Professor John Stevens Henslow, his mentor and a close friend. The samples were deposited in the Royal College of Surgeons where Richard Owen began its study. Between 1837 and 1845, Owen described eleven taxa, including: Toxodon platensis, Macrauchenia patachonica, Equus curvidens, Scelidotherium leptocephalum, Mylodon darwinii, and Glossotherium sp.

Macrauchenia, meaning “big neck,” was named by Richard Owen based on limb bones and vertebrae collected by Charles Darwin on January 1834 at Puerto San Julian, in Santa Cruz Province, Argentina. The bizarre animal had a camel-like body, with sturdy legs, a long neck and a relatively small head. Owen described as “A large extinct Mammiferous Animal, referrible to the Order Pachydermata; but with affinities to the Ruminantia, and especially to the Camelidae”Macrauchenia is now considered among the more derived native South American litopterns, an endemic order whose fossil record extends from the Paleocene to the end of the Pleistocene and includes some 50 described genera. Darwin also made inferences about the environment which Macrauchenia lived: “Mr. Owen… considers that they form part of an animal allied to the guanaco or llama, but fully as large as the true camel. As all the existing members of the family of Camelidae are inhabitants of the most sterile countries, so we may suppose was this extinct kind… It is impossible to reflect without the deepest astonishment, on the changed state of this continent. Formerly it must have swarmed with great monsters, like the southern parts of Africa, but now we find only the tapir, guanaco, armadillo, capybara; mere pigmies compared to antecedents races… Since their loss, no very great physical changes can have taken place in the nature of the Country. What then has exterminated so many living creatures?…We are so profoundly ignorant concerning the physiological relations, on which the life, and even health (as shown by epidemics) of any existing species depends, that we argue with still less safety about either the life or death of any extinct kind” (Voyage of the Beagle, Chapter IX, Jan. 1834).

Dated mitogenomic phylogenetic tree. (From Westbury, M. et al)

The unusual morphological traits displayed by extinct South American native ungulates defied both Charles Darwin and Richard Owen, who tried to resolve their relationships. Two recently published molecular studies, using protein (collagen) sequence information, found that litopterns as well as notoungulates formed a monophyletic unit that shared more recent common ancestry with Perissodactyla than with any other extant placental group.

A valuable tool for uncovering phylogenetic relationships of extinct animals is ancient DNA (aDNA), although, attempts to use standard aDNA methodologies to collect genetic material from specimens from low-latitude localities have been largely unsuccessful. However, a new study recovered a nearly complete mitochondrial genome for Macrauchenia from a cave in southern Chile. The small size of the mitochondrial genome simplifies the assembly of fossil sequences using de novo methods.

In theory, reconstructing an ancient genome de novo can be undertaken without relying on a close relative’s DNA for guidance, but due to contaminant DNA and low average fragment lengths, de novo assembly is generally considered not computationally feasible. A promising new approach is using  the genetic codes of numerous living species as reference points, allowing them to reliably predict the fossil’s likeliest genetic sequences. Using the new approach, the phylogenetic analyses place Macrauchenia as a sister taxon to all living Perissodactyla, with the origin of Panperissodactlya at 66 Ma.

 

References:

Westbury, M. et al. A mitogenomic timetree for Darwin’s enigmatic South American mammal Macrauchenia patachonicaNat. Commun. 8, 15951 doi: 10.1038/ncomms15951 (2017).

Welker, F. et al. Ancient proteins resolve the evolutionary history of Darwin’s South American ungulates. Nature 522, 81–84 (2015). doi:10.1038/nature14249

Buckley, M. Ancient collagen reveals evolutionary history of the endemic South American ‘ungulates’. Proc. Biol. Sci. 282, 20142671 (2015). DOI: 10.1098/rspb.2014.2671

Neuroanatomy of the abelisaurid theropod Viavenator exxoni

Viavenator exxoni, Museo Municipal Argentino Urquiza

The Abelisauridae represents the best-known carnivorous dinosaur group from Gondwana. Their fossil remains have been recovered in Argentina, Brazil, Morocco, Niger, Libya, Madagascar, India, and France. The group was erected by Jose Bonaparte with the description of  Abelisaurus Comahuensis. These theropods exhibit spectacular cranial ornamentation in the form of horns and spikes and strongly reduced forelimbs and hands. In South America, braincase remains are known for Carnotaurus sastrei, Abelisaurus comahuensis, Aucasaurus garridoi, Ekrixinatosaurus novasi, Skorpiovenator bustingorryi, Eoabelisaurus and Viavenator exxoni.

The holotype of Viavenator exxoni (MAU-Pv-LI-530) was found in the outcrops of the Bajo de la Carpa Formation (Santonian, Upper Cretaceous), northwestern Patagonia, Argentina. Cranial elements of this specimen include the complete neurocranium: frontals, parietals, sphenethmoids, orbitosphenoids, laterosphenoids, prootics, opisthotics, supraoccipital, exoccipitals, basioccipital, parasphenoids and basisphenoids. The cranial endocast of Viavenator measures 157.7 mm from the olfactory bulbs to the foramen magnum, with a volume of approximately 141.6 cm3. The general shape of cranial endocast is elongate and narrow, similar to Aucasaurus and Majungasaurus. The widest part of the cranial endocast of Viavenator is at the level of the cerebral hemispheres. Four blood vessel foramina are recognized in the braincase: the caudal middle cerebral vein, the rostral middle cerebral vein, the cerebral internal carotid artery and the sphenoid artery.

Figure 1. Rendering of the type braincase of Viavenator exxoni (MAU-Pv-LI-530) in dorsal (A,B), and right lateral (C,D) view. Adapted from Carabajal y Filippi, 2017.

The forebrain of Viavenator include the olfactory tracts and olfactory bulbs, the cerebral hemispheres, optic nerves, the infundibular stalk, and the pituitary body. The CT scans show that the olfatory tracts are undivided. The olfactory bulbs are oval and are separated by a median septum at the anterior region of the sphenethmoids. The optic lobes are not clearly defined. The visible features of the hindbrain in the cranial endocast include the cerebellum, medulla oblongata, and cranial nerves V–XII. The cerebellum is not clearly expanded in the endocast; however, the floccular process of the cerebellum is well defined. The general morphology of both, brain and inner ear of Viavenator is markedly similar to that of Aucasaurus.
Neurosensorial capabilities of extinct animals can be inferred in part based on the relative development of certain regions of the brain. The flocculus of the cerebellum plays a role in coordinate eye movements, and tends to be enlarged in taxa that rely on quick movements of the head and the body. The flocculus of Viavenator is particularly large compared with Majungasaurus, suggesting that Viavenator relied more on quick movements of the head and sophisticated gaze stabilization mechanisms than the African form. The dimensions of the auditory sensory epithelium of Viavenator is similar to Majungasaurus, suggesting that they had similar hearing capabilities. In large dinosaurs, hearing was restricted to low frequencies with high frequency limit below 3 kHz.

References:

Paulina-Carabajal, A., Filippi, L., Neuroanatomy of the abelisaurid theropod Viavenator: The most complete reconstruction of a cranial endocast and inner ear for a South American representative of the clade, Cretaceous Research (2017), doi: 10.1016/j.cretres.2017.06.013

Leonardo S. Filippi, Ariel H. Méndez, Rubén D. Juárez Valieri and Alberto C. Garrido (2016). «A new brachyrostran with hypertrophied axial structures reveals an unexpected radiation of latest Cretaceous abelisaurids». Cretaceous Research 61: 209-219. doi:10.1016/j.cretres.2015.12.018

Forelimb posture in Chilesaurus diegosuarezi.

 

Chilesaurus holotype cast (MACN. From Wikipedia Commons. Author: Evelyn D’Esposito)

Chilesaurus diegosuarezi is a bizarre tetanuran 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. For a basal tetanuran, Chilesaurus possesses a number of surprisingly plesiomorphic traits on the hindlimbs, especially in the ankle and foot, which resemble basal sauropodomorphs.

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. However, the hindlimbs of Chilesaurus are posteriorly extended, rather than ventrally flexed. So it seems that individuals of Chilesaurus were buried quickly and fossilized almost in life position during passive activity (e.g. feeding, resting).

Cast of SNGM-1937 specimen of Chilesaurus diegosuarezi in dorsal (1), 471 lateral (2), and anterolateral view (3).

Cast of SNGM-1937 specimen of Chilesaurus diegosuarezi in dorsal (1), 471 lateral (2), and anterolateral view (3). Scale: 20 mm.

The specimen SNGM-1937 shows an angular relation in the wrist that resembles that in Deinonychus. In fact, several coelurosaurs have the same resting position as the forelimbs of Chilesaurus, with the humerus and radius-ulna in perpendicular relation or elbow flexed in an acute angle, hands under the radius-ulna, and palmar surface posterodorsal and dorsomedial oriented with respect to the main body axis. The resting posture of the forelimbs has been studied in theropod species, in relation to the acquisition of flight. It was suggested that the presence of the forelimb folded structure in advanced theropods are related with soft structures, as patagial skin and muscles, present in several maniraptoran dinosaurs.

The cojoined flexion of wrist and elbow in living birds is mainly conducted by the action of a large number of tendons located within the propatagium. Although the existence of propatagium was considered as unique to modern birds, it have also been described for coelurosaurs and Pterosauria. The preserve of a flexed forearm in Chilesaurus, may be also regarded as an indirect indicative of the presence of propatagium in this taxon.

 

References:

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 (advance online publication) 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

 

A mid-Cretaceous enantiornithine frozen in time

Overview of HPG-15-1 in right lateral view. (From Xing et al., 2017)

Overview of HPG-15-1 in right lateral view. (From Xing et al., 2017)

Amber from the Hukawng Valley in northern Myanmar, called Burmese amber, has been commercially exploited for millennia. Of the seven major deposits of amber from the Cretaceous Period, Burmese amber has probably the most diverse paleobiota, including the tail of a non-avian coelurosaurian theropod, and three juvenile enantiornithine birds. The third specimen, HPG-15-1, is the most complete fossil bird discovered in Burmese amber. It comes from the Angbamo site, and measures approximately 86 mm x 30 mm x 57 mm, and weighs 78 g. It  was encapsulated during the earliest stages of its feather production, and  plumage preserves an unusual combination of precocial and altricial features unlike any living hatchling bird.

 Details of the head in HPG-15-1. A, x-ray µCT reconstruction in left lateral view

Details of the head in HPG-15-1. A, x-ray µCT reconstruction in left lateral view (From Xing et al., 2017)

The skull was split when the amber was cut. The rostrum is preserved in one section and the neck and most of the braincase in the other. The skull is mesorostrine. A  single tooth is visible in the left premaxilla. As in Early Cretaceous enantiornithines, the premaxillary corpus is short, forming approximately one-third of the rostrum. The exoccipitals contributed to the dorsal portion of the condyle and were unfused at the time of death. The frontals articulate for most of their length with a small gap between their rostral ends as in Archaeopteryx.  The inner ear and its semicircular canals are preserved. There are at least six articulated cervical vertebrae, including the atlas and axis, preserved in articulation with the skull. The post-axial vertebrae are rectangular with large neural canals, low and caudally displaced neural spines, and a ventral keel as in many enantiornithines. The articulated skull and series of cervical vertebrae bear plumage in dense fields. The individual feathers  are dark brown in color, and appear to consist of tufts of four or more barbs. Skin is preserved as a translucent film in unfeathered regions of both the head and neck.

Microstructure and pigmentation of feathers on wing and body of HPG-15-1. Scale bars equal 1 mm in (A, C); 0.5 mm in (B, D). From Xing et al., 2017

The new specimen also preserves a partial distal wing, the distal right tibiotarsus and complete right foot as well as part of the left pes. Both skeletal material and integumentary structures from the wing’s apex are well-preserved. The plumage consists of fragments of some of the primaries, and alula feathers, some of the secondaries and coverts, and traces of contours from the wing base. The hind limbs preserve feathers and traces of skin. The absence of fusion between the tarsals indicates that the specimen is ontogenetically immature. The proportions of the pedal digits suggest an arboreal lifestyle. Plumage within the femoral and crural tracts consists of neoptile feathers with a short or absent rachis. These feathers are nearly transparent, suggesting that they were pale or white. The skin beneath the crural tract is thin and smooth. The tip of the tail clearly preserves the remains of a single large sheathed rectrix.

The slow post-natal growth results in a protracted period of vulnerability, which is reflected in the Enantiornithes by the large number of juveniles found in the fossil record, whereas young juveniles of other Cretaceous bird lineages are unknown.

 

References:

Lida Xing, Jingmai K. O’Connor, Ryan C. McKellar, Luis M. Chiappe, Kuowei Tseng, Gang Li, Ming Bai , A mid-Cretaceous enantiornithine (Aves) hatchling preserved in Burmese amber with unusual plumage, (2017), doi: 10.1016/j.gr.2017.06.001

The American incognitum and the History of Extinction Studies

 

Georges Cuvier (1769 -1832) and the painting of Charles Wilson Peale’s reconstruction of the American incognitum

Extinction is the ultimate fate of all species. More than 95% of all species that ever lived are now extinct. But prior to the 18th century, the idea that species could become extinct was not accepted. However, as the new science of paleontology began bringing its first major discoveries to light, researchers began to wonder if the large vertebrate fossils of strange creatures unearthed by the Enlightenment explorers were indeed the remains of extinct species.

In 1739, French soldiers under the command of Baron Charles le Moyne de Lougueuil recovered a tusk, femur, and three curious molar teeth from Big Bone Lick, Kentucky, a place known in several American Indian narratives. Lougueuil sent these specimens to the Cabinet du Roi (Royal Cabinet of Curiosities) in Paris. In 1762, Louis Jean-Marie Daubenton, a zoologist at the Jardin du Roi concluded that the femur and tusk from the Longueuil’s collection were those of a large elephant, the “Siberian Mammoth,” but the three molars came from a gigantic hippopotamus.

Molar collected at Big Bone Lick in 1739 and described in Paris in 1756. (Georges Cuvier, Recherches sur les ossemens fossiles)

By the early 18 century it was inconceivable for many researchers that a species could be vanished. Naturalist Georges-Louis Leclerc de Buffon, wrote in 1749 about the extinction of marine invertebrates, but he adopted Daubenton’s view that the Siberian mammoth and the animal of the Ohio, known as the American incognitum, were both northern forms of the extant elephant rather than a vanished species. British anatomist William Hunter was the first to speculate that these remains might be from an extinct species. In 1799, the discovery of an American incognitum femur from Quaternary deposits in the Hudson River Valley led to excavations organized by Charles Wilson Peale. In 1801, the excavations resulted in the recovery of an almost complete skeleton. Peale reconstructed the skeleton with help from the American anatomist Caspar Wistar, and the displayed the mounted skeleton in public in December of that year.

In 1806 Georges Cuvier resolved the controversy about the  American incognitum demonstrating that both the Siberian mammoth and the “animal de l’Ohio” were elephants, but of different species. He described the Ohio elephant as a mastodon and he reached the conclusion that probably represented an extinct species. Cuvier was also the first to suggested that periodic “revolutions” or catastrophes had befallen the Earth and wiped out a number of species. But, under the influence of Lyell’s uniformitarianism, Cuvier’s ideas were rejected as “poor science”. The modern study of mass extinction did not begin until the middle of the twentieth century. One of the most popular of that time was “Revolutions in the history of life” written by Norman Newell in 1967.

 

References:

Macleod, N. The geological extinction record: History, data, biases, and testing. Geol. Soc. Am. Spec. Pap. 505, (2014), DOI: 10.1130/2014.2505(01)​

Marshall, Charles R., Five palaeobiological laws needed to understand the evolution of the living biota, Nature Ecology & Evolution 1, 0165 (2017), DOI: 10.1038/s41559-017-0165 .

Terrestrial floras at the Triassic-Jurassic Boundary in Europe.

Proportions of range-through diversities of higher taxonomic categories of microfloral elements over the Middle Triassic–Early Jurassic interval (From Barbacka et al., 2017)

Over the last 3 decades, mass extinction events  have become the subject of increasingly detailed and multidisciplinary investigations. Most of those events are associated with global warming and proximal killers such as marine anoxia. Volcanogenic-atmospheric kill mechanisms include ocean acidification, toxic metal poisoning, acid rain, increased UV-B radiation, volcanic darkness, cooling and photosynthetic shutdown. The mass extinction at the Triassic-Jurassic Boundary (TJB) has been linked to the eruption of the Central Atlantic Magmatic Province (CAMP), a large igneous province emplaced during the initial rifting of Pangea. Another theory is that a huge impact was the trigger of the extinction event. At least two craters impact were reported by the end of the Triassic. The Manicouagan Impact crater in the Côte-Nord region of Québec, Canada was caused by the impact of a 5km diameter asteroid, and it was suggested that could be part of a multiple impact event which also formed the Rochechouart crater in France, Saint Martin crater in Canada, Obolon crater in Ukraine, and the Red Wing crater in USA (Spray et al., 1998).

Photographs of some Rhaetian–Hettangian spores and pollen from the Danish Basin (From Lindström, 2015)

Most mammal-like reptiles and large amphibians disappeared, as well as early dinosaur groups. In the oceans, this event eliminated conodonts and nearly annihilated corals, ammonites, brachiopods and bivalves. In the Southern Hemisphere, the vegetation turnover consisted in the replacement to Alisporites (corystosperm)-dominated assemblage to a Classopollis (cheirolepidiacean)-dominated one. But there was no mass extinction of European terrestrial plants during the TJB. The majority of genera and a high percentage of species still existed in its later stages, and replacement seems to have been local, explainable as a typical reaction to an environmental disturbance. In Greenland, for example, the replacement of Triassic wide-leaved forms with Jurassic narrow-leaved forms was linked to the reaction of plants to increased wildfire. In Sweden, wildfire in the late Rhaetian and early Hettangian caused large-scale burning of conifer forests and ferns, and the appearance of new swampy vegetation. In Austria and the United Kingdom, conifers and seed ferns were replaced by ferns, club mosses and liverworts. In Hungary, there was a high spike of ferns and conifers at the TJB, followed by a sudden decrease in the number of ferns along with an increasing share of swamp-inhabiting conifers.

Although certain taxa/families indeed became extinct by the end of the Triassic (e.g. Peltaspermales), the floral changes across Europe were rather a consequence of local changes in topography.

References:

Maria Barbacka, Grzegorz Pacyna, Ádam T. Kocsis, Agata Jarzynka, Jadwiga Ziaja, Emese Bodor , Changes in terrestrial floras at the TriassicJurassic Boundary in Europe, Palaeogeography, Palaeoclimatology, Palaeoecology (2017), doi: 10.1016/j.palaeo.2017.05.024

S. Lindström, Palynofloral patterns of terrestrial ecosystem change during the end-Triassic event — a review, Geol. Mag., 1–23 (2015) https://doi.org/10.1017/S0016756815000552

Van de Schootbrugge, B., Quan, T.M., Lindström, S., Püttmann, W., Heunisch, C., Pross, J., Fiebig, J., Petschick, R., Röhling, H.-G., Richoz, S., Rosenthal, Y., Falkowski, P. G., 2009. Floral changes across the Triassic/Jurassic boundary linked to flood basalt volcanism. Nat. Geosci. 2, 589–594. doi: 10.1038/NGEO577.

N.R. Bonis, W.M. Kürschner, Vegetation history, diversity patterns, and climate change across the Triassic/Jurassic boundary, Paleobiology, 8 (2) (2012), pp. 240–264 https://doi.org/10.1666/09071.1