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 patachonica. Nat. 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). 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)

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 (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 .