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

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An avian vocal organ from the Mesozoic.

The Vegavis iaai specimen showing the location of the syrinx. (Adapted from Clarke et al., 2016)

The Vegavis iaai specimen showing the location of the syrinx. (Adapted from Clarke et al., 2016)

Birds originated from a theropod lineage more than 150 million years ago. Their evolutionary history is one of the most enduring and fascinating debates in paleontology. In recent years, several discovered fossils of theropods and early birds have filled the morphological, functional, and temporal gaps along the line to modern birds. The earliest diversification of extant birds (Neornithes) occurred during the Cretaceous period and after the mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary, the Neoaves, the most diverse avian clade, suffered a rapid global expansion and radiation. Today, with more than 10500 living species, birds are the most species-rich class of tetrapod vertebrates.

In the mid-nineteenth century, T. H. Huxley recognized that birds were most closely related to dinosaurs. He also named the unique vocal organ in birds as the syrinx. Located at the base of a bird’s trachea, the syrinx consists of specialised cartilaginous structures, connective tissue masses, membranes and muscles. The oldest known remains of a syrinx was found within the fossilised, partial skeleton of a bird, known as Vegavis iaai, from the Late Cretaceous (66 mya) of Antarctica.

Vegavis iaai by Gabriel Lio. / Photo: CONICET

Vegavis iaai by Gabriel Lio. / Photo: CONICET

The Vegavis iaai holotype specimen 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). It belonged to the clade Anseriformes, a group that includes ducks, geese and swans. Vegavis exhibits the fusion of cartilage rings and asymmetry between the left and right sides of the syrinx, that are useful for making comparisons with structural data from the present-day birds. Fused rings in Vegavis form a well-mineralized pessulus, a derived neognath bird feature, proposed to anchor enlarged vocal folds or labia. Although mineralized structures of the syrinx in Vegavis and many parts of extant Anatidae show asymmetry, Presbyornis, Chauna and Galliformes lack this feature. The absence of known tracheobronchial remains in all other Mesozoic dinosaurs may be indicative that a complex syrinx was a late arising feature in the evolution of birds, well after the origin of flight and respiratory innovations.

 

References:

Julia A. Clarke, Sankar Chatterjee, Zhiheng Li, Tobias Riede, Federico Agnolin, Franz Goller, Marcelo P. Isasi, Daniel R. Martinioni, Francisco J. Mussel and Fernando E. Novas. Fossil evidence of the avian vocal organ from the Mesozoic. Nature, 2016 DOI: 10.1038/nature19852

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.

Larsen, O. N.; Franz Goller (2002). “Direct observation of syringeal muscle function in songbirds and a parrot”. The Journal of Experimental Biology. 205 (Pt 1): 25–35.

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 Permian lagerstätte from Antarctica.

 

Vertebraria solid-stele and polyarch roots colonised by fungal spores (From Slater et al., 2014)

Vertebraria solid-stele and polyarch roots colonised by fungal spores (From Slater et al., 2014)

A lagerstätte (German for ‘storage place’) is a site exhibiting an extraordinary preservation of life forms from a particular era. The term was originally coined by Adolf Seilacher in 1970. One of the most notable  is Burgess Shale in the Canadian Rockies of British Columbia. The site, discovered by Charles Walcott in 1909, highlight one of the most critical events in evolution: the Cambrian Explosion (540 million to 525 million years ago). The factors that can create such fossil bonanzas are: rapid burial (obrution), stagnation (eutrophic anoxia), fecal pollution (septic anoxia), bacterial sealing (microbial death masks), brine pickling (salinization), mineral infiltration (permineralization and nodule formation by authigenic cementation), incomplete combustion (charcoalification), desiccation (mummification) and freezing. The preservation of decay-resistant lignin of wood and cuticle of plant leaves  is widespread, but exceptional preservation also extends to tissues.

The Toploje Member chert of the Prince Charles Mountains preserves the permineralised remains of a terrestrial ecosystem before the biotic decline that began in the Capitanian and continued through the Lopingian until the Permo-Triassic transition (Slater et al., 2014). During the late Palaeozoic and early Mesozoic, Antarctica occupied a central position within Gondwana and played a key role in floristic interchange between the various peripheral regions of the supercontinent.

permian

Singhisporites hystrix, a megaspore with ornamented surface.

The fossil micro-organism assemblage includes a broad range of fungal hyphae and reproductive structures. The macrofloral diversity in the silicified peats is relatively low and dominated by the constituent dispersed organs of arborescent glossopterid and cordaitalean gymnosperms.  The fossil palynological assemblage includes a broad range of dispersed bisaccate, monosaccate, monosulcate and polyplicate pollen. The roots (Vertebraria), stems (Australoxylon) and leaves (Glossopteris) of the arborescent glossopterid exhibited feeding traces caused by arthropods, but the identification is  difficult since plant and arthropod cuticles look similar in thin section. Tetrapods are currently unknown from Permian strata of the Prince Charles Mountains as either body fossils or ichnofossils (McLoughlin et al., 1997, Slater et al., 2014).

Times of exceptional fossil preservation are coincident with mass extinctions, oceanic anoxic events, carbon isotope anomalies, spikes of high atmospheric CO2, and transient warm-wet paleoclimates in arid lands (Retallack 2011). The current greenhouse crisis delivers several factors that can promote exceptional fossil preservation, such as eutrophic and septic anoxia, microbial sealing, and permineralization.

References:

Benton, M.J., Newell, A.J., (2013), Impacts of global warming on Permo-Triassic terrestrial ecosystems. Gondwana Research.

Rees, P.M., (2002). Land plant diversity and the end-Permian mass extinction. Geology 30, 827–830.

Retallack, G., (2011), Exceptional fossil preservation during CO2 greenhouse crises?, Palaeogeography, Palaeoclimatology, Palaeoecology 307: 59–74.

Slater, B.J., et al., (2014), A high-latitude Gondwanan lagerstätte: The Permian permineralised peat biota of the Prince Charles Mountains, Antarctica, Gondwana Research. http://dx.doi.org/10.1016/j.gr.2014.01.004

Seilacher, A., (1970) “Begriff und Bedeutung der Fossil-Lagerstätten: Neues Jahrbuch fur Geologie und Paläontologie“. Monatshefte (in German) 1970: 34–39.

Halloween special: Lovecraft and the Mountains of Madness.

Lovecraft,_Mountains_of_Madness

H. P. Lovecraft (1890-1937) was one of the most influential writers of the 20th century.  Despite leaving school without graduating, in his writings, evidences an extensive knowledge of archaeology, geology, and paleontology. According to his biographer S. T. Joshi, Lovecraft was fascinated by Antarctica since an early age. Much of this fascination is recognizable in his famous novel “At the Mountains of Madness”, written in 1931. The novel was rejected by Weird Tales and finally was published by Astounding Stories in a serial form in 1936.

At the Mountains of Madness” is told from the perspective of William Dyer, a geologist from Miskatonic University who  flies into an unexplored region of Antarctica. He’s accompanied by Professor Lake, a biologist;  Professor Pabodie, an engineer;  and some graduate students.The basic plot of the novel is the discovery of the frozen remains of bizarre entities from the deep space and their even more terrifying “slaves”:  the  shoggoths. The story could be divided in two parts. The first one is particularly rich, detailed and shows an impressive scientific erudition.

Richard E. Byrd (1888-1957). From Wikimedia Commons

Richard E. Byrd (1888-1957). From Wikimedia Commons

Lovecraft was influenced by Robert Byrd’s first  flight over the South Pole in 1928. He wrote: “On January 6, 1931, Lake, Pabodie, Danforth, all six of the students, four mechanics, and I flew directly over the south pole in two of the great planes, being forced down once by a sudden high wind which fortunately did not develop into a typical storm. This was, as the papers have stated, one of several observation flights; during others of which we tried to discern new topographical features in areas unreached by previous explorers.” 

He also was influenced by the paintings of the Himalayas by Nicholas Roerich –mentioned a total of six times in the novel-,  by the theory of continental drift by A. Wegener and by the paleontological advances of his time, like the discovery of archaeocyathids  found in rocks dated to the Cambrian Period in 1920. This is clear in the following paragraph when he describes something that Professor Lake found : “He  was strangely convinced that the marking was the print of some bulky, unknown, and radically unclassifiable organism of considerably advanced evolution, notwithstanding that the rock which bore it was of so vastly ancient a date—Cambrian if not actually pre-Cambrian—as to preclude the probable existence not only of all highly evolved life, but of any life at all above the unicellular or at most the trilobite stage. These fragments, with their odd marking, must have been 500 million to a thousand million years old”

E. Haeckel's Kunstformen der Natur (1904), plate 90: Cystoidea. From Wikimedia Commons

E. Haeckel’s Kunstformen der Natur (1904), plate 90: Cystoidea. From Wikimedia Commons

Of course, one of the most fascinating parts of the novel is the description of the Elder Things: “Cannot yet assign positively to animal or vegetable kingdom, but odds now favour animal. Probably represents incredibly advanced evolution of radiata without loss of certain primitive features. Echinoderm resemblances unmistakable despite local contradictory evidences. Wing structure puzzles in view of probable marine habitat, but may have use in water navigation. Symmetry is curiously vegetable-like, suggesting vegetable’s essentially up-and-down structure rather than animal’s fore-and-aft structure. Fabulously early date of evolution, preceding even simplest Archaean protozoa hitherto known, baffles all conjecture as to origin.”

According with  S.T. Joshi, Lovecraft based his description of the Elder Thing in the fossil crinoids drawn by E. Haeckel in  Kunstformen der Natur.

Since Lovecraft wrote his novel, several missions to Antarctica has improved our knowledge of the white continent. For instance,  we know now that the interior of East Antarctica – like East Africa- is a mosaic of Precambrian provinces affected by rifting processes. While drilling into Lakes Vostok, Ellsworth, and Whillans amplified our understanding about the ability of organisms to survive in places with minimal nutrients and without sunlight as an energy source.

References:

Lovecraft, H. P, “At the Mountains of Madness”, Random House, 2005.

Joshi, S. T. (2001). A dreamer and a visionary: H.P. Lovecraft in his time. Liverpool University Press, 302.