Sunday, April 12, 2015

One big Triassic land predator: Erythrosuchus africanus



Reconstruction of Erythrosuchus africanus. CZJ posing as scale.
When it comes to ferocious prehistoric land carnivores, there is one group that never did get much attention outside paleontological circles and it is likely that you never heard of them: the erythrosuchids. This is a pity because, as far as we know, Erythrosuchus africanus was the largest land predator in South Africa during the Middle Triassic period. At an estimated total body length of 5 meters, it possessed a massive one meter long skull  equipped with numerous sharp conical teeth, superficially resembling that of big meat eating dinosaurs such as Tyrannosaurus rex. The head was thus disproportionately large with respect to the rest of the body. Unlike T-rex however,  Erythrosuchus was quadrupedal, stalking preys on its rather short limbs. The limbs had a semi-erect posture, making it a more efficient runner than most of its contemporaries. Early representations of Erythrosuchus show it with a deep tall snout but modern reconstructions based on the discovery of a complete skull, indicate that it had a more tapered face similar to other erythrosuchids.

The creature has been first described by Scottish paleontologist Robert Broom in 1905 from fragmentary and poorly preserved remains (on a side note, Broom became famous for his later discoveries of early hominid fossils including Paranthropus robustus). Erythrosuchus (the name means “Red crocodile”)  is today known from a number of specimens all relatively incomplete but enough to provide a good picture on what the animal may have looked like in real life. All the fossils were unearthed from the upper layers of the so called “Cynognathus assemblage zone”, Upper Beaufort Series, of the Karoo basin of South Africa and date from the Early Middle (Anisian) Triassic period  (~245 MYA). Erythrosuchus was therefore contemporary with the cynodonts Cynognathus, Diademodon and Trirachodon and the large plant-eating dicynodont Kannemeyeria, the latter representing its main meal in all probabilities. Other representatives of the Cynognathus assemblage zone fauna are the small theriodont Bauria, the agile archosauriforme Euparkeria, the early rhynchosaurs Howesia and Mesosuchus and the big capitosauroid amphibians such as Watsonisuchus.

The erythrosuchids represent the first radiation of large terrestrial carnivores within the Archosauriformes. They were not quite archosaurs (the higher clade that includes crocs, dinosaurs and birds) but very close relatives. Erythrosuchids appeared in the late Early Triassic period and survived to the end of the Middle Triassic period and had likely a worldwide distribution. Erythrosuchus was the largest among them. The most primitive erythrosuchid is the medium-sized long snouted Fugusuchus hejiapensis from the Heshankou formation (Olenekian) of the Shanxi province of China. Others representatives include Garjainia prima from the Yarenskian horizon (Olenekian) of the Orenburg region of Russia, Vjushkovia triplicostata and Uralosaurus magnus both from the Donguz Formation (Anisian) of the Orenburg region of Russia,   Shansisuchus shansisuchus and Guchengosuchus shiguaiensis, both from the upper Ermaying Formation (Anisian) of the Shanxi Province of China. Chalishevia cotburnata from the Bukobay Formation (Ladinian) of the Orenburg region of Russia, is the last known member of the group. It is conjectured that the erythrosuchids went extinct because they were outcompeted in their ecological niche by more efficient archosaurian predators such as the Rauisuchians.


References:


Cruickshank, A. R. I. (1978). The pes of Erythrosuchus africanus Broom. Zoological Journal of the Linnean Society, 62(2), 161-177.


Gower, D. J. (1997). The braincase of the early archosaurian reptile Erythrosuchus africanus. Journal of Zoology, 242(3), 557-576.


Gower, D. J. (2001). Possible postcranial pneumaticity in the last common ancestor of birds and crocodilians: evidence from Erythrosuchus and other Mesozoic archosaurs. Naturwissenschaften, 88(3), 119-122.

Parrish, J. M. (1992). Phylogeny of the Erythrosuchidae (Reptilia: Archosauriformes). Journal of Vertebrate Paleontology, 12(1), 93-102.

Wednesday, April 1, 2015

Were dinosaurs responsible for their own demise by initiating a runaway climate change?



Many scenarios have been put forward to explain the sudden disappearance in the fossil record of all non-avian dinosaurs, along with many other group of animals such as plesiosaurs, mosasaurs, ammonites and pterosaurs, at the very end of the Cretaceous period, 65 million years ago. Explanations such as starvation due to the rise of flowering plants that their stomach could not process, epidemic diseases of planetary proportion or loss of genetic diversity have long been discarded by scientific evidences. Today, most scientists adhere to the idea of a catastrophic event that caused global devastation of such magnitude many group of animals were unable to cope with. A large crater off the Yucatan peninsula (the Chicxulub crater) is believed to be the site of a massive asteroid/comet impact that wiped out the dinosaurs and 75% of life on Earth (Hildebrand et al., 1991). More recently, the Deccan traps theory, volcanic activity of unprecedented scale on the Indian subcontinent, gained ground when precise dating of the event shows it happened just before the so called K-Pg (Cretaceous-Paleogene) boundary (Schoene et al., 2015). A more exotic theory proposed dark matter in our galaxy as the prime culprit for the deed (Rampino, 2015).

However, in a new study that has just been published in the Journal of Supernatural Geological Studies, Dr. A. Zierste and co-workers came up with another plausible explanation (Zierste et al., 2015). It is well known that the Earth went through an episode of extreme “global warming” with high level of greenhouse gases during the Paleocene and Eocene periods of the Tertiary that followed the Cretaceous. The famous “Messel pit” in Germany with its exceptionally well preserved fossils of a rich tropical and subtropical fauna and flora is a vivid testimony of what the climate was at this high latitude. Zierste wondered about the mechanism that caused this greenhouse effect. Using a Tunable laser Spectrometer (TLS) similar to the one employed by the Mars Curiosity Mission (Webster et al., 2015), her team measured with high precision the isotopic ratio of light elements such as carbon and oxygen contained in sedimentary rocks from the end of the Cretaceous period to the beginning of the Eocene period. The data indicate a gradual increase in methane and carbon dioxide levels in the atmosphere that peaked right at the K-Pg boundary. But what is the origin of the high level of greenhouse gases? Zierste has a surprising answer to it: dinosaurs! These beasts were reaching gigantic proportions by the end of the Jurassic. The long-necked sauropods in particular were truly titanic, with some individuals attaining an estimated weight of 100 tons. To sustain their body mass, they have to eat tremendous amount of plants. Their stomachs were formidable fermentation chambers, producing colossal amount of methane gas that were naturally released in the Mesozoic atmosphere through gastric emanations. Methane is actually 20 times better at trapping heat than carbon dioxide. A 2006 United Nations' Food and Agricultural Organization report (http://www.fao.org/ag/magazine/0612sp1.htm ) has shown that the some 1.5 billion cows bred for milk and meat by the human population on Earth today have a non-negligible contribution to the current climate change, generating 18% more greenhouse gas than transport, in CO2 equivalent unit. Now imagine the amount of methane gas that dinosaurs could have produced. Bone histology studies have shown that the biggest dinosaurs lived to very old age, maybe 100 years and their population were most probably quite large, as they were social animals living in herds. It is difficult to estimate the total population of dinosaurs that lived on Earth at any moment of time during the Mesozoic but Zierste and her colleagues using a conservative number of about 1 billion sauropods and hadrosaurs calculated that they would have generated a staggering 20 billion metric tons of methane per year, enough to provoke a fast climate change and rapid rise of the Earth atmospheric temperature. In other words, dinosaurs perished because they induced a runaway global warming triggered by their own flatulence. Without the asteroid impact, that put an end to the continuous rise in temperature by provoking a global cooling, it is likely that the remaining 25% species of animals and plants would also have vanished and we would not be around to tell the tale.


References:

Avril Z., Peter S. G., Robert T. B., Ines L. P., Lambert J. B., Stewart M.T., Fernando E. N., Oviedo T. F., Oscar J., Lewis C. C. (2015)  Geochronological correlation of NH4 level with global atmospheric temperature at the Cretaceous/Paleogene boundary. J. of Supernatural Geological Studies, 90., 1-12.

Hildebrand, A. R., Penfield, G. T., Kring, D. A., Pilkington, M., Camargo, A., Jacobsen, S. B., & Boynton, W. V. (1991). Chicxulub crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology, 19(9), 867-871.

Rampino, M. R. (2015). Disc dark matter in the Galaxy and potential cycles of extraterrestrial impacts, mass extinctions and geological events. Monthly Notices of the Royal Astronomical Society, 448(2), 1816-1820.

Schoene, B., Samperton, K. M., Eddy, M. P., Keller, G., Adatte, T., Bowring, S. A., ... & Gertsch, B. (2015). U-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction. Science, 347(6218), 182-184.

Webster, C. R., Mahaffy, P. R., Atreya, S. K., Flesch, G. J., Mischna, M. A., Meslin, P. Y., ... & Lemmon, M. T. (2015). Mars methane detection and variability at Gale crater. Science, 347(6220), 415-417.

Sunday, February 8, 2015

Dickinsonia from the Ediacara biota


Reconstruction of Dickinsonia costata
In 1946, while investigating abandoned mines in the Ediacara Hills, South Australia, geologist Reginald Claude Sprigg stumbled upon some peculiar and barely visible trace fossils of jellyfish-like creatures. He realized that they must be far older than any fossil known at the time. He published his preliminary results in the Transactions of the Royal Society of South Australia (1947, 1948). He then tried to publish his findings in the prestigious journal Nature and made a presentation at the 1948 International Geological Congress, but his results were largely dismissed by the larger scientific community. Sprigg will have to wait until 1959 for his contribution to be recognized when paleontologist Martin Glaessner evaluated the age of the Ediacaran rocks to be Precambrian  (~600 MYA) and fully appreciated the importance of the fossils found within. The Ediacara biota consists of a number of truly enigmatic trace fossils of soft bodied creatures that give us a glimpse of what life forms existed before the evolutionary “invention” of readily fossilizable parts such as hard shells.
Spriggina, another enigmatic creature from the Ediacara biota

One of the most famous fossils of the Ediacara biota is Dickinsonia,  first described by Sprigg in 1947. Dickinsonia fossils were preserved as imprints of ovoid or ribbon- like creatures with bilateral symmetry. Their sizes range from a few millimeters to practically a meter in length. The animal also appeared to be segmented. The affinities of Dickinsonia were and is, still today, highly debated. Is it related to jellyfish (Sprigg, 1947) ,  corals (Valentine, 1992), comb jellies (Zhang & Reitner, 2006)? Is it a lichen (Retallak, 1994, Retallak, 2007), a sponge, a polychaete worm (Wade, 1972), a giant single cell organism (Seilacher et al., 2003) or something else altogether? In the absence of any discernable proof of affiliation to any modern phylum, a  brand new name was erected to encompass these strange precambrian fossils, the Vendobionta (Seilacher,1992; Buss & Seilacher, 1994),  that would have evolved before the reign of the eumetazoans (all bilateral animals plus the cnidaria [jellyfish, coral, sea anemones, …] and ctenophores [comb jellies]).  The phylum Vendobionta regroups many of the familiar Ediacaran fossils such as Dickinsonia, Spriggina, and Charnia, and they are viewed as immobile creatures, possibly ancestral to the Cnidaria. Some authors have argued that Dickinsonia like other Ediacaran fauna (such as the above mentioned Spriggina) lacked true bilateral symmetry and placed them in a phylum called Proarticulata (Fedonkin, 1987; Fedonkin, 2003) that went totally extinct at the end of The Precambrian and left no descendant. Specimens of Dickinsonia indeed show that the segments do not join symmetrically at the median ridge but are made of portions called isomers that alternate along the longitudinal axis, as if one side of the body is shifted by half a segment period with respect to the other half. 

Charnia looked like modern sea pens but they were
probably unrelated.

However extensive examination of hundreds of specimens show that the lack of bilateral symmetry in those individuals is only apparent and is, like the presence of the middle line rim, a preservation artifact (Gehling et al., 2005). Dickinsonia was thus a true bilateral segmented creature. Specimens showing folds and tears also indicate that it was a soft-bodied flexible organism, contradicting earlier claims that only rigid "wood-like" bodies could have been preserved in sandstone (Retallak, 1994). The preservation of series of oval traces “following” the body mold of some Dickinsonia specimens, that are about the same size and shape as the animal itself, also indicates that it was clearly mobile, so the hypothesis of its affinity to a phylum such as sponges or lichens with a sessile life can be rejected. These traces have been interpreted as resting or feeding traces, imprints of shallow depressions left by Dickinsonia as it fed and digest the layer of organic matter underneath. The total absence of anything that look like a digestive system such as a mouth (Gehling et al., 2005) indicates that the animal was most probably feeding through ventral external digestion. And there is actually only one phylum of animals alive today that eats in this way, the Placozoans. So, is Dickinsonia a placozoan (Sperling & Vinther, 2010)? The difficulty of this hypothesis is that modern placozoans,such as Trichoplax adhaerens, appear to be much simpler creatures. Although multicellular, they do look like amoebas, with no apparent bilateral symmetry, no segments and reproduced asexually by budding. However, the genome sequencing of Trichoplax (Srivastava et al., 2008) showed that they belong to the eumetazoans. Cnidarians (jellyfish, sea anemones, corals and the like) do possess a radial symmetry rather than a bilateral symmetry. However molecular and deep morphological clues reveal that cnidarians were originally bilateral (Matus et al., 2006). Thus bilaterality was probably an ancestral feature to all eumetazoans including placozoans. So is Dickinsonia a placozoan? Well, we still can’t say for sure and the debate is far from closed, but that's one definite intriguing possibility.

Acknowledgment: I am indebted to Prof. James G. Gehling who has kindly provided me with first-hand information about Dickinsonia.

References:

Buss, L. W., & Seilacher, A. (1994). The Phylum Vendobionta: a sister group of the Eumetazoa?. Paleobiology, 1-4.

Fedonkin, M. A. (1987). Non-skeletal fauna of the Vendian and its place in the evolution of metazoans. Trans. Paleontol. Inst., 226. Nauka, Moscow 175 pp. (in Russian).

Fedonkin, M. A. (2003). The origin of the Metazoa in the light of the Proterozoic fossil record. Paleontological Research, 7(1), 9-41.

Gehling, J. G., Droser, M. L., Jensen, S. R., & Runnegar, B. N. (2005). Ediacara organisms: relating form to function. Evolving form and function: fossils and development, 43-66.

Matus, D. Q., Pang, K., Marlow, H., Dunn, C.W., Thomsen, G. H., and Martindale, M. Q. (2006). Molecular evidence for deep evolutionary roots of bilaterality in animal development. Proc. Natl. Acad. Sci., USA 103: 1195–1120.

Retallack, G. J. (1994). Were the Ediacaran fossils lichens?. Paleobiology, 523-544.

Retallack, G. J. (2007). Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil. Alcheringa, 31(3), 215-240.

Seilacher, A. (1992). Vendobionta and Psammocorallia: lost constructions of Precambrian evolution. Journal of the Geological Society, 149(4), 607-613.

Seilacher, A., Grazhdankin, D., and Legouta, A. (2003). Ediacaran biota: the dawn of animal life in the shadow of giant protists. Paleontol. Res. 7: 43– 54.

Sperling, E. A., & Vinther, J. (2010). A placozoan affinity for Dickinsonia and the evolution of late Proterozoic metazoan feeding modes. Evolution & development, 12(2), 201-209.

Srivastava, M., Begovic, E., Chapman, J., Putnam, N. H., Hellsten, U., Kawashima, T., ... & Rokhsar, D. S. (2008). The Trichoplax genome and the nature of placozoans. Nature, 454(7207), 955-960.

Valentine, J. W. (1992). Dickinsonia as a polypoid organism. Paleobiology, 378-382.

Wade, M. (1972). Dickinsonia: polychaete worms from the late Precambrian Ediacara fauna, South Australia. Memoirs of the Queensland Museum, 16(2), 171-190.

Zhang, X., & Reitner, J. (2006). A fresh look at Dickinsonia: removing it from Vendobionta. Acta Geologica Sinica (English Edition), 80(5), 636-642.

Saturday, January 31, 2015

First among the synapsids: the ophiacodonts



The synapsids constitute the second major group of reptiles after the diapsids. They are characterized by a single hole in the skull just behind each eye socket called temporal fenestrae. Synapsids include the mammals (thus including us), their reptilian ancestors (“mammal-like reptiles”) and  all their closest extinct relatives . The oldest known synapsid belongs to a group called Ophiacodontids which flourished during the Late Carboniferous and Early Permian periods. These were a rather peculiar lot with their unusually large head compared to their body size. The skull was very tall and narrow. The slightly recurved teeth were sharp, numerous and tightly packed inside the jaws, two being a bit larger than the other forming  the upper canines. These medium size animals were among the largest land carnivores of their time. The limbs were rather short, broad and bulky. 
Archaeothyris florensis


The ancestral ophiacodontid type can be seen in Archaeothyris florensis (Reisz, 1972) from the Late Carboniferous of Nova Scotia, Canada (Westphalian C; 308-311 MYA), which is by the way, the earliest known definite synapsid. It somewhat looked like a modern lizard but with already the group hallmark of a tall and elongated skull. Measuring about 50 cm, it was contemporary to other early reptiles such Palaeothyris and living in a swampy forest made of giant tree-like lycopsids (club mosses) and dominated by amphibians and giant arthropods.

Varanosaurus acutirostris (Broili, 1904) from the Early Permian of Texas, measured about 1.2 m in length and was an agile predator with a slender laterally compressed snout. It is known from decent material including the almost complete holotype skeleton. A second species, V. wichitaensis, has been described, based on isolated postcranial remains (Romer, 1937) and is virtually indistinguishable from V. acutirostris besides being smaller and geologically slightly older.
Varanosaurus acutirostris


Body proportions reached its extremes in Ophiacodon (Marsh, 1878; Cope, 1878; Case, 1907; Romer, 1925; Romer & Price, 1940) the best known and most studied ophiacodont. Many skeletons of this odd-looking animal are known from the Early Permian of Texas, New Mexico, Kansas, Oklahoma, Colorado, Utah and Ohio and it is unclear how many of the described species are actually valid. For the story, the type species, Ophiacodon mirus was first described by Othniel Marsh in 1878 based on a mandible and vertebrae in the midst of the so-called  “bone war” against his rival Edwin Cope. Marsh clearly intended to beat Cope, who had a paper in press, in the naming of this animal but his hastily written paper was so deficient that the name Ophiacodon was ignored by the paleontological community for over 30 years. In the meantime, Cope’s paper published just days after Marsh’s, described three species, Theropleura retroversa, T. uniformis and T. triangulata, based on isolated vertebrae. The type specimen of Ophiacodon was reinvestigated by Williston and a new complete skeleton described in the 1910s (Williston & Case, 1913)  and it is much later that Theropleura was found to be synonymous with Ophiacodon although the species names were retained (Romer & Price, 1940).  The following species are still recognized today: O. mirus, known from several skeletons including a nearly complete one from New Mexico and Oklahoma; O. retroversus, known from multiple materials from Texas and Oklahoma, including a near complete skelton; O. uniformis from several partial skeletons from Texas and Oklahoma; O. navajonicus frong fragmentary postcranial skeletons from New Mexico; O. hilli  known from a fragmentary skeleton from Kansas; O. major from fragmentary materials from Texas.   It appears that size difference might reflect different growth stage rather than species (Brinkman, 1988). Ophiacodon was originally thought to be a semi-aquatic animal but recent studies debunked all the supposed aquatic adaptation that the animal might have possessed and today (Felice & Angielczyk, 2014), Ophiacodon is viewed as a fully terrestrial predator. Specimens of Ophiacodon have sizes ranging from 1.5 m up to 3 m in length.
Ophiacodon mirus


Other ophiacodontids are known from rather fragmentary remains and include the Late Carboniferous genera Clepsydrops and Echinerpeton from North America and Stereorhachis from France, the Early Permian Stereophallodon, Baldwinonus from North America. Protoclepsydrops haplous (Carrol, 1964) from Nova Scotia might also have been an ophiacodontid predating Archaeothyris but the remains are so fragmentary that it is difficult to tell for sure.

Refrerences:

Brinkman, D. (1988). Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology, 8(2), 172-180.

F. Broili. 1904. Permische Stegocephalen und Reptilien aus Texas. Palaeontographica 51:1-120

Carroll, Robert L. (1964). "The earliest reptiles". Journal of the Linnean Society of London, Zoology 45 (304): 61–83

E. C. Case. 1907. Revision of the Pelycosauria of North America. Carnegie Institution of Washington 55:3-176

E. D. Cope. 1878. Descriptions of extinct Batrachia and Reptilia from the Permian formations of Texas. Proceedings of the American Philosophical Society 17:505-530

Felice, R. N., & Angielczyk, K. D. (2014). Was Ophiacodon (Synapsida, Eupelycosauria) a swimmer? A test using vertebral dimensions. In Early evolutionary history of the Synapsida (pp. 25-51). Springer Netherlands.

R. R. Reisz. 1972. Pelycosaurian Reptiles from the Middle Pennsylvanian of North America. Bulletin of the Museum of Comparative Zoology 144(2):27-60

A. S. Romer. 1925. An ophiacodont reptile from the Permian of Kansas. Journal of Geology 33(2):173-182

A. S. Romer. 1937. New genera and species of pelycosaurian reptiles. Proceedings of the New England Zo├Âlogical Club 16:89-95

A. S. Romer and L. I. Price. 1940. Review of the Pelycosauria. Geological Society of America Special Paper 28:1-538

Williston, S. W., & Case, E. C. (1913). Description of a nearly complete skeleton of Ophiacodon Marsh. Carnegie Institution of Washington Publication, 181, 37-59.