The impact winter model

The Chicxulub asteroid impact (Image credit: NASA)

The Chicxulub asteroid impact was the final straw that pushed Earth past the tipping point.  The K-Pg extinction that followed the impact was one of the five great Phanerozoic  mass extinctions. The impact released an estimated energy equivalent of 100 teratons of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. The dramatic decrease of sunlight that reached the Earth surface caused a drastic short-term global reduction in temperature. This phenomenon is called “impact winter”.

Cold and darkness lasted for a period of months to years.  Photosynthesis stopped and the food chain collapsed. Three-quarters of the plant and animal species on Earth disappeared. Marine ecosystems lost about half of their species, and freshwater environments shows low extinction rates, about 10% to 22% of genera.

This difference in extinction rates between marine and freshwater ecosystems could be explained by differential effects of an impact winter on marine and freshwater biota and the difference in the use for detrital foods.

Arkhangelskiella cymbiformis and Globotruncana linneiana (d’Orbigny, 1839).

Three factors can be associated with the impact winter in marine and fresh water enviroments. First, starvation caused by the stop of photosynthesis. Second, the loss of dissolved oxygen. Third, the low temperatures. The flux of organic detritus to the sea floor also declined abruptly and remained low for about 3 Myr after the impact.

Marine extinction rates were greater among pelagic than benthic organisms. Calcareous nanoplankton (primarily the coccolithophores) and planktonic foraminifera had the highest extinction rates among the marine plankton, possibly because they commonly lack cysts or resting stages.  About 70% of planktonic foraminifera became extinct at the K/Pg boundary. The food webs supported by plankton were severely affected. For instance, ammonites were plankton feeder and they, like mosasaurs, plesiosaurs and pliosaurs that fed on them, became extinct at the  K-Pg boundary.

Mosasaurus hoffmani, Late cretaceous of Europe by Nobu Tamura. From Wikimedia Commons.

In case of freshwater communities, they were adapted to rapidly changing environments. For instance, starvation can be offset by dormancy, which is much more common among freshwater than marine organisms. Dormancy could have lowered extinction rates in inland waters compared to marine waters.

Because the late Cretaceous climate was warm, a major challenge for aquatic organisms, especially in inland waters, may have been the persistence of low temperatures. Inland waters had an advantage for preventing extinction caused by prolonged cold:  the presence of thermal refugia in the drainage networks of inland waters derived not only from  geothermal waters, but also from groundwater.

The abundance of refugia combined with greater adaptation to stressful conditions may explain lower extinction frequencies in freshwaters than in marine waters. Even when mortality within all taxa may have been equal to or even greater in freshwaters than in marine waters, the factors that protect some survivors against extinction  are more evident in the freshwater environment.

Jeletzkytes spedeni, a fossil ammonite from USA. From Wikimedia Commons.

It seems clear that the terrestrial and marine extinction were separated in time by a matter of months to years.  These extinctions had two different mechanisms: an impact winter in the marine environment and a heat pulse and subsequent fires in the terrestrial environment, although an impact winter would also affect the terrestrial environment. A more comprehensive analysis also shows three separate spatial domains (terrestrial, marine, and freshwater), which provide us a more understandable picture of the K-Pg extinction.

References:

Douglas S. Robertson, William M. Lewis, Peter M. Sheehan and Owen B. Toon, K-Pg extinction patterns in marine and freshwater environments: The impact winter model, Journal of Geophysical Research: Biogeosciences, JUL 2013, DOI: 10.1002/jgrg.20086.

Schulte, P., et al. (2010), The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary, Science, 327, 1214–1218, doi:10.1126/science.1177265

The Dinosaurs of the Victorian Era.

Benjamin Waterhouse Hawkins unveiled the first ever sculptures of Iguanodons.

Since the End of the 17th century to the beginning of the 19th, several discoveries of dinosaur remains were reported for first time, mostly from England, France and North America. In 1677, Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum published his ‘Natural History of Oxfordshire’ where described and illustrated a distal fragment of a large femur that he interpreted as the remains of a giant man, like those mentioned in the Bible, or of some other animal. In 1677, Richard Brookes, described it again, and named “Scrotum humanum“, but the label was not considered a proper Linnaean name and was not used in posterior literature. Now is considered a nomen oblitum (forgotten name). When in 1818 George Cuvier went to England, William Buckland, Professor of Geology at the University of Oxford and dean of Christ Church, showed him the vertebrae of a large fossil animal collected at the Stonesfield quarry. Cuvier wrote: “Professor Buckland had made this great discovery years before, and I saw its pieces at his house in Oxford in 1818. I even drew some of them.”

Hawkins reconstruccion of Hylaeosaurus. Crystal Palace, 1853

Later, in 1824, William Buckland published the first report of a large carnivore animal: the Megalosaurus (the large femur found by Plot, belongs to this animal). He had a piece of a lower jaw, some vertebrae, and fragments of pelvis, scapula and hind limbs, probably not all from the same individual and estimated that the animal had 12 m (almost 40 feet) long. Megalosaurus became so popular that is mentioned in Charles Dickens’ novel Bleak House: “As much mud in the streets as if the waters had but newly retired from the face of the earth, and it would not be wonderful to meet a Megalosaurus, forty feet long or so, waddling like an elephantine lizard up Holborn Hill.” It was the first appearance of a dinosaur in popular literature.

Iguanodon battling Megalosaurus by Édouard Riou. From Wikimedia Commons

One year after the discovery of the Megalosaurus, the Iguanodon entered in the books of History. Gideon Mantell proposed the name for this fossil found in Tilgate Forest, Sussex. He could describe a relative complete skeleton by buying a large block extracted from a quarry in Maidstone, but Mantell represented the Iguanodon with a horn on its nose, that now we know is actually one of the Iguanodon’s thumbs. Later, in 1833, Mantell described the Hylaeosaurus. After those pioneering work Richard Owen, one of the best anatomist of his age and the first director of the British Museum, coined the term Dinosauria. It was in a paper published in 1841 for the Eleventh Meeting of the British Association for the Advancement of Science.

Hawkins’ Sydenham Studio. From Wikimedia Commons.

Owen used his influence with Prince Albert, Queen Victoria’s husband, to propose the financing of the three-dimensional reconstruction of the first known dinosaurs: Megalosaurus, Iguanodon and Hylaeosaurus, for the closure of the first international exposition in modern European history: the Crystal Palace exhibition, that would be placed in Sydenham Park, south of London, instead of the original site in London’s Hyde Park. Benjamin Waterhouse Hawkins, sculptor and natural history artist, was commissioned to make the full-size replicas of the dinosaurs and other extinct reptiles for Sydenham Park. Owen conceived the dinosaurs as a special group of fossil reptiles with some advanced characteristic that in some ways were similar to those of mammals, and that conception is clearly exposed on the reconstructions of Megalosaurus and Iguanodon as large quadrupeds resembling a rhinoceros. Owen also supposed that dinosaurs had four-chambered hearts and warm blood like mammals.

Woodcut of the famous banquet of New Year’s Eve. From Wikipedia Commons.

On New Year’s Eve of 1853 a banquet was held inside the reconstruction of the Iguanodon, which had not yet been completed, and under the portraits of Cuvier, Buckland, Mantell and Owen the twenty privileged guests to this unusual inauguration proposed a toast to the glory of the dinosaurs and Queen Victoria.

On 30 November 1936 the Crystal Palace burned to the ground but Hawkin’s models can still be seen today in Crystal Palace park.

References:

Jose Luis Sanz, Starring T. rex!: Dinosaur Mythology and Popular Culture, Indiana University Press, 2002.

Philippe Taquet, Dinosaur Impressions: Postcards from a Paleontologist, Cambridge University Press, 1999.

Brief introduction to Calcareous nannoplankton

Coccolithus pelagicus (Wallich, 1871) Schiller, 1930
Lower Palaeocene-Recent. From UCL

Calcareous nannoplankton represent a major component of oceanic phytoplankton, ranging in size  from 0.25 to 30 μm. The first records are from the Late Triassic. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process.  This heterogeneous group includes coccoliths, discoasters and nannoconids.

Coccolithophores are unicellular marine golden-brown algae differing from other Chrysophyta in having two flagella and a third flagella-like appendage called a haptonema. They also posses calcified scales, called coccoliths, at some stage in their life as a protective armour that eventually falls to the ocean floor to build deep-sea ooze and fossil chalks.

Micrantholithus obtusus Stradner, 1963. Berriasian-Upper Aptian. From UCL

The discoasters are an extinct group of stellate calcareous nannofossils and the nannoconids are cone-shaped microfossils very useful in Cretaceous biostratigraphy in the absence of other groups.

Typically coccolithophores are autotrophic but they can be heterotrophs under certain environmental conditions. They are restricted to the photic zone of the water column (0–200 m depth). The algal cell is generally spherical and includes  two golden-brown pigment, a nucleus, two flagella of equal length and a haptonema, mitochondria, vacuoles and the Golgi body which is the site of coccolith secretion in many species.

Diagram of a living coccolithophore cell

In some living genera there is also an alternation between a motile and a non-motile stage. The first one has a flexible skeleton with coccoliths embedded in a pliable cell membrane and in the non-motile stage, the calcification of the membrane forms a rigid shell called a coccosphere.

Coccoliths are composed of calcium carbonate in the form of calcite with a low amount of  magnesium, although it has been some of vaterite or aragonite. It is thought they are formed for  protection from intense sunlight, to concentrate light, buoyancy control, or for the biochemical efficiency of the cell.

Syracolithus (modern holococcolith) and Prediscophaera (Cretaceous heterococcolith).

The coccolith morphology is the basis for classification of both living and fossil members of the group. They can be divided in two basic morphological types: heterococcoliths and holococcoliths. While the holococcoliths are usually formed by rhombohedral calcite and always disintegrate after they are shed, the heterococcoliths provide the bulk of the microfossil record. They are built of different submicroscopic elements such as plates, rods and grains imbricated into a relatively rigid structure.

In some cases, some living coccolithophores, like Scyphosphaera produce two layers of morphologically distinct coccoliths (dithecism).

Scyphosphaera apsteinii. Credits image: Ian Probert, Markus Geisen.

Ehrenberg in 1836, was the first to use the term “coccoliths” while he was studying  the chalk from the island of Rugen in the Baltic Sea, but he thought they had an inorganic origin. G. C. Wallich  in 1860, was the first to suggest  the organic origin of coccoliths. Later, in 1872, the HMS Challenger expedition recovered coccospheres from the upper water layers and correctly concluded that they were the skeletons of calcareous algae.

Coccolithophores has a great radiation in the Early Jurassic, an event that parallels the radiation of the peridinialean dinoflagellate cysts and it’s related to the opening of the Atlantic Ocean. During the Late Cretaceous, there was a second radiation that led to the deposition of chalk in several areas of continental plataform,  but were very affected by the extinction event at the end of the Cretaceous. Since then, Coccolithophores have regained their dominance in tropical and temperate waters but are significantly less diverse than in the Mesozoic.

Coccoliths and discoasters has an extraordinary value as biostratigraphic markers for the Mesozoic and Cenozoic, and are good indicators of surface water chemistry and reflect surface productivity.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Jörg Mutterlose, André Bornemann, Jens O. Herrle, Mesozoic calcareous nannofossils — state of the art, Paläontologische Zeitschrift, March 2005, Volume 79, Issue 1, pp 113-133.

Darwin and the strangest animal, ever discovered.

Portrait of Charles Darwin painted by George Richmond (1840)

When Charles Darwin arrived to South America, he was only 22 years old. He was part of the second survey expedition of HMS Beagle. He was recommended to Captain Robert FitzRoy by John Stevens Henslow, clergyman, botanist, mineralogist, and Darwin’s mentor.
Before this journey, Darwin’s experience with Earth Sciences was limited to one field trip to the North of Wales with famous Adam Sedgwick, one of the founders of modern geology. But Darwin had a special interest in this field of knowledge and shared this interest with Captain Fitz Roy. In fact, his geological findings promoted him for the very first time, to the scientific and public consideration.

HMS Beagle in the seaways of Tierra del Fuego, painting by Conrad Martens during the voyage of the Beagle. From Wikimedia Commons.

During the first two years of the expedition, Darwin collected several fossil mammals from Argentina and Uruguay.  He sent all the specimens, to his mentor John Stevens Henslow. 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 darwini and Glossotherium sp.

Previous to this expedition, the first news of “fossils” in South American were reported by early Spanish explorers. These fossils were interpreted as the remains of an ancestral race of giant humans erased from the face of the Earth by a divine intervention.

George Cuvier, in 1796,  published the first scientific work about a South American fossil: Megatherium americanum, based on the specimen recovered by Fray Manuel Torres from Lujan, Buenos Aires, Argentina.

On September 23, 1832 Darwin recovered his first fossil at Punta Alta (Buenos Aires, Argentina). He continued collecting fossil from Buenos Aires Province until October, when he moved to Uruguay. Later in 1834, he returned to Argentina and collected  his last specimens.

It was in Uruguay where Darwin bought to a local farmer, the skull of a Toxodon used by Owen to establish the genus. He paid 18 pence for it. Darwin described it as “one of the strangest animals, ever discovered…”

Skull of Toxodon platensis in ventral view (from
Owen 1838).

Owen bestowed the name because its upper incisors were strongly arched (Toxodon means “arched tooth). He also recognized Toxodon as “A gigantic extinct mammiferous animal, referable to the Order Pachydermata, but with affinities to the Rodentia, Edentata, and Herbivorous Cetacea”. Toxodon was a puzzle that shared the massive skeleton of a rhino and the teeth had a certain resemblance to those of rodents.

Fossil Toxodon on display at Bernardino Rivadavia Natural Sciences Museum.

Nevertheless, there was no close phylogenetic relationships with the groups mentioned by Owen. Toxodonts were a group of large-sized notoungulates of South American origin, ranging from the late Oligocene to late Pleistocene.

They are now considered among the more derived native notoungulates of South America and share an ancestry with North American condylarths and a recent study, indicates that is quite possible that Toxodonts traveled to North America.

Toxodonts shares a number of dental, auditory and tarsal specializations. They had  short hippopotamus-like head with broad jaws filled with bow shaped teeth and incisors, a massive skeleton with short stout legs with three functional toes. The estimated weight is over a tonne.

About the different groups that appeared to be related to Toxodon, Darwin stated: “How wonderfully are the different orders, at the present time so well separated, blended together in different points of the structure of Toxodon ”

Despite the “erroneous” assignments of  Darwin,  it is quite possible that the observation of these characters supposedly shared significantly influenced his theory on the origin of species.

Front page of Darwin’s Journal of Researches.

By the end of the expedition, Darwin was already earned a name as a geologist and fossil collector. He narrated his experiences in his book “Journal of Researches into the Geology and Natural History of the Various Countries visited by H.M.S. Beagle, under the Command of Captain FitzRoy, R.N. from 1832 to 1836″, published in 1839 and later simply known as “The Voyage of the Beagle”.

When Darwin wrote his memories in 1858, he described the expedition in one strong and powerful sentence: “the voyage of the Beagle has been by far the most important event in my life and has determined my whole career”.

References:

Fariña, Richard A.; Vizcaíno, Sergio F.; De Iuliis, Gerry (2013). Megafauna. Giant Beasts of Pleistocene South America. Indiana University Press.

Lundelius, Jr., E., Bryant, V., Mandel, R., Thies, K., Thoms, A. 2013. The first occurrence of a toxodont (Mammalia, Notoungulata) in the United States. Journal of Vertebrate Paleontology. 33, 1: 229-232

Mysterious fossils: Chitinozoans

Scanning electron micrograph of Sphaerochitina sp.(Late Silurian of Sweden). From Wikimedia Commons

Chitinozoans are marine organic-walled microfossils,  bottle-shaped, consisting of  hollow organic vesicles of uncertain affinity.  The vesicle ranges from 30 to 1500 μm, but most are 150–300 μm long. The outer wall of the vesicle may be smooth, striate, tuberculate, folded into hollow  spines or extended into a tubular sleeve. The group was named by Eisenack in 1931 who was the first to noted that the material of the wall was composed by a pseudochitinous material.

Chitinozoans can be found as single forms or joined together in chains. They first appeared in the Early Ordovician and became an abundant and diverse group, until the Silurian, and they extinct in the Early Carboniferous. They were exclusively marine and lived in a wide range of shelf environments.

Because of their rapid evolution and wide diversity of forms,  are useful  for local and global stratigraphical correlations.

Lagenochitina sp. (right) Conochitina sp. (left). From UCL.

Systematic position of the taxon still remains problematic.  The pseudochitin wall suggests animal affinities, but whether they are metazoan or protistan is still uncertain. In 1963, Kozlowski  proposed they were the eggs of annelid worms. The co-occurrence of chitinozoa with scolecodonts is a strong argument to his proposal.

Prosomatifera, longitudinal section.

The Chitinozoans are classified based on the structure of their opening and the overall shape of flask and neck. The Order Operculatifera contains one family, the Desmochitinidae, characterized by an operculum, reduced oral tubes and a relatively small subspherical vesicle. The Order Prosomatifera contains two families the Conochitinidae and the Lagenochitinidae, distinguished by the relationship between the chamber and the neck.

Armoricochitina nigerica, from late Ordovician Hirnantian glaciation.

Most  of chitinozoan genera appear to be cosmopolitan, others show latitudinal provinciality. A reconstruction of the paleobiogeographic distribution of chitinozoan before and after the Hirnantian glaciation (∼440 Ma) showed a pattern that revealed the position of ancient climate belts. This patterns look “very modern” and suggests that ancient carbon dioxide levels could not have been as high as previously thought, but were more modest, about five times current levels.

 

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Vandenbroucke, T.R.A., Armstrong, H.A., Williams, M., Paris, F., Zalasiewicz, J.A., Sabbe, K., Nolvak, J., Challands, T.J., Verniers, J. & Servais, T. 2010. Polar front shift and atmospheric CO2 during the glacial maximum of the Early Paleozoic Icehouse. PNAS doi/10.1073/pnas.1003220107.

The Challenger expedition and the beginning of Oceanography.

Painting of the HMS Challenger by William Frederick Mitchell. From Wikimedia Commons

On December 21, 1872 the H.M.S. Challenger sailed from Portsmouth, England, for an epic voyage which would last almost three and a half years. It  was the first expedition organized and funded for a specific scientific purpose: to examine the deep-sea floor and answer questions about the ocean environment.

The expedition covered 69,000 miles (about 130.000 km) and gathered data on   currents, water chemistry, temperature, bottom deposits and marine life at 362 oceanographic stations. More than 4700 new species of marine animals were discovered during the course of the voyage, many of which were found on the seafloor – an environment that scientists originally believed to be too inhospitable to support life.

It all began in 1868, with British naturalist William B. Carpenter and Sir Charles Wyville Thomson, Professor of Natural History at Edinburgh University. They persuaded the Royal Society of London to sponsor a prolonged voyage of exploration across the oceans of the globe. But it was not until 1872 that Royal Society of London obtained the use of the HSM Challenger from the Royal Navy. The ship was modified for scientific work with separate laboratories for natural history and chemistry. The cost of expedition was £200,000 – about £10 million in today’s money.

The science and ship crew of the HMS Challenger in 1874.

The expedition was led by Captain George Nares and the scientific work was conducted by Wyville Thomson assisted by Sir John Murray, John Young Buchanan, Henry Nottidge Moseley, and the German naturalist Rudolf von Willemoes-Suhm.

They were also interested in supporting the theories of Charles Darwin and disproving the azoic theory of a dead zone below 1,800 feet.

In December 1874, Nares left the Challenger at Hong Kong to assume command of the British Arctic Expedition of 1875-1876, and Captain Frank Tourle Thomson took his place.

William Evans Hoyle, Octopus Marmoratus, from The Voyage of HMS “Challenger”, 1886.

The biological findings received a great interest. All the new species were carefully described, and were sketched by the expedition’s artist, J.J. Wild, but the most famous paintings are those from Hoyle’s monographic studies on cephalopods and the Haeckel’s serie included in “Kunstformen der Natur”.

When in 1895, the first reports of the Challenger were published John Murray summed up the significance of the voyage by calling it “the greatest advance in the knowledge of our planet since the celebrated discoveries of the fifteenth and sixteenth centuries“.

Among the Challenger Expedition’s discoveries is included the first ever rough map of the ocean floor and the finding of an enormous depression in the north-west Pacific Ocean representing the deepest places in the Earth’s crust, now called the Mariana trenches, the deepest point in it is named the Challenger Deep in honor of the expedition. But the greatest discovery of the expedition would be that of the Mid-Atlantic ridge, a mountain chain extending the entire length of the Atlantic Ocean near its center.

When the voyage came to an end in 1876, only 144 crew remained on the ship from the original 216 members. Seven people had died, 26 were left in hospitals or were unable to continue the journey, and several had deserted at the various ports of call. After the dead of Thomson in 1882, John Murray became director and edited the Expedition Reports.

Sir John Murray (1841-1914). From Wikimedia Commons.

The biggest legacy of the Challenger expedition was the establishment of the science of oceanography, based in international and interdisciplinary scientific cooperation.

References:

Deacon, M., Rice, T. and Summerhayes, C. (eds.) (2001) Understanding the oceans: a century of ocean exploration, London, UK, UCL Press.