The realm of the Tyrant

Close up of “Sue” at the Field Museum of Natural History in Chicago, IL, 2009 (From Wikimedia Commons)

After the extinction of many carnivorous crurotarsan lineages (phytosaurs, ornithosuchids, and rauisuchians) at the Triassic–Jurassic boundary, theropod dinosaurs became the primary large-bodied flesh-eaters in terrestrial ecosystems. The group reached a great taxonomic and morphological diversity during the Jurassic and Early Cretaceous. Some major groups include Ceratosauria, Megalosauroidea, Spinosauridae; Carnosauria, and Coelurosauria. In the last decades, the study of Gondwanan non-avian theropods has been highly prolific, showing that the group reached a great taxonomic and morphological diversity comparable to that of Laurasia. Notwithstanding, there is a qualitative difference between Jurassic and Early Cretaceous assemblages relative to the latest Cretaceous (Campano-Maastrichtian) assemblages with abelisaurids dominating Gondwanan continents, and tyrannosaurids ruling Asiamerican ecosystems. 

Tyrannosaurus rex, the most iconic dinosaur of all time, and its closest relatives known as tyrannosaurids, comprise the clade Tyrannosauroidea, a relatively derived group of theropod dinosaurs, more closely related to birds than to other large theropods such as allosauroids and spinosaurids. The clade originated in the Middle Jurassic, approximately 165 million years ago, and was a dominant component of the dinosaur faunas of the American West shortly after the emplacement of the Western Interior Seaway (about 99.5 Mya). Over the past 20 years, new discoveries from Russia, Mongolia and China helped to build the Tyranosaurs family tree.

Skulls of the basal tyrannosauroids Guanlong (A), Dilong (B); Skulls of juvenile (C) and adult (D)Tyrannosaurus. (Adapted from Brusatte et. al., 2010)

All large-bodied carnivorous theropod dinosaurs passed through a wide range of body sizes. Therefore, the ecological niche of any given individual shifted throughout its lifetime. From the Jurassic through the early Late Cretaceous, this transformation occurred in the context of ecosystems in which the juveniles and subadults potentially competed with other theropod species with medium adult body sizes. But sometime after the Turonian something changed.

A new study by Thomas Holtz, a principal lecturer in the University of Maryland’s Department of Geology, surveyed the record of 60 dinosaur communities from the Jurassic and Cretaceous periods, revealing a drop-off in diversity of medium-sized predator species (50–1000 kg) in communities dominated by tyrannosaurs. On the other hand, the study also showed that the diversity of prey species did not decline. The proposed explanation for this phenomenon is the “tyrannosaurid niche assimilation hypothesis”. In this scheme, juvenile and subadult members of Tyrannosauridae were the functional equivalent of earlier middle-sized theropod carnivores. This absence of other potential mid-sized competitors in Campano-Maastrichtian Asiamerica could be a factor in some evolutionary transformations in Tyrannosauridae such as bite force and agility.

 

References:

Thomas R. Holtz, Theropod guild structure and the tyrannosaurid niche assimilation hypothesis: implications for predatory dinosaur macroecology and ontogeny in later Late Cretaceous Asiamerica, Canadian Journal of Earth Sciences (2021). DOI: 10.1139/cjes-2020-0174

Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, et al. (2010) Tyrannosaur paleobiology: new research on ancient exemplar organisms. Science 329: 1481–1485. doi: 10.1126/science.1193304

Zanno, L., Makovicky, P. Neovenatorid theropods are apex predators in the Late Cretaceous of North America. Nat Commun 4, 2827 (2013). https://doi.org/10.1038/ncomms3827

 

The Spinosaurus tail

Reconstructed skeleton and caudal series of Spinosaurus aegyptiacus. From Ibrahim et al., 2020.

Spinosaurus aegyptiacus is one of the most famous dinosaur of all time. It was discovered by German paleontologist and aristocrat Ernst Freiherr Stromer von Reichenbach in 1911. This gigantic theropod possessed highly derived cranial and vertebral features sufficiently distinct for it to be designated as the nominal genus of the clade Spinosauridae. Unfortunatelly, the holotype of Spinosaurus aegyptiacus was destroyed after a British Royal Air Force raid bombed the museum and incinerated its collections. Only two photographs of the holotype of Spinosaurus aegyptiacus were recovered in the archives of the Paläontologische Museum in June 2000, after they were donated to the museum by Ernst Stromer’s son, Wolfgang Stromer, in 1995. These photographs provide additional insight into the anatomy of the holotype specimen of the animal.

Almost a century later, a partial skeleton of a subadult individual of S. aegyptiacus was discovered in the Cretaceous Kem Kem beds of south-eastern Morocco. At the time of deposition, this part of Morocco was located on the southern margin of the Tethys Ocean and it was characterized by an extensive fluvial plain dominated by northward flowing rivers and terminating in broad deltaic systems on Tethys’ southern shores. The neotype of S. aegyptiacus preserves portions of the skull, axial column, pelvic girdle, and limbs. An international team led by Nizar Ibrahim published the first description of the fossil in 2014 and suggested that Spinosaurus may have been specialised to spend a considerable portion of their lives in water.

 

Selected caudal vertebrae and chevrons of Spinosaurus. From Ibrahim et al., 2020.

Spinosaurus clearly show some adaptations to a partially or predominantly piscivorous diet (because of their morphological convergence with those of crocodilians and other fish-eating reptiles, isolated spinosaurid teeth have frequently been misinterpreted). Furthermore, the presence of a short, robust femur with hypertrophied flexor attachment and the low, flat-bottomed pedal claws are consistent with aquatic foot-propelled locomotion. Now, the description of a nearly complete and partially articulated tail of S. aegyptiacus reinforces the hypothesis that this giant theropod spent plenty of time underwater.

Proximal and distal elements of the tail are complete and preserved in three dimensions, indicating a minimal taphonomic distortion. The preserved tail is approximately 400 cm long. The zygapophyses are significantly less developed than in most tetanurans, hinting at a different functional capacity for the tail in this taxon. The neural arches are also distinctive elements of the Spinosaurus tail, while the morphology of the neural spines shows considerable variation. The elongate neural and haemal arches result in a tail shape that is markedly vertically expanded and has an extensive lateral surface area. The highly specialized morphology of the Spinosaurus tail allowed it to function as a propulsive structure for aquatic locomotion. The anterior positioning of the center of mass within the ribcage may have also enhanced balance during aquatic movement. The model proposed by Ibrahim indicates that Spinosaurus tail shape was capable of generating more than 8 times the thrust of the tail shapes of other theropods, and achieved 2.6 times the efficiency.

 

 

References:

Ibrahim, N., Maganuco, S., Dal Sasso, C. et al. Tail-propelled aquatic locomotion in a theropod dinosaur. Nature (2020). https://doi.org/10.1038/s41586-020-2190-3

Ibrahim, N., Sereno, P. C., Dal Sasso, C., Maganuco, S., Fabbri, M., Martill, D. M., Zouhri, S., Myhrvold, N., Iurino, D. A. (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science, 345(6204), 1613–1616. doi:10.1126/science.1258750 

HONE, D. W. E. and HOLTZ, T. R. (2017), A Century of Spinosaurs – A Review and Revision of the Spinosauridae with Comments on Their Ecology. Acta Geologica Sinica, 91: 1120–1132. doi: 10.1111/1755-6724.13328

The Last Mammoths

Mammuthus primigenius, Royal British Columbia Museum. From Wikipedia Commons

During the Late Pleistocene and early Holocene, most of the terrestrial megafauna became extinct. It was a deep global-scale event. The extinction was notably more selective for large-bodied animals than any other extinction interval in the last 65 million years. Among them, the mammoths offers a very complete fossil record, and their evolution is usually presented as a succession of chronologically overlapping species, including (from earliest to latest) M. meridionalis (southern mammoths), M. trogontherii (steppe mammoths), and M. columbi (Columbian mammoths) and M. primigenius (woolly mammoths).

Wrangel Island coast. From Wikipedia Commons

From Siberia to Alaska, mammoths were widespread in the northern hemisphere and their remains inspired all types of legends. Their lineage arose in Africa during the late Miocene, and first appeared in Europe almost three million years ago. The iconic M. primigenius arose in northeastn Siberia from the steppe mammoth (Mammuthus trogontherii) and their extinction has inspired an impressive body of literature. Multiple explanatory hypotheses have been proposed for this event: climatic change, overhunting, habitat alteration, and the introduction of a new disease.

The world’s last population of woolly mammoths lived on Wrangel Island going extinct around 4,000 years ago. In contrast the mammoth population from Russia disappeared about 15,000 years ago, while the mammoths of St. Paul Island in Alaska disappeared 5,600 years ago. The Wrangel Island was a part of Beringia, an ancient landmass, that included the land bridge between Siberia and Alaska. Global sea level transgression at the end of the Pleistocene isolated Wrangel Island from the mainland and broke up Beringia. Palynological and isotopic evidence suggest that present climatic conditions and floral composition were established right after the Pleistocene-Holocene transition.

A mammoth tooth on the riverbank on Wrangel Island. Image credit; Juha Karhu/University of Helsinki

Tooth specimens are about 90% of all the mammoth material for Wrangel Island. The multi-isotopic evidence (carbon, nitrogen and sulfur in collagen) measured on Wrangel Island mammoths supports the idea that this relict population mantained a typical mammoth ecology despite climate change and decreasing genetic diversity. It has been suggested that the extinction of the Wrangel Island mammoths was possibly caused by a short-term crisis, possibly linked to climatic anomalies, however the anthropogenic influence should not be dismissed despite lack of tangible evidence of hunting.

 

References:

Laura Arppe, Juha A. Karhu, Sergey Vartanyan, Dorothée G. Drucker, Heli Etu-Sihvola, Hervé Bocherens. Thriving or surviving? The isotopic record of the Wrangel Island woolly mammoth population. Quaternary Science Reviews, 2019; 222: 105884 DOI: 10.1016/j.quascirev.2019.105884

Learning from Past Climate Changes

In the last 540 million years, five mass extinction events shaped the history of the Earth. Those events were related to extreme climatic changes and were mainly caused by asteroid impacts, massive volcanic eruption, or the combination of both.  On a global scale the main forces behind climatic change are: solar forcing, atmospheric composition, plate tectonics, Earth’s biota, and of course, us. Human activity is a major driver of the dynamics of Earth system. From hunter-gatherer and agricultural communities to the highly technological societies of the 21st century, humans have driven the climate Earth system towards new, hotter climatic conditions. Until the Industrial Revolution, the average global CO2 levels fluctuated between about 170 ppm and 280 ppm. But with the beginning of the Industrial Era, that number risen above 300 ppm, currently averaging an increase of more than 2 ppm per year. The average monthly level of CO2 in the atmosphere in last April exceeded the 410 ppm for first time in history. Thus we could hit an average of 500 ppm within the next 45 years, a number that has been unprecedented for the past 50–100+ million years according to fossil plant-based CO2 estimates. This current human-driven change far exceed the rates of change driven by geophysical or biosphere forces that have altered the Earth System trajectory in the past, and it poses severe risks for health, economies and political stability. Learning from past climatic changes is critical to our future.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Microfossils from deep-sea are crucial elements for the understanding of our past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments. The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contain clues about the temperatures in which they lived. The ratio of heavy and light Oxygen in foraminifera shells can reveal how cold the ocean was and how much ice existed at the time the shell formed. Another tool to reconstruct paleotemperatures is the ratio of magnesium to calcium (Mg/Ca) in foraminiferal shells. Mg2+ incorporation into foraminiferal calcite  is influenced by the temperature of the surrounding seawater, and the Mg/Ca ratios increase with increasing temperature.

Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antarctic Oceans, especially where calcareous fossils are less abundant. Diatom assemblages are also used in reconstructions of paleoproductivity.

Scanning electron microscope image of different types of pollen grains. Image from Wikipedia.

Pollen and other palynomorphs proved to be an extraordinary tool to paleoenvironmental reconstruction too. Pollen analysis involves the quantitative examination of spores and pollen at successive horizons through a core, specially in lake, marsh or delta sediments, especially in Quaternary sediments where the parent plants are well known. This provide information on regional changes in vegetation through time, and it’s also a valuable tool for archaeologists because it gives clues about man’s early environment and his effect upon it.

Stomatal frequency of land plants, which has been shown in some species to vary inversely with atmospheric pCO2, has been used to estimate paleo-pCO2 for multiple geological time periods. Stomata are the controlled pores through which plants exchange gases with their environments, and play a key role in regulating the balance between photosynthetic productivity and water loss through transpiration.

Temple I on The Great Plaza and North Acropolis seen from Temple II in Tikal, Guatemala. From Wikimedia Commons

Paleoecological records indicate that the transition to agriculture was a fundamental turning point in the environmental history of Mesoamerica. Tropical forests were reduced by agricultural expansion associated with growing human populations. Also soil loss associated with deforestation and erosion was one of the most consequential environmental impacts associated with population expansion in the Maya lowlands. This environmental crisis ended with the collapse of the Classic Maya society.

Human activity has significantly altered the climate in less than a century. Since 1970 the global average temperature has been rising at a rate of 1.7°C per century, and the rise in global CO2 concentration since 2000 is 10 times faster than any sustained rise in CO2 during the past 800,000 years. Today the most politically unstable countries are also places where environmental degradation affected food production and water supply. Other human societies have succumbed to climate change – like the Akkadians – while others have survived by changing their behavior in response to environmental change. We have the opportunity to protect the future of our own society by learning from the mistakes of our ancestors.

References:

David Evans, Navjit Sagoo, Willem Renema, Laura J. Cotton, Wolfgang Müller, Jonathan A. Todd, Pratul Kumar Saraswati, Peter Stassen, Martin Ziegler, Paul N. Pearson, Paul J. Valdes, Hagit P. Affek. Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry. Proceedings of the National Academy of Sciences, 2018; 201714744 DOI: 10.1073/pnas.1714744115

Nicholas P. Evans et al., Quantification of drought during the collapse of the classic Maya civilization, Science (2018); DOI: 10.1126/science.aas9871 

Will Steffen, et al.; Trajectories of the Earth System in the Anthropocene; PNAS (2018) DOI: 10.1073/pnas.1810141115

Lessons from the past: Paleobotany and Climate Change

 

From 1984–2012, extensive greening has occurred in the tundra of Western Alaska, the northern coast of Canada, and the tundra of Quebec and Labrador. Credits: NASA’s Goddard Space Flight Center/Cindy Starr.

For the last 540 million years, Earth’s climate has oscillated between three basic states: Icehouse, Greenhouse (subdivided into Cool and Warm states), and Hothouse. The “Hothouse” condition is relatively short-lived and is consequence from the release of anomalously large inputs of CO2 into the atmosphere during the formation of Large Igneous Provinces (LIPs), when atmospheric CO2 concentrations may rise above 16 times (4,800 ppmv), while the “Icehouse” is characterized by polar ice, with alternating glacial–interglacial episodes in response to orbital forcing. The ‘Cool Greenhouse” displays  some polar ice and alpine glaciers,  with global average temperatures between 21° and 24°C. Finally, the ‘Warm Greenhouse’ lacks of any polar ice, and global average temperatures might have ranged from 24° to 30°C.

Reconstructions of Earth’s history have considerably improved our knowledge of episodes of rapid emissions of greenhouse gases and abrupt warming. Several episodes of global climate change were similar in magnitude to the anthropogenically forced climate change that has occurred during the past century. Consequently, the development of different proxy measures of paleoenvironmental parameters has received growing attention in recent years. Paleobotany, the study of fossil plants in deep geological time, offers key insights into vegetation responses to past global change, including suitable analogs for Earth’s climatic future.

Monthly average atmospheric carbon dioxide concentration at Mauna Loa Observatory, Hawaii.

The main forces of climatic change on a global scale are solar forcing, atmospheric composition, plate tectonics, Earth’s biota, and of course, us. Human activity is a major driver of the dynamics of Earth system. Until the Industrial Revolution, the average global CO2 levels fluctuated between about 170 ppm and 280 ppm. But with the beginning of the Industrial Era, that number risen above 300 ppm, currently averaging an increase of more than 2 ppm per year. The average monthly level of CO2 in the atmosphere on last April exceeded the 410 ppm for first time in history. Thus we could hit an average of 500 ppm within the next 45 years, a number that have been unprecedented for the past 50–100+ million years according to fossil plant-based CO2 estimates. Therefore, the closest analog for today conditions is the Eocene, meaning greater similarities in continental configuration, ecosystem structure and function, and global carbon cycling.

Some of the best-studied intervals of global change in the fossil plant record include the Triassic–Jurassic boundary, 201.36 ± 0.17 Mya; the PETM, 56 Mya; and the Eocene–Oligocene boundary, 33.9 Mya.The first two events represent rapid greenhouse gas–induced global warming episodes; the last coincides with the initiation of the Antarctic ice sheet and global cooling leading to our current icehouse.

Time line of plant evolution (From McElwain, 2018)

During the PETM, compositional shifts in terrestrial vegetation were marked but transient in temperate latitudes and long-lived in the tropics. The PETM is characterized by the release of 5 billion tons of CO2 into the atmosphere, while temperatures increased by 5 – 8°C. High temperatures and likely increased aridity in the North American temperate biomes resulted in geologically rapid compositional changes as local mixed deciduous and evergreen forest taxa (such as Taxodium) decreased in relative abundance. These suggest that global warming has a marked effect on the composition of terrestrial plant communities that is driven predominantly by migration rather than extinction. However, it’s difficult to draw parallels with Anthropocene warming and vegetation responses because they are occurring at a minimum of 20 times faster than any past warming episode in Earth’s history.

In the early Eocene (56 to 49 Mya), a time of peak sustained global warmth, the Arctic Ocean was ice free, with a mosaic of mixed deciduous, evergreen (Picea, Pinus), and swamp forests, and with high densities of the aquatic fern Azolla. The Azolla bloom reduced the carbon dioxide from the atmosphere to 650 ppm, reducing temperatures and setting the stage for our current icehouse Earth. The eventual demise of Azolla in the Arctic Ocean is attributed to reduced runoff and a slight salinity increase.

The modern fern Azolla filiculoides (From Wikipedia)

The Earth’s poles have warmed and will continue to warm at a faster rate than the average planetary warming, because the heat is readily transported poleward by oceans and the atmosphere due to positive feedback effects involving snow cover, albedo, vegetation, soot, and algal cover in the Arctic and Antarctic. This phenomenon is known as “polar amplification”.

Recent studies about the greening of the Arctic indicates that increasing shrubiness has likely already had an unexpected negative impact on herbivore populations, such as caribou, by decreasing browse quality. Thus, it is important to predict how short-term temporal trends in Arctic vegetation change will continue under CO2-induced global warming. The paleobotanical record of high Arctic floras may provide broad insight into these questions.

References:

Jennifer C. McElwain, Paleobotany and Global Change: Important Lessons for Species to Biomes from Vegetation Responses to Past Global Change, Annual Review of Plant Biology  (2018), DOI: 10.1146/annurev-arplant-042817-040405

 

Brief history of the Ocean Acidification through time: an update

Corals one of the most vulnerable creatures in the ocean. Photo Credit: Katharina Fabricius/Australian Institute of Marine Science

About one third of the carbon dioxide released by anthropogenic activity is absorbed by the oceans. Once dissolved in seawater, most of the CO2 is transported into deep waters via thermohaline circulation and the biological pump. But a smaller fraction of the CO2, forms carbonic acid and causes a decline in pH in the surface ocean. This phenomenon is called ocean acidification, and is occurring at a rate faster than at any time in the last 300 million years.

Acidification affects the biogeochemical dynamics of calcium carbonate, organic carbon, nitrogen, and phosphorus in the ocean and interferes with a range of processes, including growth, calcification, development, reproduction and behaviour in a wide range of marine organisms like planktonic coccolithophores, foraminifera, pteropods and other molluscs,  echinoderms, corals, and coralline algae.

The pH within the ocean surface has decreased ~0.1 pH units since the industrial revolution and is predicted to decrease an additional 0.2 – 0.3 units by the end of the century. An eight-year study carried out by the Biological Impacts of Ocean Acidification group (Bioacid), with the support of the German government, has contributed to quantifying the effects of ocean acidification on marine organisms and their habitats. Among the many effects of ocean acidification on marine organisms are including: decreased rate of skeletal growth in reef-building corals, reduced ability to maintain a protective shell among free-swimming zooplankton, and reduced survival of larval marine species, including commercial fish and shellfish. Even worse, the effects of acidification can intensify the effects of global warming, in a dangerous feedback loop.

Coccolithophores exposed to differing levels of acidity. Adapted by Macmillan Publishers Ltd: Nature Publishing Group, Riebesell, U., et al., Nature 407, 2000.

The geologic record of ocean acidification provide valuable insights into potential biotic impacts and time scales of recovery.  Rapid additions of carbon dioxide during extreme events in Earth history, including the end-Permian mass extinction (252 million years ago) and the Paleocene-Eocene Thermal Maximum (PETM, 56 million years ago) may have driven surface waters to undersaturation. But, there’s  no perfect analog for our present crisis, because we are living in an “ice house” that started 34 million years ago  with the growth of ice sheets on Antarctica, and this cases corresponded to events initiated during “hot house” (greenhouse) intervals of Earth history.

The end-Permian extinction is the most severe biotic crisis in the fossil record, with as much as 95% of the marine animal species and a similarly high proportion of terrestrial plants and animals going extinct . This great crisis ocurred about 252 million years ago (Ma) during an episode of global warming. The cause or causes of the Permian extinction remain a mystery but new data indicates that the extinction had a duration of 60,000 years and may be linked to massive volcanic eruptions from the Siberian Traps. The same study found evidence that 10,000 years before the die-off, the ocean experienced a pulse of light carbon that most likely led to a spike of carbon dioxide in the atmosphere. This could have led to ocean acidification, warmer water temperatures that effectively killed marine life.

Taxonomic variation in effects of ocean acidification (From Kroeker et al. 2010)

The early Aptian Oceanic Anoxic Event (120 million years ago) was an interval of dramatic change in climate and ocean circulation. The cause of this event was the eruption of the Ontong Java Plateau in the western Pacific, wich led to a major increase in atmospheric pCO2 and ocean acidification. This event was characterized by the occurrence of organic-carbon-rich sediments on a global basis along with evidence for warming and dramatic change in nanoplankton assemblages. Several oceanic anoxic events (OAEs) are documented in Cretaceous strata in the Canadian Western Interior Sea.

The Paleocene-Eocene Thermal Maximum (PETM; 55.8 million years ago) was a short-lived (~ 200,000 years) global warming event. Temperatures increased by 5-9°C. It was marked by the largest deep-sea mass extinction among calcareous benthic foraminifera in the last 93 million years. Similarly, planktonic foraminifer communities at low and high latitudes show reductions in diversity. The PETM is also associated with dramatic changes among the calcareous plankton,characterized by the appearance of transient nanoplankton taxa of heavily calcified forms of Rhomboaster spp., Discoaster araneus, and D. anartios as well as Coccolithus bownii, a more delicate form.

The current rate of the anthropogenic carbon input  is probably greater than during the PETM, causing a more severe decline in ocean pH and saturation state. Also the biotic consequences of the PETM were fairly minor, while the current rate of species extinction is already 100–1000 times higher than would be considered natural. This underlines the urgency for immediate action on global carbon emission reductions.

References:

David A. Hutchins & Feixue Fu, Microorganisms and ocean global change, Nature Microbiology 2, Article number: 17058 (2017) doi:10.1038/nmicrobiol.2017.58 

Kump, L.R., T.J. Bralower, and A. Ridgwell. 2009. Ocean acidification in deep time. Oceanography 22(4):94–107, http://dx.doi.org/10.5670/oceanog.2009.100

Parker, L. M. et al. Adult exposure to ocean acidification is maladaptive for larvae of the Sydney rock oyster Saccostrea glomerata in the presence of multiple stressors. Biology Letters 13 (2017). DOI: 10.1098/rsbl.2016.0798

Kristy J. Kroeker, Rebecca L. Kordas, Ryan N. Crim, Gerald G. Singh, Meta-analysis reveals negative yet variable effects of ocean acidification on marine organisms, Ecology Letters (2010) 13: 1419–1434
DOI: 10.1111/j.1461-0248.2010.01518.x

 

Solving the mystery of Megatherium diet.

Megatherium americanum, MACN.

Around 10,000 years ago, Argentina was home of numerous species of giant Xenarthrans, giant ground sloths (relative to tree sloth) and glyptodontids (relative to tiny extant armadillo). Sloths, characteristic of the mammal fauna of the Pleistocene of South America, show a great diversity with more than 80 genera, grouped in four families: Megatheriidae, Megalonychidae, Nothrotheriidae and Mylodontidae.

For more than a century different hypotheses on the dietary preferences of giant ground sloths have been proposed. In 1860, Owen gave an extensive explanations about their possible diet and behavior. He based his conclusions on the morphology of the skull, combined with peculiarities of the rest of the skeleton, but always by analogy with living tree sloth. He wrote: “Guided by the general rule that animals having the same kind of dentition have the same kind of food, I conclude that the Megatherium must have subsisted, like the Sloths, on the foliage of tree…”. In 1926, Angel Cabrera discussed the diet of Megatherium, rejecting some theories on myrmecophagy or insectivory, and agreed with Owen’s statements about a folivorous diet.

Megatherium americanum lower right tooth series. Scale bar: 5 cm (From M.S. Bargo and S.F. Vizcaíno, 2008)

The dietary preferences of extinct mammals can usually be evaluated through their tooth morphology, but the application of stable isotopes on fossil bones has yielded very important information to solve debates about the diet of extinct large mammal groups, by comparing the carbon and nitrogen isotopic composition of their bone collagen with those of coeval herbivorous and carnivorous taxa. Another isotopic approach is to mesure the difference between the carbon isotopic abundances of the collagen and the carbonate fractions of skeletal tissues. An animal with a herbivorous diet, exhibits significantly larger differences than a carnivore. The values measured on bone collagen from Megatherium, clearly fall in the same range as the large herbivores such as the equid Hippidion, the notoungulate Toxodon and the liptoptern Macrauchenia, for which there is no doubt about their herbivorous diet. Therefore, the hypotheses of insectivory or carnivory for these extinct mammals are not supported by the isotopic data.

 

References:

Hervé Bocherens et al. Isotopic insight on paleodiet of extinct Pleistocene megafaunal Xenarthrans from Argentina, Gondwana Research (2017). DOI: 10.1016/j.gr.2017.04.003

Bargo, M.S., Vizcaíno, S.F., 2008. Paleobiology of Pleistocene ground sloths (Xenarthra, Tardigrada): biomechanics, morphogeometry and ecomorphology applied to the masticatory apparatus. Ameghiniana 45: 175-196

Liaodactylus primus and the ecological evolution of Pterodactyloidea.

Skull of the newfound species Liaodactylus primus (Credit: Chang-Fu Zhou)

Skull of the newfound species Liaodactylus primus (Credit: Chang-Fu Zhou)

Pterosaurs are an extinct monophyletic clade of ornithodiran archosauromorph reptiles from the Late Triassic to Late Cretaceous. The group achieved high levels of morphologic and taxonomic diversity during the Mesozoic, with more than 150 species recognized so far. During their 149 million year history, the evolution of pterosaurs resulted in a variety of eco-morphological adaptations, as evidenced by differences in skull shape, dentition, neck length, tail length and wing span. Pterosaurs have traditionally been divided into two major groups, “rhamphorhynchoids” and “pterodactyloids”. Rhamphorhynchoids are characterized by a long tail, and short neck and metacarpus. Pterodactyloids have a much larger body size range, an elongated neck and metacarpus, and a relatively short tail. Darwinopterus from the early Late Jurassic of China appear to be a transitionary stage that partially fills the morphological gap between rhamphorhynchoids and pterodactyloids.

Pterodactyloidea, the most species-diverse group of pterosaurs, ruled the sky from Late Jurassic to the end of Cretaceous. Liaodactylus primus, a new specimen, discovered in northeast China’s Liaoning province, documents the only pre-Tithonian (145–152 Ma) pterodactyloid known with a complete skull, shedding new light on the origin of the Ctenochasmatidae, a group of exclusive filter feeders, and the timing of the critical transition from fish-catching to filter-feeding, a major ecological shift in the early history of the pterodactyloid clade. The holotype specimen is a nearly complete skull (133 mm long) and mandibles, with the first two cervical vertebrae preserved in articulation with the skull. The elongation of the rostrum, almost half the length of the skull, is accompanied by a significant increase in the number of marginal teeth, giving a total of 152 teeth in both sides of the upper and lower jaws. The teeth are closely spaced to form a ‘comb dentition’, a filter-feeding specialization.

Pterodaustro guinazui cast (Museo Argentino de Ciencias Naturales)

Pterodaustro guinazui cast (Museo Argentino de Ciencias Naturales)

Liaodactylus is the oldest known ctenochasmatid, predating the previously Tithonian (152 Ma) record (Gnathosaurus and Ctenochasma from Germany) by at least 8–10 Myr . The Ctenochasmatidae, represents a long-ranged clade (160–100 Ma), and the only pterodactyloid clade that crossed the Jurassic-Cretaceous transition. The group includes the Early Cretaceous Pterodaustro from Argentina. Popularly called the ‘flamingo pterosaur’, Pterodaustro represents the most remarkable filter-feeding pterosaur known from the fossil record, with a huge number (more than 1000) of densely spaced ‘teeth’ (elastic bristles) in its lower jaws, for filtering small crustaceans, microscopic plankton or algae from open water along lake shores.

Pterosaurs display an extraordinary eco-morphological disparity in feeding adaptations, expressed in skull, jaws and dentition. The Late Triassic Eopterosauria, the basalmost pterosaur clade, were mainly insectivorous. Jurassic insectivores include the Dimorphodontia, Campylognathoididae and Darwinoptera, whereas the Anurognathidae were the only Jurassic insectivores that survived the Jurassic–Cretaceous transition, but became extinct in the Early Cretaceous. The rise of the ctenochasmatid clade was the first major ecological shift in pterosaur evolution from insectivorous-piscivorous to filter-feeding. During Cretaceous time,  the Eupterodactyloidea, a group of advanced pterodactyloids, engaged in a variety of feeding adaptations, including filter-feeding, fish-eating, carnivory and scavenging, herbivory including frugivory, durophagy and omnivory. The Early Cretaceous tapejarids may have been herbivorous, while the pteranodontids, with large skull but tapering and toothless jaws were suitable for seizing fish in open-water environments. Finally, the Late Cretaceous azhdarchids have been hypothesized as foragers feeding on small animals and carrion in diverse terrestrial environments.

Time-calibrated cladogram showing stratigraphic range, eco-morphological diversity of pterosaur clades. (Adapted from Zhou et al., 2017)

Time-calibrated cladogram showing stratigraphic range, eco-morphological diversity of pterosaur clades. (Adapted from Zhou et al., 2017)

References:

Chang-Fu Zhou, Ke-Qin Gao, Hongyu Yi, Jinzhuang Xue, Quanguo Li, Richard C. Fox, Earliest filter-feeding pterosaur from the Jurassic of China and ecological evolution of Pterodactyloidea, 

Andres, B., Clark, J., & Xu, X. (2014). The earliest pterodactyloid and the origin of the group. Current Biology, 24(9), 1011-1016.

WITTON, M. P., 2010 Pteranodon and beyond: the history of giant pterosaurs from 1870 onwards. In: Moody, R.T.J., Buffetaut, E., Naish, D., Martill, D.M. (Eds.), Dinosaurs and Other Extinct Saurians: A Historical Perspective. Geological Society, London, Special Publications 343, 287–311.

 

Climate model simulations at the end of the Cretaceous.

gg_60212w_crater

Artist’s reconstruction of Chicxulub crater 66 million years ago.

About thirty years ago, the discovery of anomalously high abundance of iridium and other platinum group elements in the Cretaceous/Palaeogene (K-Pg) boundary led to the hypothesis that a 10 km asteroid collided with the Earth and caused one of the most devastating events in the history of life. The impact created the 180-kilometre wide Chicxulub crater causing widespread tsunamis along the coastal zones of the surrounding oceans and released an estimated energy equivalent of 100 teratons of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere.

To model the climatic effects of the impact, a team of scientist from the Potsdam Institute for Climate Impact Research (PIK), use literature information from geophysical impact modeling indicating that for a 2.9 km thick target region consisting of 30% evaporites and 70% water-saturated carbonates and a dunite projectile with 50% porosity, a velocity of 20 km/s and a diameter between 15 and 20 km, a sulfur mass of 100 Gt is produced. This is about 10,000 times the amount of sulfur released during the 1991 Pinatubo eruption. Additionally, for a sulfur mass of 100 Gt, about 1400 Gt of carbon dioxide are injected into the atmosphere, corresponding to an increase of the atmospheric CO2 concentration by 180 ppm. There could be additional CO2 emissions from ocean outgassing and perturbations of the terrestrial biosphere, adding a total of 360 ppm and 540 ppm of CO2. The main result is a severe and persistent global cooling in the decades after the impact. Global annual mean temperatures over land dropped to -32C in the coldest year and continental temperatures in the tropics reaching a mere -22C. This model is supported by a migration of cool, boreal dinoflagellate species into the subtropic Tethyan realm directly across the K–Pg boundary interval and the ingression of boreal benthic foraminifera into the deeper parts of the Tethys Ocean, interpreted to reflect millennial timescale changes in the ocean circulation after the impact (Vellekoop, 2014).

A time-lapse animation showing severe cooling due to sulfate aerosols from the Chicxulub asteroid impact 66 million years ago (Credit: PKI)

A time-lapse animation showing severe cooling due to sulfate aerosols from the Chicxulub asteroid impact 66 million years ago (Credit: PKI)

References:

Brugger J., G. Feulner, and S. Petri (2016), Baby, it’s cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous, Geophys. Res. Lett., 43,  doi:10.1002/2016GL072241.

Alvarez, L. W., W. Alvarez, F. Asaro, and H. V. Michel (1980), Extraterrestrial Cause for the Cretaceous-Tertiary Extinction, Science, 208 (4448), 1095{1108, doi: 10.1126/science.208.4448.1095.

Galeotti, S., H. Brinkhuis, and M. Huber (2004), Records of post Cretaceous-Tertiary boundary millennial-scale cooling from the western Tethys: A smoking gun for the impact-winter hypothesis?, Geology, 32, 529, doi:10.1130/G20439.1

Johan Vellekoop, Appy Sluijs, Jan Smit, Stefan Schouten, Johan W. H. Weijers, Jaap S. Sinningh Damsté, and Henk Brinkhuis, Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary, PNAS (2014) doi: 10.1073/pnas.1319253111

Sea level regulated tetrapod diversity dynamics through the Jurassic/Cretaceous interval.

The Jurassic/Cretaceous Boundary

Map of the Jurassic/Cretaceous Boundary by C. Scotese.

The Jurassic/Cretaceous (J/K) boundary, 145 Myr ago, remains as the less understood of major Mesozoic stratigraphic boundaries. Sedimentological, palynological and geochemical studies, indicate a climatic shift from predominantly arid to semi arid conditions in the latest Jurassic to more amicable humid conditions in the earliest Cretaceous. The continued fragmentation of Pangaea across the Late Jurassic and Early Cretaceous led to large-scale tectonic processes, on both regional and global scale, accompanied by some of the largest volcanic episodes in the history of the Earth; eustatic oscillations of the sea level; potentially heightened levels of anoxia, oceanic stagnation, and sulphur toxicity; along with two purported oceanic anoxic events in the Valanginian and Hauterivian. There’s also evidence of three large bolide impacts in the latest Jurassic, one of which might have been bigger than the end-Cretaceous Chicxulub impact.

Painting of a late Jurassic Scene on one of the large island in the Lower Saxony basin in northern Germany (From Wikimedia Commons)

Painting of a late Jurassic Scene on one of the large island in the Lower Saxony basin in northern Germany (From Wikimedia Commons)

The J/K interval represents a period of elevated extinction, and involves the persistent loss of diverse lineages, and the origins of many major groups that survived until the present day (e.g. birds). The magnitude of this drop in diversity ranges from around 33% for ornithischians to 75–80% loss for theropods and pterosaurs. Mammals suffered an overall loss of  diversity of 69%. Crocodyliforms suffered a major  decline across the Jurassic/Cretaceous boundary in both the marine and terrestrial realms; while non-marine turtles declined by 33% of diversity through the J/K boundary. In contrast, lepidosauromorphs greatly increased in diversity (48%) across the J/K boundary, reflecting the diversification of major extant squamate clades, including Lacertoidea, Scincoidea and Iguania.

In the marine realm, sauropterygians and ichthyosaurs show evidence for a notable decline in diversity across the J/K boundary, which continued into the Hauterivian for both groups.

Stratigraphic ranges of major Jurassic–Cretaceous theropod (A), sauropod (B), and ornithischian (C) dinosaur clades through the Middle Jurassic to Early Cretaceous (From Tennant et al,. 2016)

Stratigraphic ranges of major Jurassic–Cretaceous theropod (A), sauropod (B), and ornithischian (C) dinosaur clades through the Middle Jurassic to Early Cretaceous (From Tennant et al,. 2016)

Eustatic sea level is the principal mechanism controlling the Jurassic–Cretaceous diversity of tetrapods. Rising sea levels leads to greater division of landmasses through creation of marine barriers, modifying the spatial distribution of near-shore habitats and affecting the species–area relationship, which can lead to elevated extinctions. This fragmentation can also be a potential driver of biological and reproductive isolation and allopatric speciation, the combination of which we would expect to see manifest in the diversity signal. Additionally, the diversity of fully marine taxa was more probably affected by the opening and closure of marine dispersal corridors, whereas that of terrestrial and coastal taxa was more probably dependent on the availability of habitable ecosystems, including the extent of continental shelf area (Tennant, et al., 2016).

References:

Tennant J,P., Mannion P. D., Upchurch P., Sea level regulated tetrapod diversity dynamics through the Jurassic/Cretaceous interval, Nature Communications, ISSN: 2041-1723 DOI: 10.1038/ncomms12737

Tennant, J. P., Mannion, P. D., Upchurch, P., Sutton, M. D. and Price, G. D. (2016), Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover. Biol Rev. doi:10.1111/brv.12255 

Butler, R. J., Benson, R. B. J., Carrano, M. T., Mannion, P. D. & Upchurch, P. Sea level, dinosaur diversity and sampling biases: investigating the ‘common cause’ hypothesis in the terrestrial realm. Proc. R. Soc. B 278, 1165–1170 (2011).