Author: Shaena Montanari

Paleontology in a Sink Hole: Spring Break Edition

Last week, I spent time at the Bahamas Natural History Symposium in Nassau, Bahamas. Seeing policy makers, ecologists, educators, geologists, and anthropologists come together was awesomely inspiring for the future of The Bahamas, a wonderful place with a magnificent natural environment! If you are fossil hunting ,The Bahamas might not immediately come to mind as a paleontological destination, but you would be wrong! The Bahamas is a large carbonate platform that may have started forming as early as the Cretaceous. The carbonate rock was exposed to the air during the Pleistocene, and during this time sand dunes lithified and formed the islands. During the last glacial maximum, the Bahamas bank was exposed greatly increasing the landmass and connectivity of the Caribbean, fostering biotic interchange between the islands. Now this shelf is submerged again so the islands remain isolated. The limestone that the islands are based on is porous and can be dissolved, so during glacial maxima, weathering of this limestone lead to features that make these islands so special today—underground caves, sink holes, and pits.

The Bahamas. The light blue shallow water is the carbonate shelf that was once exposed during the Last Glacial Maximum. From NASA on Wikimedia Commons

When sink holes and caves fill with water, these underground networks become submerged ethereal mazes that preserve unique life forms. Besides looking beautiful, blue holes as they are called, become a quiet resting place for anything –animal or human—that might end up in there. The blue holes of Abaco in The Bahamas have preserved a rich paleontological and anthropological history spanning thousands of years. On Great Abaco Island, there are a few of these astonishing features, but especially notable is Sawmill Sink. Sawmill Sink, one of the most famous blue holes in The Bahamas, contains the bones of numerous vertebrates that no longer live on the islands or are completely extinct. It is a spectacularly preserved graveyard of tortoises, birds, and crocodiles. Even human bones have found in Sawmill Sink, a remnant of the original prehistoric inhabitants of The Bahamas, the Lucayan people. Since these underwater caves were not always filled with water—as I previously mentioned, sea level was lower—the Lucayans were able to bury individuals in these limestone caves. The bones in the cave are perfectly preserved and also contain organic collagen, so they can be dated using carbon-14. The human remains in Sawmill Sink date from 1050 to 920 calendar years before present, making this the earliest evidence of human inhabitance of the Bahamian Archipelago.

320px-Dean_Blue_Hole_Long_Island_Bahamas_20110210

Dean’s Blue Hole in Long Island, The Bahamas. By Ton Engwirda Wikimedia Commons

Up to 54 individual crocodile specimens have been found in Sawmill Sink that are dated to 2780 years before present, predating any human occupation of The Bahamas. The species of crocodile is Crocodylus rhombifer, the Cuban crocodile. There are currently no crocodiles in The Bahamas, and the Cuban crocodile is limited only to a very small region of Cuba. How the Cuban crocodiles found in Sawmill Sink are genetically related to the current Cuban population remains unknown, but it is quite possible ancient DNA can be obtained from the well preserved specimens. Additionally, why did they go extinct from most of the West Indies? It is supposed that the extinction was caused by human encroachment and hunting, which make sense, because C. rhombifer bones are found in archaeological middens, along with other extinct tortoises that would have been a tasty meal, throughout the islands of The Bahamas.

Cuban crocodile in a Miami zoo. By Alexf Wikimedia Commons

Cuban crocodile in a Miami zoo. By Alexf Wikimedia Commons

Birds, such as the now extinct Bahamas caracara, and the Bermuda petrel (now only in Bermuda) are found preserved in blue holes indicating the avifauna of the Caribbean was once very different and perhaps a lot more diverse. These fossil deposits preserve a unique time in geological history when both humans and large vertebrates were inhabiting space-limited environments. Detailed study of these fossils can potentially help us see at a fine-scale the impact human settlement can have on biodiversity. Since blue holes are so spectacular, there are a myriad of great photographs out there, specifically through National Geographic. I suggest you take a look because the images are just fantastic.

Now if you’ll excuse me I am off to spring break. Paleontologists need some beach time too!

Additional reading:

Morgan, G.S. and N. A. Albury. 2013. The Cuban crocodile (Crocodylus rhombifer) from the Late Quaternary fossil deposit in The Bahamas and Cayman Islands. Bulletin of the Florida Museum of Natural History 52(3): 161-236.

 

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The asteroid started the fire (or did it?)

Poor guys didn’t know what hit em. Public Domain via Wikimedia Commons by Donald E. Davis

In December, I listened to the Radiolab “Apocolyptical” show which was all about the Cretaceous-Paleogene boundary event. Famously, in 1980, Walter Alvarez and colleagues described an iridium anomaly at the K/Pg (also known at K/T) boundary which was subsequently specifically tied to an extraterrestrial impact event. The crater from this impact is around 200-km wide off the coast of the Yucatán in Mexico, known at the Chicxulub Crater.

But what happens when a 10 km (6 mile) wide rock smashes into Earth’s surface? When this object made impact, hot, melted rock was ejected and these little rock balls (spherules) rained down and can be found in deposits of K/Pg age rocks worldwide. There is little debate that this enormous disruption to the entire Earth was responsible for the large-scale extinction we see at the K/Pg boundary. But what, more specifically, did the molten rock and rock vapor have to do with it?

Back to the Radiolab. I was surprised as I listened to it that the hosts were taking an angle of “everything you have learned about dinosaur extinctions is wrong!” From my own experience, it is taught that the final nail in the coffin for dinosaurs, plants, and other animals was a long-lasting “impact winter.” The amount of debris kicked up into the atmosphere would have been so extreme, it would have blocked out the sun, reduced photosynthesis, and caused a cooling period. This cooling period, although it probably only lasted about 2,000 years, would be devastating to ecosystems worldwide. I don’t think there is much doubt an impact winter actually happened, but the stance this radio show took was that this was absolutely not the cause of the massive vertebrate extinction.

Their source on this was work done last year, mainly through computer modeling, that calculated the infrared (IR) radiation heat pulse and subsequent probability of global wildfires caused by molten ejecta re-entering the atmosphere. Douglas Robertson published a comprehensive review of the heat-fire hypothesis, noting that by calculating the kinetic energy converted to IR radiation by ejecta re-entering the atmosphere, a temperature could be reached on the surface of the Earth that is sufficient to ignite plant matter and tinder, causing global wildfires.

Much of this review is dedicated to addressing the problems with this hypothesis raised by other researchers. This often comes down to the morphology and fine structure of the soot found in K/Pg age rocks. While proponents of the heat-fire hypothesis say that soot found is clearly from widespread forest fires, the research of scientists like Claire Belcher suggests otherwise. Her research indicates that these soot deposits are not soot from the burning of plant matter, but actually hydrocarbon combustion from the impact site. Additionally, she puts forth compelling evidence that while there would have been IR radiation coming from the spherules, it mainly would have been shielded from the Earth by the spherules actually settling and forming a cloud in the atmosphere. The spherules’ interaction with the atmosphere potentially prevented the surface of the Earth from being completely incinerated. Of course, the other camp argues the charcoal deposits have signs of coming from ignition of plants and not of hydrocarbons. The debate rages on.

So was every living thing not underground or in the water burned up almost immediately by an IR radiation pulse? Or was there not truly enough heat reaching the Earth’s surface to cause such widespread fire, and extinction was driven mainly by other factors such as acid rain or impact winter? It is hard to know if we will ever be 100% certain, but I find the debate fascinating. Do I think it is right to tell the public everything the know about the dinosaur extinction is wrong? Honestly, I don’t think it is the best way to go about things, because there is still so much debate on this topic and scientists have not reached a consensus, and may never have a unified theory. What do you think about this debate? Any strong opinions either way? If you are an educator, what do you teach your students?

Photo representation of what the non-avian dinosaur extinction may have looked like

Photo representation of what the non-avian dinosaur extinction may have looked like

Category: Climate Change, Dinosaurs, Paleontology | Tagged , , , | 5 Comments

Rodents of the Caribbean: The Curse of the Quaternary Extinction

The Caribbean is typically thought of as a lovely spring break destination. If you are an animal lover, the area is great for diving and birding, but there are not many land mammals to be found. Sure, you will find bats, some endemic rodents, and of course invasive cats and rats, but besides that you will not find anything larger, like, oh, a sloth.

Where's my piña colada? Public Domain- Wikimedia Commons

Where’s my piña colada?
Public Domain- Wikimedia Commons

That’s right, up until a few thousand years ago, sloths weren’t just a South American thing. There were also primates, specifically New World monkeys like the Jamaican monkey, on islands throughout the Greater Antilles. The Greater Antilles consist of Cuba, Hispaniola, Puerto Rico, Cayman Islands, and Jamaica.

Map of the Greater Antilles in green. Other Caribbean islands like the Bahamas and Turks and Caicos have had mammalian extinctions Wikimedia Commons

Map of the Greater Antilles highlighted in green. Other Caribbean islands like the Bahamas and Turks and Caicos have also had mammalian extinctions
Wikimedia Commons

This now extinct terrestrial fauna of the Greater Antilles still remains very mysterious for a variety of reasons. As previously mentioned, only a few species of endemic rodents remain on Caribbean islands. The two biggest questions in this evolutionary conundrum are how did the mammals arrive? And then, how did they go extinct? The arrival debate is probably the most contentious- if we know the ancestors of extinct Antillean sloths, primates, and rodents originated in South America, how did the end up on islands in the middle of the Caribbean Sea? Did they all come from South America? It is possible, but seemingly unlikely, these different groups of mammals came in a series of overwater dispersals. A sloth floating its way to Jamaica may seem unbelievable, but overwater dispersals can be viable migration methods.

There is another theory though—perhaps there was a late Eocene- early Oligocene land bridge that connected the Greater Antillean Ridge and South America via the Aves Ridge. Welcome to:

Gaarlandia

GAARlandia was proposed by Iturralde-Vincent and MacPhee in 1999 as a way to explain the dispersal of land mammals from South America to the Caribbean islands in one continuous event, rather than a series of random overwater dispersals. Phylogenetic evidence has both supported and refuted the potential for the existence of GAARlandia, so the debate can still continue.

This month, in the Journal of Vertebrate Paleotology, Vélez-Juarbe et al. describe rodent incisors from Puerto Rico that date to the Oligocene- making them the earliest rodent fossils in the Caribbean. They are just incisors, so we cannot learn too much about the actual animal they belonged to (besides the fact their enamel structure indicates they belonged to a caviomorph rodent), but the existence of this fossil pushes back the date of rodent arrival in the Greater Antilles by approximately 9 million years. This new date is consistent with other molecular divergence estimations of caviomorph rodent groups in South America. In addition, a dated molecular phylogeny indicates there was a split between a Greater Antillean toad and its sister taxon in South America during the late Eocene- early Oligocene.  The coincidence between molecular divergence dates, paleontological finds, and the hypothesized date of the land-bridge adds support to the idea there may have been a short lived sub-aerial land bridge bringing non-flying mammals to the Greater Antilles.  Of course, this new evidence still does not preclude the possibility of overwater dispersal! These biogeographic hypotheses are very difficult to disprove so the debate rages on.

So where have the Caribbean monkeys and sloths gone? The Caribbean mammalian fauna experienced extremely high rates of extinction during the Quaternary, and these extinctions seemed to occur both before and after humans were a factor in the environments. During the Holocene, climate change could have potentially altered the environments these mammals were living in and this caused their extinction. But with the arrival of humans, overhunting, habitat destruction, disease, and introduction of invasive predators could also have been extremely damaging to biodiversity. The zooarchaeological and paleontological records on these islands can be spotty, but much more remains to be done to figure out the complex history of the ancient life of the Caribbean. Ancient DNA, stable isotopes, and re-examination of the taxonomy of fossils and sub-fossils from the Caribbean will continue to inform biogeographic theory and help us understand what drove such a unique mammalian biota to extinction.

References:

Iturralde-Vincent, M. A., R. MacPhee. 1999. Paleogeography of the Caribbean region: implications for Cenozoic biogeography. Bulletin of the American Museum of Natural History 238:1-95.

Velez-Juarbe, J., T. Martin, R. D. E. MacPhee, D. Ortega-Ariza. 2014. The earliest Caribbean rodents: Oligocene caviomorphs from Puerto Rico. Journal of Vertebrate Paleontology 34:157-163.

Category: Geology, Paleontology | Tagged , , , , | 3 Comments

Ichthyosaur is the New Black

Just yesterday, a group of 2nd graders asked me what color dinosaurs were. I was pretty excited to tell them we actually do know this through looking at tiny structure on feathers called melanosomes. Melanosomes are sub-millimeter sized round or cigar shaped organelles inside of animal cells that contain melanin, the substance that causes pigmentation. When we examine melanosomes in modern organisms, their chemical profiles (known as a spectra) tells us what type of melanin is in that melanosome. Eumelanin is responsible for black and brown pigment and pheomelanin is responsible for red or pink coloration. Carotenoids and porphyrins are other non-melanin pigments, and can produce other colors in birds like bright yellow and green. Interestingly, a blue coloration is produced not by pigment but the scattering of light from unique keratin ‘air pockets’ in certain type of bird feathers. This means blue is a structural, not a pigmented color.

Different types of melanin are really the easiest for us to distinguish in fossil specimens due to the fact their chemical profiles are preserved in melanosomes. From this type of research on dinosaurs, we have found out that Microraptor was shiny and black. A feather from the 1861 Archaeopteryx specimen indicates it too was black. This research, up until this point, has mostly focused on examining the melanosomes in birds, but this week, in Nature, this same idea was used to determine the colors of ancient marine reptiles.

This time, Johan Lindgren and coauthors were interested in the colors of extinct marine giants like ichthyosaurs, the mosasaur Tylosaurus, and an extinct leatherback turtle. Scrapings of “skin” were taken from these specimens and put into some special microscopes. The skin of these specimens is a blackish sort of film that, to the naked eye, doesn’t look like anything special. But up close in the scanning electron microscope, it is immediately apparent that the telltale cigar-shaped melanosomes are present. It is important to note fossil bacteria can look very similar to fossilized melanosomes, but distinct chemical signatures only on the body areas of the fossils and not on the sediment outside that zone seem to indicate they were present in life and did not form after death.

The chemical profiles of the melanosomes in all three of these marine reptiles indicated they were black in color because eumelanin was detected. Interestingly, it seems the ichthyosaur was black on the entire body- not just on the dorsal side. The mosasaur and turtle possibly had a dark top and a lighter underside. You may be wondering why any of this matters- what difference does it make if an ichthyosaur was black all over? In the paper, the authors aptly mention coloration is a trait that is subject to natural selection. Coloration in animals influences sexual displays, thermoregulation, and camouflage.  Certain animals can be better adapted to colder environments if their dorsal sides are darker in color, which will allow them to absorb more heat. Cordylid lizards in South Africa that had lower skin reflectance (more melanin) had a higher fitness than species that had lighter, more reflective skin.

Ichthyosaur specimen from the Alf Museum. Courtesy of Andy Farke. CC-BY

Ichthyosaur specimen from the Alf Museum. Courtesy of Andy Farke. CC-BY

Ichthyosaurs may have had eumelanin on their entire body due to a benefit in thermoregulation. If they spent time close to the surface, they could absorb more sunlight and stay warmer in cold environments. Extant leatherback turtles are dark on the top of their shells, possibly for this reason. This pigmentation pattern is called thermal melanism. On the other hand, most turtles and cetaceans are countershaded–dark on top and light on the bottom. This shading in cetaceans seems to obscure their own shadows while they are diving, making it easier to sneak up on prey. Species of whales that possess uniform dark coloration could be better adapted for diving to extreme depths where there is no light, but the evidence for this is mostly anecdotal at this time.

This study indicates for the first time there is convergent evolution of melanism in secondarily aquatic tetrapods, but more importantly expands fossil melanosome analysis beyond just feathers. Not that long ago, determining the color of extinct animals seemed impossible, but now it is very clearly possible for more organisms than we ever imagined!

References: 

Carney, R. et al. 2012. New evidence on the colour and nature of the isolated Archaeopteryx feather. Nature Communications. 3 (637). doi:10.1038/ncomms1642

Clusella-Trullas, S., Hvan Wyk, J., Spotila, J.R. 2009. Thermal benefits of melanism in cordylid lizards: a theoretical and field test. Ecology 90:2297–2312. http://dx.doi.org/10.1890/08-1502.1

Li et al. 2012. Reconstruction of Microraptor and the Evolution of Iridescent Plumage. Science 335 (6073) 1215-1219. DOI: 10.1126/science.1213780

Lindgren, J. et al. 2014. Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles. Nature. doi:10.1038/nature12899

Category: Dinosaurs, Geology, Paleontology | Tagged , , , , | 3 Comments

Prehistoric Platypus: Revenge of the Monotremes

(I tried to make the title of the post sound like a SyFy movie title, let me know how I did)

 Obdurodon tharalkooschild is not your garden-variety platypus.

Today, the platypus is one of two remaining varieties of living monotreme (the other being echidnas). I could go on and on about the platypus- it does too many cool things to mention them all.  Ok, ok, I will name one. The male platypus both has pointy ankle spurs that contain venom (females have the spur, but no venom).  This venom isn’t fatal to humans, but is, of course, extremely painful. How is this known? Well apparently a guy wanted to test it out by getting stabbed in the hand by a platypus in the early 90s (paper here) and it really, really hurt. Additionally, “significant functional impairment of the hand persisted for three months”. So no need to try that at home people.

Foot spur! Copyright (c) 1995 E.Lonnon Wikimedia Commons

Foot spur! Copyright (c) 1995 E.Lonnon Wikimedia Commons

We don’t know if O. tharalkooschild was venomous, but we know that it was huge! This new species is described on the basis of one singular tooth. The beautiful part about mammalian teeth is that even one tooth can be so different than any other known specimen and it diagnoses an entire species.  When then-Honors student Rebecca Pian (now a PhD student at Columbia University/AMNH) was looking through the vast amount of fossil material from Riversleigh, she adeptly noticed that this monotreme tooth was not like the others. The tooth in question is a lower first molar, and based on its size, O. tharalkooschild was at least two times larger than its modern counter part Ornithorhyncus anatinus! An adult male platypus is ~50cm long, so O. tharalkooschild was at least 1 meter long!

Reconstruction of O. tharalkooschild by Peter Schouten

Reconstruction of O. tharalkooschild by Peter Schouten

Riversleigh, the fossil locality this specimen was discovered at, is a famous World Heritage site in the outback of Queensland, Australia. Some of the most spectacular fossil marsupials and monotremes that define the evolutionary history of Oz’s most iconic animals have been found here. Previously, the only fossil platypus known from Riversleigh was Obdurodon dicksoni and all vaguely platypus-y material was referred to that species.

The variation in morphology of this tooth really shakes things up in the world of monotreme relationships. All of the platypus relatives had very similar looking molars, indicating the evolutionary history of these animals was fairly simple. This big ol’ platypus throws a wrench in things—it is lacking twin lophs (ridges) on a certain part of the tooth, which indicates this is a separate radiation from the previously discovered modern and fossil platypuses.

The molar. Photo by Rebecca Pian.

The molar. Photo by Rebecca Pian.

The age of this specimen is a bit vague, based on current stratigraphic knowledge it could be middle Miocene (~15 Ma) or as young as Pliocene (~5 Ma). Either way, it is still an extremely unique specimen that changes what we know about fossil monotremes. The oldest fossil monotremes include animals like Steropodon and Kollikodon, both Cretaceous (~100 Ma) in age, from the Lightning Ridge locality in New South Wales, Australia. Besides being some of the oldest known monotremes, these specimens are unique because most of the fossils at Lightning Ridge are opalised—meaning the minerals in the bones and teeth have been replaced with shiny opal.

Steropodon opalised jaw. (c) Carl Bento Australian Museum

Steropodon opalised jaw. (c) Carl Bento Australian Museum

Besides the fossils I’ve mentioned, there are a few other fossil monotremes, but the record is relatively sparse. The origin of these animals is still a mystery, although the presence of yet another platypus relative fossil in Argentina (Paleocene in age) gives the indication there was, at one point, a Gondwanan distribution of these enigmatic creatures. Rebecca says “Other than the Patagonian platypus Monotrematum sudamericanum, we have nothing in between the early Cretaceous and the late Oligocene. No fossils have be found from Antarctica either despite evidence that they should be there.” There are a few fossil localities in Australia that seem promising for the discovery of monotreme teeth, so hopefully the record will be filled in in the coming years.

(This research is from an article in the Journal of Vertebrate Paleontology that is not yet available online)

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Paleoecology of Magnificent Megafauna: The Moa

There was a time not so long ago (thousands of years ago, not millions, which is not so long ago to a paleontologist) when ecosystems around the world had something that is generally lacking today: really big herbivores. I’m talking beasts such as giant armadillos, giant sloths, mastodons, and mammoths. Obviously there aren’t any mastodons lumbering around your neighborhood today, so what happened to them? In general, there are two main hypotheses: either humans ended their reign or climate change eliminated their preferred habitats. These hypotheses are hotly debated in the literature, but as it is difficult to prove any absolute cause in certain cases, the debate still rages on.

No matter what caused their extinction, extinct herbivore megafauna can still be extremely useful in studying climate change. What evidence is there about the structure of habitats where megafauna lived? Just this week a paper in PNAS seeks to answer this questions in great detail. And how did they do it? Ancient bird poop, of course.

The New Zealand moa was a giant herbivorous bird that has been extinct since 1400 AD. The two largest species of moa would have been around 12 feet tall and weighed up to 500lbs! Their extinction, like other megafauna, has been debated to be caused by overhunting and/or habitat loss. Due to the fact the moa only lived with one other animal that was big enough to eat it- Haast’s eagle- it is likely humans caused their extinction. The moa is a relatively well studied animal for being extinct: we have even been able to sequence their genome and see what kinds of parasites lived in their guts. The nine extinct species of moa were endemic to New Zealand and they likely shaped the plant communities they lived in, but it is difficult to know for sure when we were unable to critically assess their impact. The study of ancient poop (coprolites, which I also talked about here) can remedy these issues.

Artists rendition of a Haast's eagle attacking a moa. By John Megahan. Distributed under CC-BY license, PLOS Biology

Artist’s rendition of a Haast’s eagle attacking a moa. By John Megahan. Distributed under CC-BY license, PLOS Biology

In this study, the authors analyzed 51 coprolites rigorously: they were carbon dated, and individually assessed for ancient DNA (aDNA), plant macrofossils, and fossil pollen. Ancient DNA is critical in this type of study because not only does the DNA tell us that these coprolites are from four different moa species, but they can also provide DNA identification of the plant remains contained in the coprolite. Additional pollen and macrofossil study provide a detailed list of what plants were present at this site 400 years before the extinction of the moa. This combination of methods is extremely rigorous, and an incredible way of measuring paleoecology quantitatively. The results indicate that there were four species of moa living at the same time in the same area but occupying different dietary niches.

Figure 3 from Wood et al. 2013. Plant data from coprolite aDNA, fossils, and pollen. The different species had distinct diets.

Figure 3 from Wood et al. 2013. Plant data from coprolite aDNA, fossils, and pollen. The different species had distinct diets.

It stands to reason that ecosystems can be vastly changed when key organisms are removed. Large birds like moa likely had an impact on habitats through altering vegetation composition and seed dispersal. The species of moa from this study had dietary preferences from forest plants to grasses, and everything in between. In response to the idea that other herbivores could be introduced into ecosystems to mimic the effect a moa might have had on the vegetation community, the authors note modern ostriches and emus do not occupy the same dietary niche spaces as the extinct moa, and would be a poor replacement herbivore in a New Zealand ecosystem.

I think this study is so great because it really drives home the power of a multiproxy approach for understanding paleoecology. Combining traditional methods of fossil analysis with “newer” methods of aDNA and carbon creates a very clear picture of an ancient environment that otherwise would be impossible to obtain.

Reference:

Resolving lost herbivore community structure using coprolites of four sympatric moa species (Aves: Dinornithiformes). Jamie R. Wood, Janet M. Wilmshurst, Sarah J. Richardson, Nicolas J. Rawlence, Steven J. Wagstaff, Trevor H. Worthy, and Alan Cooper. PNAS 2013 ; published ahead of print September 30, 2013, doi:10.1073/pnas.1307700110

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Dinosaurs Come Through In The Clutch

In the last few months, a lot has been going around about a pretty interesting topic—dino sex. Besides the mechanics of dinosaur sex and reproduction (some people love talking about that)- there are some more interesting questions to consider. For example, how many eggs did most dinosaurs lay? Some have suggested that enormous dinosaurs, such as sauropods, had many offspring per year, and this contributed to their gigantism. Previously, the only things we were able to discern about dinosaur reproduction were what was uncommonly preserved in the fossil record—such as complete fossil egg clutches, very rarely with a brooding parent on top. These finds are amazing and interesting, but because they are so rare, it is difficult to use these fossils to make broad generalizations about reproduction in different groups of dinosaurs.

Although rare, this brooding Citipati osmolskae is probably my favorite fossil ever. Uploaded by Dinoguy2, distributed under Creative Commons ShareALike 1.0 license.

In a paper out yesterday in PLOS ONE, authors Jan Werner and Eva Maria Griebeler looked at extant birds and reptiles in order to better understand how many eggs some dinosaurs may have laid during the year. They constructed a really cool allometric regression model based on modern species and found a close correlation between body mass, egg mass, clutch mass, and annual clutch mass.

Figure 2 from Werner and Griebeler 2013 showing the allometries of reptile, bird, and turtle body mass and egg mass.

Figure 2 from Werner and Griebeler 2013 showing the allometries of reptile, bird, and turtle body mass and egg mass.

When there is a clear idea of how extant animal size relationships function, the same equation can be applied to dinosaurs using extant phylogenetic bracketing. In other words, non-avian dinosaurs will be plugged into the same allometric equation as their closest ancestor. Theropods were plugged in to the “bird model” because they are most closely related to modern birds. Hadrosaurs and sauropodomorphs were plugged into the “reptile model” because the authors decided these dinosaurs were less related to birds than to reptiles since they were outside Therapoda. The results indicate that theropods probably only had one clutch per year, but hadrosaurs and sauropodomorphs may have had several. Surprisingly, using these calculations, most of the dinosaurs studied had less than 200 eggs per year. Enormous 75,000kg sauropods had around 400 eggs per year, which is less than extant sea turtles that can lay over 500 eggs per year!

But why did these different types of dinosaurs have different egg-laying strategies? Well, that is fairly open to interpretation. It is possible that the environments these animals lived in were so different that theropods were able to get by on one clutch per year, while other dinosaurs may have lived in environments where their clutches did not experience good survival rates. It is definitely clear though that there was a shift at some point during the evolution of non-avian dinosaurs to birds relating to egg and clutch size. Theropods had larger eggs than sauropodomorphs relative to body size, but had fewer eggs in their clutch. Modern birds have the largest eggs compared to body size of all, but by far the smallest clutch size, only laying 2.2-4.5 eggs per clutch. It will be interesting to explore and hypothesize about the drivers of this trend.

I always enjoy elegant studies like this one that utilize mostly published data to make new inferences about extinct animals. There are already so many data out there that paleontologists can utilize without relying on new fossil finds, so get (data)mining!

References:

Werner J, Griebeler EM (2013) New Insights into Non-Avian Dinosaur Reproduction and Their Evolutionary and Ecological Implications: Linking Fossil Evidence to Allometries of Extant Close Relatives. PLoS ONE 8(8): e72862. doi:10.1371/journal.pone.0072862

Category: Dinosaurs, Paleontology, PLOS ONE | Tagged , , , , | 2 Comments

Climate Change and Paleontology: Back to the Future

This week and next at PLOS Blogs, we are doing a focus on climate change. This is leading up to a great collection of papers that will be released at the Ecological Society of America meeting very soon (the link to the collection will be live on August 5th).

As paleontologists, we are frequently thinking about changes in climates over longer time spans that most people ever consider. Decades and centuries seem like small drops in a vast, overflowing bucket. Even though we often deal with thousands or millions of years of climate data obtained from proxies, we still are very concerned with how our science can help understand the current state of rapidly accelerated climate alteration and its impact on global ecology. Paleontology is indeed relevant to this discussion because we can gain perspective on the magnitude of events, such as extinction, in the face of normal climate change, but also in response to rarer, more extreme climate events.

Certain proxies in paleontology allow us to understand how past climate change impacted biodiversity, and we can use this information to make an attempt at predicting the future. As I have previously posted, ancient and modern small mammal communities are strong indicators of how climate change will impact the survival of vertebrates around the world. For example, research has shown certain rodents that feed mainly on seeds dominated past desert faunal assemblages during warm periods. Since warming is predicted to increase in many of the world’s deserts, this gives a strong indication of what may happen to certain key species of rodents over the coming decades.

It is no secret my favorite proxy for studying how climate change has impacted animals in the past is stable isotope paleoecology. By quantitatively studying what vertebrates ate and the types of environments they lived in at different points in time, we can visualize how changes in environments will impact the diets, ranges, and survival of certain species. Climate change will strongly impact the food webs of both marine and terrestrial ecosystems, and looking into how these changes have played out in the fossil record could give us an idea of what will happen in vulnerable areas in the future.

Using stable isotopes to assess diet in a unique, changing environment was the basic premise behind my most recent paper that came out on June 12th in PLOS ONE. My co-authors and I used carbon and oxygen stable isotopes to characterize the environment these extinct large marsupials lived in 3 million years ago based on the diets reflected in their tooth enamel. This locality that we analyzed was Pliocene in age, and represents the time preceding large scale megafauna extinctions in Australia. Being able to understand the environment these animals lived in is critical for assessing how the subsequent shift to grasslands in the region may have impacted biodiversity and factored in to future extinction events.

Euryzygoma dunense, one of the animals I sampled tooth enamel from for stable isotope analysis. Image by Nobu Tamura, distributed under GNU Free Documentation.

Euryzygoma dunense, one of the animals I sampled tooth enamel from for stable isotope analysis. Image by Nobu Tamura, distributed under GNU Free Documentation.

In this study, I compared the stable isotope values in the tooth enamel of fossil marsupials, such as kangaroos and giant wombats, to that of modern kangaroo tooth enamel in order to characterize the ancient environment. According to stable isotope data, this area of Queensland was much wetter than previously thought. Today, this same region is classified as grassland when, during the Pliocene, it probably more closely resembled a temperate or subtropical forest. Now that this is known about the past environment, with more sampling of later localities, we can better understand how climatic and environmental factors could have lead to a decline of certain species. While doing this research, I was struck by the vast range of plant fodder modern kangaroo species consume—it spans a vast range of carbon stable isotope values over their different habitats. While kangaroos may be generalist herbivores that can survive over a range of habitats, other animals may not be, which exposes them to a greater danger of losing their food source in areas strongly impacted by climate change.

Figure 2 from Montanari et al. 2013. This illustrates the range in plant diet in modern kangaroos, and also that the stable isotope values in the paleoenvironment indicate it may have been similar to a modern temperate or subtropical environment.

Figure 2 from Montanari et al. 2013. This illustrates the range in plant diet in modern kangaroos, and also that the stable isotope values in the paleoenvironment indicate it may have been similar to a modern temperate or subtropical environment. (Click to enlarge)

Typically in the megafauna extinction debate we talk about a binary cause of extinction: climate change or humans? In this day and age, it isn’t so clear. We have caused an accelerated shift in the carefully orchestrated conditions that allow us to have such a great diversity of life on Earth. In the future, we cannot really think of extinction debates in the same binary fashion—as almost all extinction can now be traced back to humans. The work of paleontologists to understand past climate change and its impact on global ecology and biodiversity will clearly only become more relevant in the near future.

 

References:

Montanari S, Louys J, Price GJ (2013) Pliocene Paleoenvironments of Southeastern Queensland, Australia Inferred from Stable Isotopes of Marsupial Tooth Enamel. PLoS ONE 8(6): e66221. doi:10.1371/journal.pone.0066221

Category: Climate Change, Paleontology, PLOS ONE | Tagged , , , , , , | 1 Comment

What’s My Age Again?

Earlier this month, an almost 40 pound rockfish was caught in Alaska that was allegedly 200 years old. The angler that caught the enormous fish based this age on body size estimates. Then, earlier this week when the fish was officially aged, it was found to only be 64 years old. Many of you at home may be wondering: how was this fish officially aged? How do we know how old any vertebrate is when all we have a carcass (or in the case of paleontology, just some dry bones)?

The term ‘sclerochronology’ was coined in the 70s to describe the study of accreted hard parts in invertebrates, but this study can be applied to any hard parts that have specific growth patterns. In corals, mollusks, and even vertebrate teeth and bones, we see growth lines that can reflect annual, weekly, or daily increments of time. Studying these lines and how often they form can tell us how old an animal is, since body size estimates are clearly unreliable once an animal reaches adulthood.

The aforementioned rockfish was aged by analyzing its otoliths- small bony structures in the ears used for sensing movement. These structures grow incrementally, and these increments form at known times. When they are counted, the fish can be assigned an age- a method that works in both modern and fossil organisms. The growth lines take on different characteristics in summer and in winter due to nutrient cycling and diet, so a pair of summer and winter bands can be counted as one year of life. Trace elements and isotopes can be measured in each band, which gives us an idea about migration and seasonal dietary change.

Otoliths from a Pacific cod.  Uploaded by Edgewise, distributed under GDFL Wikimedia Commons.

Luckily there are also similar growth lines in vertebrate teeth, and sometimes bones, although these lines can be slightly more difficult to interpret. Sometimes their formation is not regular, so they cannot simply be counted the same as tree rings to determine age. In vertebrate teeth, there are a variety of types of growth lines known as perikymata and striae of Retzius, that can be a valuable chronometer if the time between these increments is known. In humans, certain growth lines in teeth are essentially weekly, but since we are unable to perform experiments to study the growth of extinct organisms, the length of time between growth increments of fossils can be difficult to discern.

Last week, Kevin Uno and co-authors published a study in PNAS relating to growth lines in elephant tusks. This paper makes me so excited for a variety of reasons, but mainly because they use carbon-14 dating and stable isotope analysis to make vital points about animal growth, ecology, and wildlife forensics.

Basically, Uno et al. were interested in utilizing the carbon-14 “bomb curve” to date ivory and thereby show if it was legal or not, as ivory harvested before a certain date can be legal for sale in the United States. Nuclear weapons testing from 1952-1962 doubled the atmospheric concentration of carbon-14. Recognizing this spike in radiocarbon in a biological sample can give us an idea of when it was formed.

Figure 3 from Uno et al. 2013 illustrating the C-14 age of growth bands in an elephant molar.

Figure 3 from Uno et al. 2013 illustrating the C-14 age of growth bands in an elephant molar.

The work of Uno et al. illustrates that the dating can help discern whether the ivory is legal or not, but also, their detailed research into the dating of growth lines in elephant tusks and other mammalian molars allow us to be able to make more detailed paleoecological estimates with stable isotopes. Serial sampling fossilized materials for stable isotopes, such as molars, can tell us about seasonal changes in diets and water use. Understanding how often growth lines are deposited in hard parts of vertebrates by dating them is necessary if we want to have an accurate chronometer for assessing the age at death of a fossil organism. The work in this paper illustrated that elephants also have approximately week long increments of growth, and this knowledge now allows us to have a detailed chronometer for other extinct elephant relatives. I am of the opinion that serial sampling for stable isotopes and dating should be done on growth lines whenever possible so we have quantitative data on just how old these fossils are.

And next time the media throws us a fish story, we can all be a bit more skeptical.

Reference:

Kevin T. Uno, Jay Quade, Daniel C. Fisher, George Wittemyer, Iain Douglas-Hamilton, Samuel Andanje, Patrick Omondi, Moses Litoroh, and Thure E. Cerling. Bomb-curve radiocarbon measurement of recent biologic tissues and applications to wildlife forensics and stable isotope (paleo)ecology. PNAS 2013 : 1302226110v1-201302226.

Category: Paleontology, Zoology | Tagged , , , | Comments Off

How to Find a Fossil

Since the summer is the field season for many of us, I thought I would write a little bit about the first step in paleontological discovery: actually going out and looking for fossils. I will admit, when I first started graduate school and had never gone on any sort of expedition, I had no idea how anyone ever found a fossil. When you ask a non-paleo person, many times, they think you just show up somewhere and indiscriminately start digging a hole hoping to find bones. I’m happy to say sometimes we are a little more efficient than that…but sometimes not by much.

Humans have been discovering fossils for thousands of years, and ancient societies have in fact recorded finding bones in the ground accidentally (a topic I am interested in writing more about for a future post!) Similarly, in the modern era, serendipity is often needed to find great fossils. Sometimes it is a farmer tilling a field, workers digging coal mines, or children out exploring; some lucky catalyzing event usually clues us in to an area’s fossil bearing possibilities.

Flaming Cliffs, Gobi Desert, Mongolia

Flaming Cliffs. (S. Montanari, 2010)

I started my field work in the Gobi Desert of Monoglia. The tradition of finding great fossils in Mongolia is a long one, dating back to the 1920s and the expeditions of Roy Chapman Andrews. He went to Mongolia hoping to find human fossils, but actually stumbled upon a vast trove of fossils, including the first specimens of animals such as Velociraptor and Indricotherium. Because of this detailed history, we have a good idea of what specific rock formations contained fossils.

When I first went out into the field in Mongolia, I heard the story of how Ukhaa Tolgod, one of the greatest Cretaceous Mongolian fossil localities, was discovered. Basically, one of the many vehicles had gotten stuck in the sand, and so it was going to take a while to tow. The leaders of the expedition decided to go check out a red hill in the distance that looked like the same sort of rock as the famous Flaming Cliffs. When they pulled up and saw skulls and skeletons dotting the ground, they knew they had hit the jackpot. The crew has since visited hundreds of similar looking sites that appear to be the same formation, but found no fossils, so it goes to show you that if you keep your boots on the ground long enough you might only be lucky enough to find a handful of great sites!

Ukhaa Tolgod, Mongolia. Looks similar to Flaming Cliffs. (S. Montanari, 2010)

Fossil localities are often discovered as the by-products of construction. Some famously important fossils have been found in tailing piles of coal mines, opal mines, and road cuts. But once you have found the locality, and know there are fossils there, how do you find more of them? Sometimes people get an image in their heads of paleontologists using sophisticated tools like Ground Penetrating Radar in order to find fossils. While this method does work (check here for a paper on this topic by the late great Derek Main) many scientists don’t have the resources to use these machines. They also don’t work in certain rock formations as well. Most of the time, we just put our boots (and eyes) to the ground and walk around for hours in the hot sun, rain, or cold until we see a little scrap of bone on the surface that can look like a boring rock. You need to learn what bone looks like at your locality, which can take a while. Sometimes it is white, sometimes brown, or even black. If you see a scrap of bone, it is useful to explore if it is just a small part of something big by digging around it carefully. If you see bone rolling down a small slope, you walk to the top to see if there is a fully exposed skull waiting for you to collect it (doesn’t often happen, but I wish it did).

This is what it is like in Mongolia. We don’t get out the big digging equipment unless we are fairly certain we are digging into something nice. In some places I have worked, like an Eocene locality in Australia, the heavy machinery was brought out immediately because the fossils were contained in a silty layer that was exposed meters below the topsoil. This is also a locality where you really never see the fossils you are collecting, you merely take loose piles of poorly consolidated sediments, throw them in bags, and then sieve them in the lab later to find very precious small teeth and bones. While you don’t find whole, articulated skeletons this way, sieving is a valuable methods for understanding smaller fauna at a locality.

Eocene Australian fossil locality (S. Montanari 2012)

Eocene Australian fossil locality (S. Montanari 2012)

If you are on top of a rich bonebed, you can set up a quarry, where basically everyone huddles in a glorified hole, very carefully exposing piles of bones. The way bones are found in a bonebed can be important to interpreting behavior later, so it is important to be careful. It can take weeks, months, or even years (decades!) for a fossil to make it from the field into the preparation lab, so sometimes the fossils you read about in recent literature were discovered by previous generations of scientists. For my next post, I will talk more about preparation, or everything that happens after we find an exciting fossil in the field!

A photo I snapped on my iPhone in Mongolia of a bit of exposed bone. (S. Montanari 2010).

A photo I snapped on my iPhone in Mongolia of a bit of exposed bone. (S. Montanari 2010).

Does anyone collect fossils in an interesting locality? Underwater for example? Anyone use blasting at their site to move rocks? I know some people who collect in extreme environments like Antarctica have interesting stories. Post a comment if you would like!

Category: Background, Dinosaurs, Paleontology | Tagged , | 4 Comments