DinoByte Wednesday: What Happened to the Dinosaurs?

The Cretaceous-Paleogene (K/Pg) extinction event wiped out up to 75% of ALL species on earth, including the non-avian dinosaurs! Why the “non-avian” distinction and what does it mean? The fossil record indicates that birds (avians) evolved from dinosaurs by the end of the Jurassic Period, meaning birds ARE dinosaurs. Birds obviously survived the K/Pg mass extinction, but what happened to the all of the “non-avian” dinosaurs? The K/Pg mass extinction was rapid, global, and severe. Evidence from 66 million-year-old rocks around the world support various theories and scientists today continue to debate whether the K/Pg mass extinction was due to a single cause, or to multiple causes. In fact, DIG teachers help scientists answer this very question during their time at the DIG Field School. Let’s examine each of these ideas and the evidence supporting them.

Artistic rendition of a dinosaur looking on as an enormous asteroid crashes into earth.
Artistic rendition of a dinosaur looking on as an enormous asteroid crashes into earth.

Single Cause Theory (or the Asteroid Impact Theory)

If you ask most people how the dinosaurs went extinct they would probably tell you it was from an asteroid impact. This idea was first proposed in 1980 by Luis and Walter Alvarez, a nobel-prize winning physicist father and geologist son team from UC Berkeley. In the 1970s Walter was a young professor doing fieldwork in Italy in sediment that straddles the Cretaceous-Paleogene boundary. In sediments older than 66 million years ago (mya), he discovered abundant marine microfossils (including various forms of plankton), above these was a distinct clay layer lacking any fossils, and in the layers above the clay, (younger than 66 mya), there were very few microfossils. Further investigation revealed that the clay layer contained extremely high concentrations of iridium (30x normal levels!), a rare earth element found in high concentrations in asteroids and comets. Walter’s father, Luis, suggested this “iridium anomaly” could be due to some sort of extraterrestrial impact, and the hunt was on for more evidence to support this theory for non-avian dinosaur extinction, or the “Alvarez Hypothesis.”

A few pieces of evidence to corroborate an extraterrestrial event were found in Italy and ultimately worldwide. Shocked quartz results from impact shock waves (extraterrestrial, nuclear bomb) penetrating quartz crystals with such force that the crystalline structure is disrupted. Produced at the site of K/Pg impact, shocked quartz would have floated into the atmosphere, and dispersed across the globe. Additionally, the impact would have been forceful enough to liquefy nearby rock and sand, creating tiny “glassy spherules” that would also have been dispersed globally.

It appeared the rocks were telling Walter the story of how the dinosaurs met their fate! At the site of the DIG Field School, rocks tell the same story. We find a clay layer at the K/Pg boundary, with shocked quartz, glassy spherules, and high levels of iridium, as well as a coal layer immediately above the clay that contains high levels of spores from ferns. This “fern spike” is a common indicator of “primary succession” following an ecological disturbance, and even today, as on Mount St. Helens, ferns are the first to colonize devastated areas. At the K/Pg boundary, the fern spike tells us plants were growing again and the environment was recovering after the mass extinction.

Shock waves from an impact disrupt the crystalline pattern of quartz causing lines to form in the rock (left). DIG 2013 participant, Siri, examines the layer of rock that marks the extinction of non-avian dinosaurs and the end of the Mesozoic (right). Shocked quartz and the iridium anomaly are found at the K/Pg boundary at this DIG field site. In the rocks below the Hell Creek Formation one finds abundant non-avian dinosaurs, but the only dinosaurs found in the Tullock Formation above are birds! Photo credit (right): Lauren DeBey
Shock waves from an impact disrupt the crystalline pattern of quartz causing lines to form in the rock (left). DIG 2013 participant, Siri, examines the layer of rock that marks the extinction of non-avian dinosaurs and the end of the Mesozoic (right). Shocked quartz, glassy spherules, and  the iridium anomaly are found at the K/Pg boundary at this DIG field site. In the rocks below where Siri is pointing, in the Hell Creek Formation, one finds abundant non-avian dinosaurs, but the only dinosaurs found in the above Tullock Formation are birds! Photo credit (right): Lauren DeBey.

There was ample evidence to suggest an extraterrestrial impact, but where was the crater? Separately but at approximately the same time, geophysicists searching for oil off the coast of the Yucatan Peninsula region in the Gulf of Mexico found a 110-mile wide circular feature. Working with geologist Alan Hildebrand, they determined it was a crater that was the result of an asteroid impact, and they named it the Chicxulub Crater, after a nearby town.

Shortly after Walter Alvarez proposed his impact theory, geologists from a Mexican oil company discovered the 110-mile wide Chicxulub crater off the Yucatan Peninsula shown above.
Shortly after Walter Alvarez proposed his impact theory, geologists from a Mexican oil company discovered the 110-mile wide Chicxulub crater off the Yucatan Peninsula shown above.

Based on the size of the enormous crater, it is estimated that the asteroid was 6-miles wide! Such a large impact would have had approximately the energy of 100 trillion tons of TNT, or about 2 million times greater than the most powerful thermonuclear bomb ever tested. An impact of this size would have produced many cascading environmental effects in addition to distributing iridium, glassy spherules, and shocked quartz globally. First, the collision of the asteroid with the earth’s crust likely triggered earthquakes, tsunamis, and wildfires. In some places deposits from this time preserve giant trees that suggest these monster tsunamis from the Gulf of Mexico penetrated all the way to Texas and Brazil! Second, the impact would have ejected huge amounts of debris and rock into the atmosphere, which would have globally darkened the skies and cooled the planet for approximately a year, ultimately inhibiting photosynthesis and collapsing ecosystems dependent upon plants

Scientists generally agree that this enormous impact was a significant contributing cause to the K/Pg mass extinction. However, many scientists argue that evidence of environmental change and disturbance BEFORE the impact suggests the asteroid impact was not the ONLY cause, but was potentially one of MANY causes resulting in such a devastating extinction event.

Multiple Causes Theory

During the last few million years of the Cretaceous, and the last ~10 million years that non-avian dinosaurs were in existence, the earth was a very dynamic place. Volcanic activities in India, known as the Deccan Traps, were erupting 1.5 million square kilometers of lava (thats half the size of India!) and releasing huge amounts of dust and sulfurous gases into the atmosphere. These factors caused decreases in sunlight, as well as global cooling that would have affected plant-dependent food chains worldwide before the K/Pg mass extinction. Fossil evidence from our field sites suggests that during these Deccan Trap eruptions in India, ecosystems in the Hell Creek of northeastern Montana were stressed. Stressed ecosystems are analogous to the whole pile of straw on a camel before the last piece added (the asteroid) will break its back. Much of our work during the field season and during the DIG is designed to investigate which groups of animals were suffering decreased numbers before the asteroid impact (like dinosaurs and mammals), and which animals were thriving or doing just fine (like amphibians).

Deccan traps are one of the largest volcanic features on Earth measuring approximately 6,500 feet thick and covering around 193,000+ square miles. The term “trap” is derived from the Swedish word for stairs (trapp, trappa) and refers to the step-like hills forming the landscape of the region.
Deccan traps are one of the largest volcanic features on earth, measuring approximately 6,500 feet thick and covering around 193,000+ square miles. The term “trap” is derived from the Swedish word for stairs (trapp or trappa) and refers to the step-like hills forming the landscape of the region.

Another contributing factor was regression of the Western Interior Seaway (remember the Western Interior Seaway?) that we can trace in the changing rock formations in northeastern Montana. As the Western Interior Seaway regressed, or receded, areas that were once marine or near shore would have dried up and been replaced by more inland ecosystems and different species. Also during this time, global climate change was also occurring and the once warm, mild climate became more varied. So although the asteroid impact at ~66 mya would have had a major effect on the planet and its inhabitants, the geological and biological evidence suggests a much more complicated explanation for the end of the non-avian dinosaurs. Will YOU help us find more evidence to answer these questions?

The Aftermath

Regardless of the exact cause, the K/Pg mass extinction wiped out three quarters of the species on earth and led to a major transition in floras and faunas. Not all groups were affected equally, but non-avian dinosaurs, pterosaurs, ammonites, and many mammals went completely extinct during this time. A loss of different species at various levels of the food chain can result in empty “ecological niches,” and with these previously occupied ecological niches now open, other groups can evolve to fill them. What followed the K/Pg mass extinction were a series of radiations (rapid diversification of organisms resulting from environmental change), with Cenozoic mammals ultimately replacing the niches left empty by the Mesozoic non-avian dinosaurs. We can even think of today’s tigers, cows, and rodents as the modern day version of the Mesozoic’s theropod carnivorous dinosaurs, Triceratops, and multituberculates, respectively. Most dinosaurs went extinct 66 mya, but a few survived and have been very successful. If you had chicken for dinner last night, you ate one!

Artist Rudolph Zallinger’s famour “Age of Mammals” mural at the Yale Peabody Museum of Natural History depicts the evolution of mammals over the past 66 million years. At 60 feet wide, it is one of largest murals in the world.
Artist Rudolph Zallinger’s famous “Age of Mammals” mural at the Yale Peabody Museum of Natural History depicts the evolution of mammals over the past 66 million years. At 60 feet wide, it is one of largest murals in the world.

*If you are interested in learning more about Walter Alvarez’s scientific journey and his process of science, the Understanding Science website has an excellent interactive narrative describing his discovery.

DinoByte Wednesday: Fossils and the DIG Field School

It is thrilling to find fossils and know you are the first to uncover those remains of an ancient world. Word of new, large fossil discoveries, like the recent uncovering of a sauropod dinosaur in Argentina, makes news headlines across the globe. However, much of the information paleontologists use to reconstruct paleoenvironments comes from the study of microfossils. Microfossils, as you might imagine, are very small, and therefore require a microscope to properly examine. The picture below will give you a better idea of the scale of fossils we find.

A sample of fossils that might be found on the outcrop in the Hell Creek area in northeastern Montana. Photo credit: Greg Wilson.
A sample of fossils that might be found on the outcrop in the Hell Creek area of northeastern Montana. Photo credit: Greg Wilson.

Many different animals are represented here, including dinosaurs, turtles, and fish. Let’s take a closer look at the type of animals we find fossilized during the DIG.

Mammals

The mammal fossils found at the DIG field site are from three major groups: marsupials, placentals, and multituberculates (called the “rats of the Mesozoic,” see below). Due to the fragility of bones of these animals, we primarily find their teeth and jaw bones. The main focus of DIG Executive Director Dr. Greg Wilson’s lab is the evolution and ecology of early mammals in the context of major earth history events. Specifically, Greg investigates change across the K/Pg boundary by examining mammalian tooth shape and diet, and relative abundances of different species through time. Although teeth are by far the most commonly found elements, there are a few other bones of mammals we can find in Hell Creek. Lauren DeBey, a graduate student in the Wilson lab, and DIG Field School Assistant Director, studies the limb elements (e.g., femur, humerus) of these small mammals to assess changes in locomotion in relation to the K/Pg extinction event.

(http://ircamera.as.arizona.edu/NatSci102/NatSci102/text/extpaleocene.htm) Depiction of the multituberculate mammal, Ptilodus (top), that lived during the Paleogene Period. In Hell Creek, many fossilized mammalian jaws are found like those shown here (bottom) from a multituberculate.
Depiction of the multituberculate mammal, Ptilodus (top), that lived during the Paleogene Period. In the Hell Creek, many fossilized multituberculate teeth are found like the one shown here (bottom). Multituberculates are named for the shape of these molar teeth, which are composed of “many tubercles” or “many bumps.” Fossil photo credit: Greg Wilson.

Dinosaurs

In the Hell Creek Formation, we find representatives of both major dinosaur groups, the Saurischians (“lizard-hipped” dinosaurs), and the Ornithischians (“bird-hipped” dinosaurs). As with mammals, the most common dinosaur microfossils we find are teeth because dinosaurs constantly shed their teeth, and teeth are the hardest substance in the body. Carnivorous saurischian dinosaurs from the Hell Creek Formation include the raptors Dromaeosaurus and Saurornitholestes, and the Tyrannosaurus rex. Ornithischian dinosaurs we find include the herbivorous Triceratops, which was so common on the Cretaceous landscape their nickname is the “cows of the Mesozoic.” We also find duck-billed ornthiscian dinosaur remains, often toe bones from Edmontosaurus.

(dinosaur: http://www.theguardian.com/science/2013/jul/15/t-rex-tooth-embedded-prey-dinosaur); Teeth from Tyrannosaurus rex (left) and Triceratops (right) are common finds at the DIG field site. Recently, a T. rex tooth (link T. rex tooth with guardian article) was found embedded in the vertebra of a plant eating dinosaur, suggesting the scavenging T. rex also actively hunted its prey. Photo credit: Dave DeMar.
Serrated teeth from Tyrannosaurus rex (left) and leaf-shaped teeth from Triceratops (right) are common finds at the DIG field site. Recently, a T. rex tooth was found embedded in the vertebra of a plant eating dinosaur, suggesting the scavenging T. rex also actively hunted its prey (middle). Fossil photos credit: Dave DeMar.

Reptiles

In addition to dinosaurs, many other reptilian groups are preserved in the Hell Creek, including common finds like turtles, crocodiles, champsosaurs, and more rare finds like lizards, snakes, birds, and winged pterosaurs (related to Pterodactyls). Fossilized turtle shells are very common, and the majority come from soft-shelled aquatic species. Crocodile microfossils include teeth, vertebrae, and scutes (flat-plate-like bones embedded in the skin). Champosaurs were mostly aquatic, crocodile-like reptiles and we find mainly teeth and vertebrae from these creatures that went extinct over 50 million years ago (mya). Lizards and snakes are more rare finds in the Hell Creek, most often found as jaws and vertebrae.

Amphibians

We know of three groups of amphibians, two living and one now extinct, that inhabited the Hell Creek region. Of the groups still living today, we find the jaws and vertebrae of salamanders, and less commonly the jaws, skull parts, and hip bones from frogs. We also find jaws and vertebrae of extinct, salamander-like amphibians called albanerpetontids. A graduate student in the Wilson Lab at UW, Dave DeMar, studies the fate of amphibian groups across the K/Pg extinction boundary.

A sampling of sirenid and albanerpetonid tooth bearing elements found by Dave DeMar in the Hell Creek Formation in northeastern Montana (Wilson et. al 2014).
A sampling of sirenid and albanerpetontid tooth-bearing elements (maxillae and mandibles) from the Hell Creek Formation in northeastern Montana (Wilson et. al 2014; photo credit: Dave DeMar).

Fishes

The Hell Creek region preserves fossils from both cartilaginous and bony fish. The two most abundant cartilaginous fishes found here are sharks and rays, (yes, sharks and rays are as old as the dinosaurs!). Since the skeletons of these fish are made of cartilage, we generally only find their teeth and placoid (or “tooth-like”) scales in the fossil record.

The most common cartilaginous fish fossil found in Hell Creek is the flat, hexagonal shaped, double rooted tooth (left) of the ray Myledaphus bipartitus depicted as a cartoon on the right (https://cumuseum-archive.colorado.edu/Exhibits/BioLounge/HarvesterAnts/ray.html).
The most common cartilaginous fish fossil found in Hell Creek is the flat, hexagonal shaped, double rooted tooth (left) of the ray Myledaphus pustulosus depicted as a cartoon on the right. (Fossil photo credit: Dave DeMar).
Outer and inner views of fossilized scales from a gar fish found in Hell Creek.
Outer and inner views of fossilized scales from a gar fish found in the Hell Creek. Photo credit: Dave DeMar.

 

Remains of bony fish found in our field area include scales, vertebrae, jaws, teeth, and skull elements from primitive bony fish (some that are still alive today!) like the paddlefish, gar, and bowfin, and more derived teleost fish like Coriops. The most common fossil we find in the Hell Creek area is a gar fish scale, which are easily recognized by their (usually) black color, and flat, shiny surfaces.

Different anatomical views of fossilized vertebrae, from top to bottom row, from bowfin, gar, and teleost fish.
Different anatomical views (columns) of fossilized vertebrae of common fish found in the Hell Creek. The rows of fish are: bowfin (top), gar (middle), and a teleost fish (bottom). If you find a fossil that resembles a hockey puck, you can be fairly certain you’ve found a fish vertebrae! Photo credit: Dave DeMar.

In total, the vertebrate microfossils found at our field site represent over 125 different species! Because these microfossils are so abundant they provide a more complete picture of the vertebrate fauna. And, since they are from multiple fossil horizons spanning different geologic time periods they allow us to paint a detailed picture of the last two million years of the Cretaceous Period and first one million years of the Paleogene.

If there are fossils from 125+ species at the DIG field site, how will you know what you’ve found? Well, fossils can be distinguished based on characteristic shapes (circular, thin and flat, cone-shaped, flat with pegs) or textures (smooth, pitted, bumpy). By observing the shape and texture of the fossils, it quickly becomes easier to pinpoint what kind of fossil you have found.

Next week we will DIG deeper into the causes of the K/Pg mass extinction, and the evidence found in the Hell Creek for the end of the dinosaurs and some 75% of species on earth!