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!




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