Scientists Describe Therizinosaur and Hadrosaur Tracks from Alaska

A team of international scientists have published a report on a series of dinosaur trackways found in Upper Cretaceous rocks located in Denali National Park, Alaska.  The tracks show the presence of duck-billed dinosaurs (Hadrosaurs) and bizarre herbivorous Theropods (Therizinosaurs), in the same location at potentially the same time.  If this is the case, then it could be speculated that these very different dinosaurs benefited from an association, just as some animals today congregate together for mutual benefit.

The Hadrosaur and Therizinosaur trackways represent the first report from North America of co-occurrence between these very different dinosaurs and the trace fossils support the idea that the dinosaur population in Alaska in the Late Cretaceous was similar in composition to that associated with Asia (Nemegt Formation).  Alaska could have been a “highway” linking the dinosaur faunas of Asia with the North American continent.

Did Therizinosaurs and Hadrosaurs Live Side by Side?

A series of Hadrosaur and Therizinosaur tracks have been discovered together in the Denali National Park of Alaska.

Picture Credit: Masato Hattori

It Started with a Single Footprint

The lower Cantwell Formation represents a series of sedimentary deposits laid down in a terrestrial environment and fossil pollen analysis suggests a Late Campanian to early Maastrichtian temporal setting (73-71 million years ago).  Numerous vertebrate and plant fossils are associated with this strata, vertebrates including fishes, pterosaurs and numerous dinosaurs.  Lead author of the paper, published in the journal “Scientific Reports”, Anthony Fiorillo, (Perot Museum of Nature and Science, Dallas, Texas), had previously described a single four-toed print found in the area back in 2012.  This track was identified as having been made by a Therizinosaur and was the first evidence found of these strange Theropods living at such high latitudes.

To read more about the 2012 fossil discovery: Potential Therizinosaur Track Discovered in Alaska

The discovery of a single fossil leaf, resembling that of a waterlily from the same site as the Therizinosaur and Hadrosaur trackways suggest the prints were made by dinosaurs as they crossed a shallow body of water away from the main river channels.  During the Late Cretaceous this part of North America was a vast wetland habitat.

A Photograph and Line Drawing of the Waterlily Fossil

A fossil of a waterlily leaf found in Upper Cretaceous rocks of Alaska.  Photograph (a) and line drawing (b) of nymphaceous leaf found in study area.  This plant is indicative of a body of standing water.

Picture Credit: Scientific Reports

A More Detailed and Thorough Mapping of the Area

A field team returned to this location in 2013 and 2014 an mapped a series of dinosaur tracks, unearthing dozens more four-toed tracks that were identified as Therizinosaur prints.  The researchers were surprised to discover that the Therizinosaur tracks seem to co-occur with lots of tracks indicating herds of Hadrosaurs.

Commenting on the significance of these trace fossils, Dr Fiorillo stated:

“Hadrosaurs are very common and found all over Denali National Park.  Previously, they had not been found alongside Therizinosaurs in Denali.  In Mongolia, where Therizinosaurs are best known, though no footprints have been found in association, skeletons of Hadrosaurs and Therizinosaurs have been found to co-occur from a single rock unit so this was a highly unusual find in Alaska and it prompted my interest.”

A Photograph and Accompanying Diagram Showing Some of the Associated Trace Fossils

A photograph of a large block within the study area showing the co-occurrence of Hadrosaur and Therizinosaur tracks.

Picture Credit: Scientific Reports

The picture above (a) shows a large block of stone representing a single bedding plane with two distinctive trackways highlighted (note the geology hammer, closest to the uppermost yellow circle that provides scale).  Diagram (b) illustrates the two types of trackway found, yellow prints and circles indicate Therizinosaur, whilst blue prints and circles indicate trace fossils made by Hadrosaurs.

The scientists deduced that from the range of sizes of the Hadrosaur tracks (pes prints), these trace fossils represented groups of duck-billed dinosaurs of different ages, with younger animals associating with larger, fully-grown adults.

A Highway Linking North America to Alaska – A Dinosaur Driveway

The scientists state that this co-occurrence of Therizinosaurs and hadrosaurids at this single locality within the lower Cantwell Formation has not been documented elsewhere in North America.  This dinosaur co-occurrence is more characteristic of dinosaur biota associated with contemporaneous rocks found in central Asia.  The team speculate that the Alaska of the Late Cretaceous represented a gateway for faunal exchange between the two continental landmasses.  The existence of a Cretaceous Beringian land bridge prompted this mixing of faunas, which was encouraged as similar habitats were present within continental North America and Asia.

Co-author of the scientific paper, Dr Yoshitsugu Kobayashi (Hokkaido University Museum, Japan), reaffirmed the team’s conclusion stating:

“This study helps support the idea that Alaska was the gateway for dinosaurs as they migrated between Asia and North America.”

A Fondness for Marshland?

A report from Asia has commented on the presence of both Therizinosaurs and hadrosaurids at the same location.  Fossils of both these types of dinosaur being found in the same strata, in a sequence of sediments that indicate the palaeoenvironment was very wet at the time, relative to the sequence of rocks deposited above and below the fossil layer.  The authors of this newly published study, suggest that Therizinosaurs and Hadrosaurs liked to live in wetter locations, such as marshland.

Did Therizinosaurs and Hadrosaurs Co-exist?

It is possible the tracks were made at different times and the these two different types of herbivorous dinosaur did not interact.  However, given the similarity of track preservation, the research team conclude that it is likely that these two taxa occupied the same environment at the same time, but why would two dinosaurs want to hang out together?

There are probably a number of reasons for this co-existence with one group of dinosaurs at least tolerating the presence of another megaherbivore, two possibilities are speculated upon within the scientific paper and summarised below:

1. Not competing with each other for food – the teeth and jaws of hadrosaurids and Therizinosaurs are very different.  The two herbivores probably fed on different types of plant food and therefore they were not in direct competition with each other for food.
2. Mutual protection/spotting predators – studies have suggested that Therizinosaurs had excellent hearing and a well-developed sense of smell, their long necks gave them an excellent field of view.  Although, an excellent sense of smell has been proposed for at least some duck-billed dinosaurs (Lambeosaurinae), re-evaluation of the nasal cavities of some Hadrosaurs has suggest that their sense of smell was not that remarkable.  It is tempting to consider the differences in sensory adaptations and capabilities in these taxa that might have served a role as a mutually beneficial predatory avoidance mechanism for the more inclusive herd.  Zebras and ostriches are often found feeding together, the sharp, colour vision of the ostrich compliments the zebra’s better developed sense of smell, helping to detect the approach of predators, thus benefiting all the grazing animals.

Examples of Mutual Association (Extinct and Extant)

Zebras and ostriches benefit from the presence of each other when it comes to sensing predators.  Perhaps, duck-billed dinosaurs and Therizinosaurs had a similar mutually beneficial relationship.

Picture Credit: Everything Dinosaur

Although the scientists speculate on the reasons why these plant-eaters might have associated, they suggest it is more likely that Therizinosaurs and Hadrosaurs gathered together simply because they preferred the same habitat.

Study Suggests Shift from Lamniform to Carcharhiniform Dominated Shark Populations

The Cretaceous mass extinction event saw the demise of the non-avian dinosaurs on land, but in the seas there was a massive faunal turnover too.  For example, many of the marine reptiles became extinct.  However, sharks seem to have come through the K-Pg extinction event largely unscathed, but a new study, published in the journal “Current Biology” suggests a subtle change in the diversity of shark species, a change that is reflected in extant shark species today.

The researchers, which include scientists from Uppsala University (Sweden) and the University of New England (Australia), examined hundreds of fossilised shark teeth across the Cretaceous through to the Palaeogene and their study concludes that modern shark biodiversity was triggered by the mass extinction event that took place approximately 66 million years ago.

A Late Cretaceous (Maastrichtian) Marine Faunal Assemblage

A Late Cretaceous sea scene.

Picture Credit: Julius Csotonyi

Hell’s Aquarium

The Upper Cretaceous marine deposits of Kansas have provided palaeontologists with an insight into the number of large predators that inhabited the sea during the last few million years of the Mesozoic.  There were numerous types of Mosasaur, such as Hainosaurus which was as long as Tyrannosaurus rex.  In addition, you had enormous marine turtles as well as the elasmosaurids, giant Plesiosaurs that measured up to fifteen metres in length.  Then you have the fish, brutes like Xiphactinus (zie-fak-tin-us) with a mouth lined with needle-sharp teeth and the sharks, lots of species, some of which were apex predators, such as Squalicorax and Cretoxyrhina.

Hell’s Aquarium – Many Different Types of Predator in Late Cretaceous Seas

Typical Western Interior Seaway marine life.

Picture Credit: Everything Dinosaur

Carcharhiniformes and Lamniformes – Two Different Types of Shark

Today, there are two major groups of predatory sharks.  Firstly, there are the Carcharhiniformes, otherwise called “ground sharks”, typically represented by species such as the dangerous bull and tiger sharks, as well as the enigmatic hammerhead shark and closer to home, the lesser spotted dogfish (Scyliorhinus canicula) which is resident in British waters.  There are around 270 extant species within this clade.

Secondly, there are the  Lamniformes, or the “mackerel sharks” which has around fifteen species and includes the great white, mako and another resident of British waters, the Porbeagle shark (Lamna nasus).  Back in the Cretaceous, things were very different, the Lamniform sharks, in particular, a diverse group of great-white-like sharks, members of the family Anacoracidae, were much more numerous.  The fearsome Squalicorax was an anacoracid and therefore along with Cretoxyrhina, members of the Lamniformes clade.

Lead author of the study, PhD student at Uppsala University, Mohamad Bazzi stated:

“Our study found that the shift from Lamniform to Carcharhiniform-dominated assemblages may well have been the result of the end-Cretaceous mass extinction.  Unlike other vertebrates, the cartilaginous skeletons of sharks do not easily fossilise and so our knowledge of these fishes is largely limited to the thousands of isolated teeth they shed throughout their lives.  Fortunately, shark teeth can tell us a lot about their biology, including information about diet, which can shed light on the mechanisms behind their extinction and survival.”

Fossil Shark Teeth

A successful fossil hunt.  Shark teeth study reveals shift from Lamniform to Carcharhiniform dominated shark populations.

Picture Credit: Everything Dinosaur

Cutting-edge Analytical Techniques

The researchers used state-of-the-art analytical techniques to explore the variation of tooth shape in Carcharhiniformes and Lamniformes and measured diversity by calculating the range of morphological variation, also called disparity.

Dr Nicolás Campione, (University of New England), a co-author of the study added:

“Going into this study, we knew that sharks underwent important losses in species richness across the extinction.  But to our surprise, we found virtually no change in disparity across this major transition.  This suggests to us that species richness and disparity may have been decoupled across this interval.”

A More Complex Picture

Despite this seemingly stable pattern, the study found that extinction and survival patterns were substantially more complex.  Morphologically, there were differential responses to extinction between Lamniform and Carcharhiniform sharks, with evidence for a selective extinction of Lamniformes and a subsequent proliferation of Carcharhiniformes in the immediate aftermath of the extinction.

Student Bazzi explained:

“Carcharhiniforms are the most common shark group today and it would seem that the initial steps towards this dominance started approximately 66 million years ago.”

Although the reasons for the shift in shark species are not that clear, the researchers hypothesise that the extinction of various types of prey such as the marine reptiles and ammonites may have played a significant role.  In addition, it is likely that the loss of apex predators (such as Lamniformes and marine reptiles) benefited secondary predator sharks, a role fulfilled by many Carcharhiniforms.

Dr. Campione concluded:

“By studying their teeth, we are able to get a glimpse at the lives of sharks and by understanding the mechanisms that have shaped their evolution in the past, perhaps we can provide some insights into how to mitigate further losses in current ecosystems.”