Paleontology: The Crocodyliforms of Prehistoric Montana

A Journey Through Time: The Crocodyliforms of Prehistoric Montana

Venture back millions of years to a Montana teeming with life, a landscape both alien and vaguely familiar, where giants roamed and the waters concealed formidable predators. This was a world dominated by dinosaurs, but lurking in the rivers, swamps, and even on land, was another dynasty of reptiles: the crocodyliforms. These ancient relatives of modern crocodiles and alligators were a diverse and successful group, playing a crucial role in the ecosystems of prehistoric Montana for tens of millions of years. Their story, etched in the fossil-rich sedimentary rocks of the state, is one of remarkable adaptation, survival, and a testament to the enduring legacy of the archosaurs.

This extensive exploration will transport you through the vast geological timeline of Montana, from the lush, warm environments of the Late Cretaceous to the world that emerged from the ashes of a cataclysmic extinction event. We will delve into the various geological formations that have made Montana a paleontological treasure trove, uncovering the unique crocodyliforms that called these ancient environments home. From a newly discovered, diminutive terrestrial crocodyliform to the robust, turtle-crushing jaws of another, the fossil record of Montana reveals a fascinating chapter in the evolutionary history of these incredible reptiles. We will examine their anatomy, their presumed behaviors, their interactions with the famed dinosaurs they lived alongside, and their ultimate fate in a changing world.

The World of the Western Interior Seaway: A Crocodyliform Paradise

During much of the Late Cretaceous, from roughly 100 to 66 million years ago, North America was a continent divided. A vast, shallow sea known as the Western Interior Seaway stretched from the Arctic Ocean to the Gulf of Mexico, splitting the landmass into two smaller continents: Laramidia to the west and Appalachia to the east. Montana was situated on the eastern coastal plain of Laramidia, a dynamic landscape of rivers, deltas, and swamps bordering the seaway. This environment, characterized by a mild, subtropical climate with no prolonged freezes, was an ideal habitat for a wide array of life, including a flourishing population of crocodyliforms.

The constant shifting of the seaway's coastline, along with the sediment washed down from the newly forming Rocky Mountains to the west, created the perfect conditions for the preservation of fossils. As a result, geological formations from this period in Montana, such as the Blackleaf, Two Medicine, Judith River, and the world-renowned Hell Creek Formation, have yielded an astonishing wealth of paleontological information, offering a detailed glimpse into these ancient ecosystems.

A Startling Discovery in the Blackleaf Formation: The Unexpected Terrestrial Hunter

Our journey begins approximately 95 million years ago, in the middle of the Cretaceous period, within the rocks of the Blackleaf Formation in southwestern Montana. This formation preserves an environment that was at the edge of the Western Interior Seaway. While dinosaurs have been found in the Blackleaf Formation, other vertebrate fossils are exceedingly rare, making any new discovery particularly significant.

In the summer of 2021, a remarkable find was made by a student from Montana State University. A tiny fossil, no bigger than the tip of a pinkie finger, with an unusual texture, turned out to be part of a small, exquisitely preserved crocodyliform skull. This discovery led to the identification of a completely new species, and indeed a new family, of crocodyliforms.

Named Thikarisuchus xenodentes, meaning "strange-toothed sheath crocodile," this creature challenges many preconceived notions about ancient crocodilians. The juvenile specimen, nicknamed "Elton," was a mere two feet in length, and it is estimated that an adult would have reached only about three feet long, making it significantly smaller than most of its neosuchian relatives, the group that includes modern crocodiles and their closest extinct kin.

What truly sets Thikarisuchus apart is its inferred lifestyle. Unlike the vast majority of neosuchians, which are semi-aquatic or marine carnivores with simple, conical teeth, Thikarisuchus was a terrestrial animal. Its name, "strange-toothed," refers to its varied and specialized dentition, which suggests a diet that was not limited to meat. Paleontologists believe that Thikarisuchus was likely an omnivore, feeding on a combination of plants, insects, and small animals. This is a striking departure from the typical image of a crocodyliform as a purely carnivorous aquatic ambush predator.

The way in which the fossil was preserved also offers clues to its behavior. The bones of "Elton" were found in a densely concentrated manner, a pattern consistent with animals that have been preserved within burrows. This suggests that Thikarisuchus may have been a burrowing animal, a behavior that would have offered protection from predators and the elements in its terrestrial environment. This discovery highlights the incredible diversity of crocodyliforms in the past, with some adapting to niches that are completely unoccupied by their modern relatives. The existence of a small, land-dwelling, omnivorous crocodyliform in the Cretaceous of Montana underscores just how much there is still to learn about these fascinating reptiles.

The Judith River Formation: A Diverse Crocodyliform Assemblage

Moving forward in time to the Campanian stage of the Late Cretaceous, between 79 and 75.3 million years ago, we find ourselves in the world preserved by the Judith River Formation. This historically significant formation, where some of the first dinosaur remains in North America were discovered, represents a coastal plain environment with freshwater continental deposits, including crossbedded channel sandstones, gray siltstones, and carbonaceous shales. The climate was warm and seasonally dry, transitioning to a more consistently wet regime in the later part of the formation's deposition. This environment supported a rich and diverse fauna, including a variety of dinosaurs, fish, amphibians, lizards, mammals, and, of course, crocodyliforms.

The crocodyliforms of the Judith River Formation were a varied group, reflecting the diverse aquatic habitats available. While the fossil record of crocodyliforms from this formation is not as completely understood as that of the later Hell Creek Formation, several key players have been identified. Among them are members of the genus Leidyosuchus, a medium-sized alligatoroid with a skull length of over 40 centimeters (16 inches). Leidyosuchus was once a "wastebasket taxon," with many different species assigned to it. However, a reevaluation of the genus in 1997 led to the reassignment of many of these species to a new genus, Borealosuchus. The type species, Leidyosuchus canadensis, is known from the Dinosaur Park Formation in Alberta, which is equivalent in age to parts of the Judith River Formation, and it is likely that this or a closely related species inhabited the rivers and swamps of prehistoric Montana. With its robust build, Leidyosuchus was likely a generalist predator, preying on fish, turtles, and any small dinosaurs or mammals that ventured too close to the water's edge.

Another significant crocodyliform from this time was Brachychampsa montana, an alligatoroid with a particularly interesting diet. Brachychampsa is characterized by a short, blunt snout and robust, bulbous teeth in the back of its jaws. This specialized dentition was not designed for piercing flesh, but for crushing hard objects. For this reason, Brachychampsa is believed to have been a durophagous predator, specializing in eating hard-shelled prey like turtles and large mollusks, which were abundant in the Hell Creek Formation. While the idea of a turtle-eating crocodile is the prevailing hypothesis, some researchers have pointed out that many of the contemporary turtles were quite large, suggesting that Brachychampsa may have preyed on juveniles or smaller turtle species. It has also been proposed that Brachychampsa may have had a more generalist diet that included a variety of invertebrates and small vertebrates.

The presence of these different crocodyliforms in the Judith River Formation paints a picture of a complex ecosystem with a high degree of niche partitioning. The more generalized predator Leidyosuchus likely occupied the main waterways, while the specialized Brachychampsa may have foraged in shallower waters or areas with a high concentration of its hard-shelled prey.

The Two Medicine Formation: A Glimpse into a More Upland Environment

Contemporaneous with the Judith River Formation, the Two Medicine Formation, deposited between 82.4 and 74.4 million years ago, offers a look at a slightly different environment. While the Judith River Formation represents a more coastal plain setting, the Two Medicine Formation, located further to the west, preserves a more upland environment, with a seasonal, semi-arid climate. This formation is world-famous for its dinosaur nesting sites, particularly those of the hadrosaur Maiasaura, which have provided invaluable insights into dinosaur social behavior.

The crocodyliform fauna of the Two Medicine Formation is less well-documented than that of other Montana formations. However, given its geographical and temporal proximity to the Judith River Formation, it is highly probable that similar crocodyliforms, such as species of Leidyosuchus and possibly even Brachychampsa, inhabited the rivers and lakes of this region. The presence of crocodyliforms is a key indicator of a warm climate without prolonged freezing temperatures.

The paleoenvironment of the Two Medicine Formation was a mix of forested areas, with conifers, ferns, and horsetails, and more open plains. The presence of volcanic ash layers indicates that this was a tectonically active region. The rivers and lakes of the Two Medicine Formation would have provided a suitable habitat for semi-aquatic predators, and crocodyliforms would have played a vital role in this ecosystem, preying on fish and other aquatic life, as well as being a potential threat to the dinosaurs that came to the water's edge to drink.

The Hell Creek Formation: A World on the Brink

Perhaps the most famous of all of Montana's fossil-bearing formations is the Hell Creek Formation. Dating to the very end of the Cretaceous period, from about 68 to 66 million years ago, this formation captures the last moments of the age of dinosaurs. The Hell Creek Formation preserves a coastal floodplain environment, with a complex network of rivers, swamps, and forests. The climate was subtropical, and the landscape was home to an iconic cast of dinosaurs, including Tyrannosaurus rex, Triceratops, and Edmontosaurus. In this world on the brink of a cataclysmic extinction, crocodyliforms were not only present but thriving.

The crocodyliform diversity in the Hell Creek Formation was impressive, with several species occupying different ecological niches. One of the most common crocodyliforms of this time and place was Borealosuchus sternbergii. Originally assigned to the genus Leidyosuchus, Borealosuchus was a medium-sized crocodyliform, reaching lengths of up to 2.8 meters. It had a generalized, crocodile-like snout and was likely an opportunistic predator, feeding on fish, turtles, and small dinosaurs. Some paleontologists have speculated that Borealosuchus may have preferred the more brackish, coastal environments of the Hell Creek ecosystem, similar to the modern American crocodile, while other crocodyliforms dominated the more inland, freshwater habitats.

Sharing the waterways with Borealosuchus was the specialized turtle-eater, Brachychampsa montana. As in the earlier Judith River Formation, the presence of this durophagous predator in the Hell Creek Formation indicates a healthy population of hard-shelled prey. The coexistence of the generalist Borealosuchus and the specialist Brachychampsa is a classic example of niche partitioning, allowing multiple similar species to thrive in the same environment by exploiting different resources.

A third crocodyliform from the Hell Creek Formation was Thoracosaurus neocesariensis, a long-snouted reptile that was once thought to be a close relative of the modern gharial. While its exact evolutionary relationships are still debated, Thoracosaurus was a significant component of the Late Cretaceous and Early Paleocene aquatic ecosystems. Its long, slender snout was well-suited for catching fish, suggesting a piscivorous diet.

The presence of these diverse crocodyliforms highlights the health and complexity of the Hell Creek ecosystem. They were a vital part of the food web, acting as both predators and prey. While a large adult Tyrannosaurus rex would have had little to fear from a Borealosuchus, young or unwary dinosaurs that came to drink at the water's edge would have been vulnerable to ambush. Evidence of such interactions is rare in the fossil record, but the possibility of dinosaur predation by these ancient crocodyliforms is a tantalizing one.

The Great Extinction and the Dawn of a New Era

Approximately 66 million years ago, the world of the Late Cretaceous came to an abrupt and catastrophic end. An asteroid, estimated to be at least 10 kilometers (6 miles) in diameter, slammed into the planet in the region of the present-day Yucatán Peninsula in Mexico. The impact triggered a global environmental crisis, including widespread wildfires, a "nuclear winter" caused by dust and debris blocking out the sun, and a collapse of food chains. This event, known as the Cretaceous-Paleogene (K-Pg) extinction, led to the demise of an estimated 75% of all species on Earth, including all non-avian dinosaurs.

Yet, amidst this devastation, some groups of animals managed to survive, and among the most notable survivors were the crocodyliforms. How did these reptiles weather a storm that wiped out the mighty dinosaurs? The answer lies in a combination of their unique physiology and their habitat.

As ectotherms, or "cold-blooded" animals, crocodyliforms have a much slower metabolism than endothermic ("warm-blooded") animals like dinosaurs. This means they require significantly less food to survive. Modern crocodiles can go for months, and in some cases over a year, without eating. In the post-impact world, where food was scarce, this ability to endure long periods of starvation would have been a crucial advantage.

Furthermore, many crocodyliforms were semi-aquatic, and freshwater ecosystems appear to have been less severely affected by the extinction event than marine or terrestrial environments. Aquatic food webs are often based on detritus – dead organic matter – which would have been plentiful in the aftermath of the impact. This provided a food source for the small fish and invertebrates that a starving crocodyliform could have scavenged.

The ability to burrow, as suggested for Thikarisuchus, would have also offered protection from the immediate effects of the impact, such as the intense heat from the initial blast and the subsequent environmental fallout. This combination of a slow metabolism, a versatile diet that could include carrion, and a protective aquatic or burrowing lifestyle allowed some crocodyliforms to make it through the K-Pg extinction.

The fossil record of Montana provides a clear picture of this survival and subsequent recovery. In the rocks of the Fort Union Formation, which were deposited during the Paleocene epoch immediately following the extinction event, we find the remains of crocodyliforms that are either direct descendants of or closely related to the species of the Hell Creek Formation. The Fort Union Formation preserves a world in which the dinosaurs are gone, and mammals are beginning their rise to dominance. This was a time of extensive swamps and river systems, providing ample habitat for the surviving crocodyliforms.

Borealosuchus was one of the success stories, with species of this genus persisting well into the Eocene. In the absence of large dinosaurian predators, Borealosuchus may have even stepped into the role of a top predator in some early Paleocene ecosystems, preying on the burgeoning populations of mammals. Brachychampsa also survived the extinction, though it disappeared from the fossil record in the early Paleocene.

The Curious Case of the Champsosaurus

No discussion of the crocodile-like reptiles of prehistoric Montana would be complete without mentioning Champsosaurus. Fossils of this creature are found in several of Montana's Late Cretaceous and Paleocene formations, often alongside true crocodyliforms. With its long, slender snout, Champsosaurus bears a striking resemblance to a gharial, and for a long time, it was thought to be a type of crocodile. However, Champsosaurus belongs to a completely separate order of reptiles called the Choristodera.

Choristoderes were a diverse group of semi-aquatic diapsid reptiles that first appeared in the Jurassic and survived until the Miocene. The similarity in appearance between Champsosaurus and gharials is a remarkable example of convergent evolution, where two unrelated groups of animals evolve similar features to adapt to a similar lifestyle.

Champsosaurus was a highly specialized aquatic predator, preying on fish in the freshwater rivers and swamps of prehistoric Montana. Its powerful jaw muscles and long, tooth-lined snout were perfectly adapted for catching fast-moving prey. Like the true crocodyliforms, Champsosaurus was a survivor of the K-Pg extinction, and its fossils are found in Paleocene deposits in Montana and elsewhere. Some species of Champsosaurus that lived after the extinction event even grew larger than their Cretaceous ancestors, a rare occurrence among reptiles in the post-impact world.

The story of Champsosaurus serves as a reminder that the prehistoric world was full of evolutionary experiments, and that looks can be deceiving. While not a true crocodyliform, its presence alongside them in the fossil record of Montana adds another layer of complexity and fascination to the story of these ancient ecosystems.

The Enduring Legacy of Montana's Prehistoric Crocodyliforms

The crocodyliforms of prehistoric Montana were a diverse and resilient group of reptiles. From the small, terrestrial omnivore Thikarisuchus to the powerful, turtle-crushing Brachychampsa and the common and adaptable Borealosuchus, these ancient relatives of modern crocodiles and alligators played a significant role in the ecosystems of the Late Cretaceous and beyond. They were a testament to the evolutionary creativity of the archosaurs, adapting to a wide range of ecological niches and interacting in complex ways with the dinosaurs and other fauna of their time.

Their survival of the K-Pg extinction, an event that wiped out so many other magnificent creatures, is a powerful story of resilience. Their ability to endure in a world of devastation highlights the advantages of their unique physiology and lifestyle. The fossil record of Montana, with its rich and continuous sequence of rock formations spanning this critical period in Earth's history, provides an unparalleled window into the world of these incredible survivors. The ongoing discoveries of new species and the re-evaluation of existing ones ensure that the story of Montana's prehistoric crocodyliforms will continue to unfold, offering new insights into the deep history of life on our planet.