The Rise of Mammals in New Mexico

A warm-blooded look at New Mexico's fossil record

Dr. Matt Heizler (New Mexico Geochronology Lab, New Mexico Tech, Socorro) looks for minerals that can be radioisotopicallly dated. Photograph courtesy Thomas E. Williamson.
by Thomas E. Williamson

Walking alone over the uneven ground of the badlands of the San Juan Basin, David Baldwin tugged again at the bridle of his mule, leading it around a hoodoo—a tall pinnacle of banded rock. Baldwin could see his breath in the cold, crisp New Mexico winter air. It was 1879. Baldwin preferred to search for fossils in the winter. He could gather snow from the north- facing slopes and melt it for drinking water in this parched land.

Something unusual caught his eye. He stooped down for a closer look. It was a small bit of jawbone with black teeth—a fossil jaw of an early Paleocene mammal—jutting from the mudstone. Baldwin removed his gloves and blew warm air on his cold hands before carefully lifting it from the rock. He reached under his poncho and removed an empty rectangular tobacco tin from his shirt pocket, carefully placing the jaw inside. Then he saw another jaw, and another…

At least that’s what I imagine happened as I scan the badlands before me. I’m looking over the same exposures that Baldwin trod 140 years ago. Baldwin discovered the first mammal fossils from these particular rocks, a spectacular set of badland exposures in the Nacimiento Formation. The name refers to a traditional nativity scene, which is appropriate considering that the fossils that Baldwin collected proved to be the first early Paleocene mammal fossils found anywhere in the world. Moreover, the Paleocene marks the first appearance of placental mammals, the group of mammals that now dominates the Earth.

Baldwin shipped these fossils to Othniel Charles Marsh at Yale, but after having to wrangle with Marsh over payment, he shipped later discoveries to Marsh’s major competitor and nemesis Edward Drinker Cope. Shortly thereafter, Cope and Marsh engaged in a long, bitter, and highly public fight over the new fossil discoveries made in the West, known as the “Bone Wars.” Cope was quicker in recognizing the importance of the fossils discovered by Baldwin and published a flood of papers over the next ten years that described and named many extinct animals. New Mexico fossils provided much of the earliest evidence for the rise of mammals following the extinction of the dinosaurs.

Near the end of the nineteenth century, the American Museum of Natural History in New York, the preeminent museum of its kind, opened its doors to a public fascinated by the latest fossil discoveries made out west. Henry Fairfield Osborn, the museum’s first curator of the new Department of Vertebrate Paleontology, authorized the purchase of Cope’s collection, including his thousands of Paleocene mammals from New Mexico. This became the kernel of the museum’s paleontological collection. Osborn sent several expeditions from New York to New Mexico to collect more Paleocene fossils over the next several decades. These crews collected many thousands of additional mammal specimens that still reside in the museum’s metal cabinets in the heart of Manhattan.

Western North America contains the very best record in the world of what happened to life on land during the last few million years of the Cretaceous and the first few million years of the Paleocene, the first epoch of the Cenozoic—the beginning of the Age of Mammals. At this time, the lands of the West were undergoing a bout of mountain-building called the Laramide Orogeny. This resulted in a buckling of the crust behind the proto Rocky Mountains that accumulated thick sequences of sediment, called strata.

New Mexico’s San Juan Basin is one of these structural basins, one of the southernmost of North America, that accumulated strata through this time. These strata were subsequently lifted and are now eroding in spectacular badlands, revealing the story of the distant past to those who are able to read it.

Mammals originated at about the same time as the dinosaurs—about 230 million years ago, near the end of the Triassic Period. The first mammals were small, unassuming animals and they remained relatively small through the Age of Reptiles. Early in the Mesozoic, mammals evolved some key adaptations that allowed them to thrive in a world dominated by reptiles. It has been said that during the Mesozoic, mammals lived in the shadows of the dinosaurs. They became warm-blooded, which allowed them to be active even in cooler temperatures; they developed whiskers and fur (whiskers are special hairs that are connected to nerves and are used to sense in the dark). They evolved exquisite hearing via a unique system that incorporates bones that previously had been part of the lower jaw. They also evolved the ability to hear a very wide range of sounds, from low to high frequencies. These adaptations allowed mammals to be active at night, when most reptiles slept.

Other evidence to support this scenario is offered by the study of the mammalian genes that govern vision. Most mammals, for example, lack color vision. The most light-sensitive cells in the retina can distinguish only black and white. The ability to distinguish color among our distant ancestors was partly lost during this time when mammals took to a life in the dark. Only much later did a few mammals, such as some primates, reacquire color vision.

Mammals also developed a unique way to process food, probably connected to their need to extract as much nutrition as possible given the high energy demands required to maintain warm-bloodedness. They evolved a precision bite in which the upper and lower teeth each have a complicated pattern of raised cusps and ridges that interlock when the jaws are closed. Mammals masticate; that is, they chew their food to help break it down into smaller pieces before it is swallowed. In order to retain this interlocking pattern, mammals have reduced the replacement of their teeth so that most mammals, including humans, only replace some teeth once, replacing an initial set of deciduous (“baby”) teeth gradually with a permanent set as they mature.

The first mammals probably laid eggs, similar to just a few living mammals (the monotremes, which include the echidna and the platypus). Some mammals went on to evolve live birth. Marsupials and placentals are the only other surviving groups of these mammals. Marsupials give birth to poorly developed young that further develop in a pouch, or marsupium. Placental mammals, the predominant group of living mammals, retain the developing fetus in the body until it is much more developed before giving birth. All mammals share the trait of lactation.

The fossils of Mesozoic mammals are generally very rare, and while exceptional skeletons have been recovered, most fossils are represented only by jaws and teeth. Fortunately, thanks to their quickly evolving, complex, interlocking dentitions, various Mesozoic mammal species can often be identified just by their distinctive teeth. The teeth also give us clues as to their diets and we can look among living mammals for exemplars. We can deduce that small mammals with high, sharp cusps and crests probably ate insects; those with lower, more blunt cusps likely ate seeds and fruit. No Mesozoic mammals appear to have been adapted for chewing leaves.

Through the Mesozoic, mammals may have mostly remained in their burrows by day, venturing out only after nightfall to prey on insects. Until recently, it was thought that mammals remained unspecialized through this time; but some spectacularly well-preserved fossils from China have shown that some mammals had evolved some remarkable adaptations for gliding, digging, and swimming. Some mammals were relatively large. Repenomamus, from the Early Cretaceous (about 125 million years ago) of China, is the largest known Mesozoic mammal, reaching a length of about 3 feet (1 meter) and weighing about 30 pounds (14 kilograms). One specimen was found with a gut containing a juvenile dinosaur (the small ceratopsian dinosaur Psittacosaurus). Most Mesozoic mammals, however, were far smaller—smaller even than mice.

Despite their small size, mammals thrived in the warm Mesozoic world. In the latter half of the Mesozoic, flowering plants, the angiosperms, appeared, and by the end of the Cretaceous era they had become the dominant plants in tropical forests. These fast-growing plants provided new food sources for animals in the form of fruits, abundant seeds, and leaves. Mammals continued to prosper, feeding on the insects and other animals that dined on the flowering plants. A few mammals began eating fruits and seeds of the new plants.

The Cretaceous came to a sudden and fiery end about 66 million years ago. Dinosaurs had dominated the world for over 150 million years, and were still thriving. Tyrannosaurus rex and the giant sauropod (“long-neck”) Alamosaurus roamed the swampy forests of New Mexico.

Then they all came to an end. A large space rock the size of Mount Everest collided with the Earth, hitting the Yucatan peninsula near the present town of Chicxulub, at a velocity about ten times faster than that of a rifle bullet. The impact released the energy of a billion nuclear bombs.

The effects were catastrophic. Huge earthquakes shook the earth. Tsunamis raced from the impact site to splash inland miles from shore. Perhaps the worst bit of luck for the dinosaurs is that the rock hit an area underlaid by a thick deposit of sediments rich in sulfur. The impact vaporized these deposits and blasted them into trajectories that would encircle the earth. The fine dust and condensing droplets of sulfur compounds eventually settled into the atmosphere and drifted aloft, blocking sunlight for years. Sulfuric acid rained down and acidified the oceans, causing further environmental damage. The results were extreme. About 75 percent of all species on Earth went extinct.

Mammals didn’t survive unscathed. Virtually all the mammals known from the latest Cretaceous of North America went extinct. Only one or two species appear to have survived unchanged. A few other mammals also survived, but only after evolving into new species—or perhaps they descended from some unknown species that lived elsewhere.

How were these mammals able to survive? That is still some- thing of a mystery. Cold-blooded reptiles, such as turtles and crocodiles, may have fared better than warm-blooded animals and larger animals—it appears that anything over about 25 kilo- grams (about 55 pounds) went extinct. It may be that the survivors were those animals that were small enough to take shelter in burrows and then later take advantage of the detrital food web, a network of feeding that ultimately relies on consuming decaying organic matter. (There was no shortage of that after the impact.) Presumably, the surviving mammals were feeding primarily on the insects that ate the fungi that were consuming the rotting plants and animals.

What happened in the next few million years is nothing short of amazing. Mammals, now no longer living in the shadow of the dinosaurs, began diversifying and evolving at an explosive rate. They were free to exploit the various environmental niches left vacant by the mass extinction. Mammals didn’t waste any time. It’s this critical time, when mammals first took charge of this new world, that is recorded in the sediments of the Nacimiento Formation of New Mexico.

I have been working for many years now on trying to decipher the geological and paleontological record of Paleocene strata in order to better understand this critical time. I studied the beginning of the Age of Mammals in New Mexico for my doctoral dissertation at the University of New Mexico, and then was hired as a curator at the New Mexico Museum of Natural History and Science soon after. After twenty-five years of fieldwork, the museum has amassed the largest collection of early Paleocene mammals from New Mexico in the world, far surpassing even the collections of the American Museum of Natural History. For many years, I have also been working collaboratively with like-minded scientists on the early Paleocene. We are also now training a new generation of students who will hopefully continue this work in the coming decades. Each year, usually in late spring and early summer, we gather in the San Juan Basin to conduct fieldwork, exploring the badlands. In 2019, we had our largest crew ever.

The summer 2019 field crew. From left to right, back row: Zoi Kynigopoulou, Neil Adams, Paige dePolo, Dr. Sarah Shelley (Carnegie Museum of Natural History, Pittsburgh), Dr. Tom Williamson (New Mexico Museum of Natural History, Albuquerque), and Hans Puschel Rouliez. From left to right, front row: Dr. Stephen Brusatte (University of Edinburgh), Sofia Holpin, and Dr. Ornella Bertrand (University of Edinburgh). Photograph courtesy Thomas E. Williamson.

Dan Peppe of Baylor University in Texas arrived with Andrew Flynn, a PhD student, and Jeremiah Robinson, an undergraduate assistant. They were here to improve our under- standing of how vegetation changed here in the basin through the early Paleocene.

Peppe and Flynn are paleobotanists. They looked like old- time coal miners as they lugged their large pickaxes up the steep hillsides. They would stop, and with a few quick strokes of their pickaxes, they laid bare fresh bedding planes bearing complete Paleocene leaves. They wrapped up the best sandstone or shale blocks with leaves in toilet paper before stuffing them into their already-heavy backpacks. Analyses of the leaves has revealed not only that forests recovered quickly from the mass extinction, but also that forests were far more diverse here in New Mexico than they were at other sites farther north, such as up in the Dakotas.

A careful study of the plants has also revealed something really exciting about the climate of this time: The basin was covered with tropical rainforests, or seasonal tropical forests, similar to what we find today in Central America. This part of New Mexico received over 80 inches of rain a year—far more than the 8 inches or so that it receives today. Peppe and his team also measured strata and collected samples of rock that preserve a faint Paleocene magnetic signature. These will be used to correlate the rocks to the global time scale so that we can precisely date the rocks to an unprecedented degree.

Matt Heizler from the New Mexico Geochronology lab at the New Mexico Bureau of Geology and Mineral Resources in Socorro tagged along, collecting samples that he can date using radioisotopic dating. He would occasionally pick up a block of sandstone chipped up by Flynn’s pickaxe and whip out his hand lens to scrutinize it. “This is a keeper,” he would say, and bag the sample. The block of sandstone might contain tiny grains of the mineral sanidine that was once part of a volcanic ash that belched from a nearby volcano and settled over the landscape nearly 65 million years ago. These samples will be blasted by a laser back in Heizler’s lab. The gases from the vaporized mineral will be carefully measured to yield a precise age for the rocks and ultimately for the fossils that are contained within.

Steve Brusatte, my long-time friend, colleague, and fellow vertebrate paleontologist, is originally from the Chicago area, but is now part of the faculty at the University of Edinburgh, Scotland. He and I are part of a large collaborative project investigating Paleocene mammals. We are also founding members of the Paleocene Mammal Working Group, or PalM. We are studying the Paleocene mammals, primarily to get a better understanding of mammalian evolution during their initial radiation in the early Paleocene.

Paleocene mammals tend to be enigmatic. They are often so odd, and usually lacking in the specializations that are typically used to characterize the various groups of living mammals, that we don’t know where they fit in the mammalian family tree. Ultimately, we want to understand how and when the initial radiation of mammals occurred and how this radiation led to the origin of all the groups that are with us today. Finding how Paleocene mammals fit into the early part of the radiation of mammals is crucial to understanding the rise of placental mammals.

Sarah Shelley, a former PhD student of Brusatte whom I co-supervised, is now a postdoctoral research fellow at the Carnegie Museum of Natural History in Pittsburgh. She recently spearheaded the publication of a monographic description of her favorite Paleocene mammal, Periptychus. As Shelley has recently explained to an audience of other paleontologists, Paleocene mammals have previously been described as “archaic” or “maladapted.” However, this does not adequately describe them.

Periptychus, for example, does not closely resemble any single living mammal. It was about the size of a large dog, and had a heavy body and thick tail, similar to an aardvark; powerful limbs like a bear; and five-toed feet, each tipped with a small hoof. The skull of Periptychus is elongated and its molars resemble those of a pig. However, its premolars are absolutely unique. They are large, cone-shaped, and marked by distinctive ridges. The premolars typically become blunted with wear, so that the enamel at the tips are worn away and the softer dentine is exposed as a cup-shaped depression, functioning similar to a mortar and pestle for crushing hard objects like seeds and tough plants. Periptychus was among the largest mammals of its day. Large numbers of this animal once lumbered clumsily through the forests of the American West for millions of years! How bizarre!

Brusatte has brought a number of students with him this year. Each member of this cosmopolitan crop of students has been assigned a different group of Paleocene mammals to study.

Sofia Holpin, a PhD student from Italy, is studying the mammal Tetraclaenodon, which is thought to be near the ancestry of perissodactyls, the group of mammals that includes rhinos, horses, and tapirs. The fossils of Tetraclaenodon are particularly abundant and include several skulls and skeletons, which are rare for Paleocene mammals. The large numbers of fossil specimens allows us to examine how this animal varied over time. We have found evidence that it decreased in size during episodes of warming—a somewhat baffling phenom- enon that has also been seen among some other mammals in other warming periods.

Zoi Kynigopoulou is a PhD student hailing from Greece. She is passionately studying a strange group of mammals called the Taeniodonta. Taeniodonts are a group of stout mammals that were good at digging. They evolved massive forelimbs with large claws. One of the strangest features of taeniodonts is their teeth. The front teeth became very large, forming a sort of hard beak made of teeth; one of our Paleocene taeniodonts was even named Psittacotherium, meaning “parrot mammal.” The relationship of taeniodonts to living mammals remains obscure. We are hoping to find their place within the mammalian family tree.

Paige dePolo is originally from Reno, Nevada, but ironically, she has moved to Edinburgh to study fossils from New Mexico. She is studying one of my favorite groups of mammals: the Pantodonta. Pantodonts were the largest mammals of the early Paleocene. They were large, lumbering animals, some attaining the size of a small cow. Pantodonts often have canines that are enlarged into tusks, and their cheek teeth are specialized for eating leaves—among the first of placental mammals to do this.

Hans Püschel is a doctoral student from Chile exploring the evolutionary relationships of Paleocene South American mammals. The continent of South America was isolated for many millions of years during most of the Cenozoic, and during this time, an endemic mammal fauna arose there. The founding members of this fauna, however, probably came from North America during the early Paleocene. There are mammals in the early Paleocene of South America that are stunningly similar to some, such as those of a group we call Mioclaenidae, which we find in New Mexico.

Ornella Bertrand is a postdoctoral research fellow at the University of Edinburgh working on the brains and sensory systems of Paleocene mammals. Did mammals with larger brains better survive the end-Cretaceous mass extinction? Did mammalian brain size rapidly increase in those first few million years of the mammalian radiation as mammals evolved to explore new ways to live? In order to examine the brains of early mammals, Bertrand is examining fossil skulls using high- resolution CT scans. This technology allows us to peer inside the fragile fossils without damaging them. She has become a master of creating digital models of the normally hidden cranial cavities that once firmly held those small brains. The relative sizes of parts of the brain can tell us how important different parts of the brain, such as those involved with the sense of smell, were. Bertrand is also examining the sensory systems of early mammals, particularly the senses of hearing and balance. The ear regions of the skulls contain the middle ear—a region that houses the bony labyrinth—that part of the inner ear that contains the cochlea, the part of the ear that senses sound, and the semicircular canals, the looping cavities that are involved with balance. Did these senses change among various groups of mammals as they evolved in different ways?

Neil Adams is a doctoral student from the University of Leicester, England, whom Brusatte and I are co-supervising. Adams is looking at tooth wear in Paleocene mammals. The fossil enamel preserves the pits and scratches produced through chewing that give clues to the foods they ate. His study should tell us how mammals shifted their diets as they passed from the Mesozoic into the early Paleocene and how those ranges of diets changed as mammals evolved to occupy new environmental niches.

Ross Secord, an assistant professor of geology from the University of Nebraska, could not make it out this year. Secord is also a vertebrate paleontologist who studies fossil mammals, but he is also an expert on using stable isotopes to measure past climates. He has sampled both sediments and even the enamel from fossil mammal teeth to extract and measure the proportions of various isotopes of oxygen and carbon. These can tell us such disparate aspects of the early Paleocene environment as temperature, what part of the forest animals occupied, and even what kind of greenhouse gases were present in the atmosphere. His research has revealed that the early Paleocene was marked by brief episodes of increased levels of the heavier isotope of carbon: C-13. Times when the air was enriched in C-13 are indicative of hyperthermal event; times in the distant past when massive amounts of the greenhouse gas methane were somehow injected into the atmosphere, apparently leading to a sudden spike in global temperature.

Dr. Dan Peppe (left; Baylor University) and students (Andrew Flynn, above, and Jeremiah Robinson, below) recording data on a new fossil leaf locality in badland exposures of the lower Paleocene Nacimiento Formation in the San Juan Basin, northwestern New Mexico. Photograph courtesy Thomas E. Williamson.

Distant times in the past, such as during the early Paleocene, are seen as natural laboratories for us to study how past ecosystems responded to sudden climate change. They could help to prepare us for what may befall us soon. At least one of the hyperthermal events that we have detected in the early Paleocene appears to correspond to the sudden disappearance of several Paleocene mammals, suggesting that these brief episodes had significant impacts on Paleocene ecosystems.

As I survey the stark beauty of the desert badland landscape, I wonder at how little the landscape has changed since David Baldwin tramped across these exposures so many years ago, yet how much has come to pass over the past 60 million years. I am also amazed by how much we’ve learned about the early Paleocene mammals during my career. I think of all the wonderful fossils I have collected and all the great times I’ve experienced in this fantastic place.

Some things never grow old. I’m giddy with anticipation of what we’re going to discover next.

Dr. Thomas E. Williamson has been a curator of paleontology at the New Mexico Museum of Natural History & Science for over 25 years. He has worked extensively on the fossil record preserved in the San Juan Basin of northwestern New Mexico.