Out of the Shadows

Not all early mammals were shy and retiring

Only a few years ago, it was easy to pity the mammals that lived during the Age of Dinosaurs. Most paleontologists presumed that those tiny, shrewlike creatures, ecologically marginalized by their reptilian oppressors, thrived only by remaining out of sight. Perhaps relegated to a nocturnal lifestyle, these creatures scurried about furtively and ate insects, worms, and other invertebrates.

THE WATER’S FINE. Webbed feet and a beaverlike tail hint that Castorocauda, a close relative of mammals, led a semiaquatic lifestyle. M. Klingler/Carnegie Museum of Natural History

But a recent flurry of fossil finds is giving lie to that image. New and more nearly complete specimens of mammalian species and their close kin reveal that they had expanded into ecological niches beyond the forest floor. While some turned to burrowing and others became predators, one close relative of mammals took to the water beaver-style.

Toothy tales

Until recently, most known fossils of early mammals consisted only of teeth or fragments of teeth. Indeed, paleontologists sometimes joke that many early mammals were nothing but teeth, which mated with other teeth to produce yet more teeth.

“For 200 years or more, ancient mammals have been categorized by their teeth,” says Richard L. Cifelli, a paleontologist at the University of Oklahoma in Norman.

Teeth are the hardest materials in the mammalian body, so they are most readily preserved as fossils. Although they’re distinctive, the information that can be gleaned from teeth is limited, Cifelli notes. Their shape and features provide some clues about an animal’s diet, and their sizes afford a rough idea of how big the creature was.

Now, newly discovered fossils are triggering a reevaluation of how mammals eked out an existence in the shadows of the dinosaurs, which died out about 65 million years ago.

Last year, for instance, researchers analyzed the lower jaw, skull fragments, and 40 percent of the skeleton of a chipmunk-size creature that lived 150 million years ago (SN: 4/30/05, p. 285: Available to subscribers at Early mammal had newfangled fangs). The size, shape, and arrangement of its foot and limb bones suggest that the creature spent a lot of time digging. Previously, no mammals of that era had been known to have such a lifestyle.

Earlier last year, paleontologists described two nearly complete fossils of Repenomamus, presumably predatory mammals that lived in China about 130 million years ago (SN: 1/15/05, p. 36: Reptilian Repast: Ancient mammals preyed on young dinosaurs). The larger species of the genus, a 1-meter-long, badgerlike animal that weighed up to 14 kilograms, is the largest mammal yet discovered from the 170-million-year-long Age of Dinosaurs. The preserved stomach contents of its smaller cousin, an opossum-size creature, included remains of a hatchling dinosaur. “These are spectacular discoveries,” says Jason A. Lillegraven, a paleontologist at the University of Wyoming in Laramie. “They show a degree of diversification [of early mammals] that we hadn’t recognized before.”

In the swim

The latest entry in the early mammalian diversity parade is Castorocauda lutrasimilis—which, translated from Latin, means “beaver-tailed creature that looks like an otter.” The 50-centimeter-long creature, about the size of a modern-day platypus, lived about 164 million years ago in what is now northeastern China. It belongs to a group of animals called mammaliaforms, a dead-end lineage that branched off near the base of the mammal family tree, says Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh.

The well-preserved remains of Castorocauda show that its body was covered with a thick underfur and an outer coat of longer, stiffer guard hairs—the earliest fossilized fur found to date. The discovery hints that the closest common ancestor of this creature and all living mammals also sported fur, Luo notes.

Castorocauda probably tipped the scale at around 800 grams, at least 10 times the weight of its known mammalian contemporaries. The outermost three-quarters of the creature’s tail, like that of today’s beaver, was covered with leathery scales and some guard hairs. Many of the tail vertebrae are shaped like those of beavers and otters, which use their tails to move through water.

Castorocauda‘s rearmost molars served double duty, shearing food apart with their triangular peaks and then grinding it. In front of those molars were thin, blade-like teeth with five ragged cusps, some slightly curved toward the back of the mouth. Modern-day seals have both these types of teeth, so Castorocauda probably dined on fish and aquatic invertebrates.

Impressions of soft tissue around Castorocauda‘s rear feet suggest that those feet were webbed. Some features of the forelimbs would have made the creature well suited for digging, so Castorocauda probably lived in burrows along riverbanks and lake shores. Luo and his colleagues describe the creature in the Feb. 24 Science.

“This exciting fossil is a further jigsaw puzzle piece … demonstrating that the diversity and early evolutionary history of mammals were much more complex than perceived less than a decade ago,” says Thomas Martin, a paleontologist at the Senckenberg Research Institute in Frankfurt, Germany.

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