Beyond Bones

Trace fossils yield important clues to ancient life

11:46am, June 5, 2001
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When detectives investigate a murder, they don't just look at the dead body. They also examine the wealth of clues nearby: tooth marks on partially eaten food in the kitchen, fingerprints that don't belong to the victim, bloody footprints from a size 12 Bruno Magli shoe. Each of these could be a critical piece of evidence in the search for the killer.

Similarly, when paleontologists unearth a dinosaur's bones, they can pick up extra tips if they expand the scope of their analysis. While bones and other fossilized body parts may indicate the animal's size and shape, different types of fossil can reveal an animal's interactions with its environment (SN: 12/19 & 24/98, p. 398).

For instance, the pattern and spacing of tooth marks on a bone can help identify the species and size of the chewer. Furthermore, if the bite marks show signs of healing, it's almost certain that the attacked animal escaped and lived for an extended period afterward. The animal whose bite matches the marks, therefore, was probably a predator, not merely a scavenger.

Bite marks are a type of evidence known as trace fossils. Others include footprints, nests, burrows, boreholes, and gastroliths–the gizzard stones that plant-eating dinosaurs swallowed that helped them grind tough vegetation into a more digestible pulp. Another type of trace fossil is coprolite, or fossilized feces. Ichnology, the analysis of such trace fossils, can reveal subtle details of an animal's environment, behavior, and relationships with other species. When added to information gained from body fossils, these clues to lifestyles help scientists conjure a much richer picture of prehistoric life.

Fossil tracks

The analysis of trace fossils is vital to understanding prehistoric life because many ancient animals are known only from a single fossil of their body, says Anthony J. Martin, an ichnologist at Emory University in Atlanta. In fact, scientists know some life forms only from trace fossils. The problem is especially keen among invertebrates, such as worms, which have very few, if any, hard parts that become fossilized.

Sometimes, even vertebrates can be elusive. Last year, scientists described the fossil tracks of an otherwise unknown web-footed bird that lived in eastern Asia more than 110 million years ago (SN: 8/12/00, p. 111).

Dinosaur footprints are some of the most common trace fossils. There are millions of them worldwide, and detailed analysis of a well-preserved set of tracks can reveal much more than the size and shape of the feet that made them, says Martin.

Paleontologists can measure the length of a dinosaur's stride from a series of footprints and then estimate the animal's speed. The shape and depth of individual footprints hint at a dinosaur's posture, and changes in the impressions from one footprint to the next can indicate grazing behaviors or stalking techniques.

The methods used to make these detailed inferences aren't much different from those developed by skilled trackers in groups such as the Apache Indians of North America or the Kalahari bushmen of Africa, says Martin. Their techniques of reading animal behavior from footprints may seem intuitive, but they're actually based on good science, he adds.

The information gained from trace fossils complements that garnered from body fossils, says Martin. Two fossil burrows recently excavated in South Africa yielded an array of clues about a hamster-size reptile called Trirachodon. The finds suggest that 240 million years ago, the animals lived communally in a surprisingly social style. Researchers had previously thought this type of behavior had appeared only in mammals millions of years later.

South African paleontologists found one well-preserved burrow in a layer of fine-grained yellow sandstone on a hillside about 300 kilometers south of Johannesburg. James A. MacEachern, a sedimentologist at Simon Fraser University in Burnaby, British Columbia, helped interpret the fossils. MacEachern and his South African colleagues describe the tunnel systems in the April/May Palaios.

Trirachodon reptiles apparently dug the complex system of sandy tunnels on a floodplain along a river. At least three different surges of sediment-laden floodwaters inundated the burrow, says MacEachern. The sediment filled the tunnels and preserved them.

That burrow didn't include any remains of animals, possibly because the floodwaters rose slowly and gave the occupants time to escape. However, the same researchers found a similar, but less well-preserved, system of tunnels about 2.5 km away that contained about 20 Trirachodon skeletons. This burrow complex, which workers uncovered while building a road, seems to have filled with a single surge of floodwater and sediments. So, at least some of its inhabitants drowned inside it.

Each of the burrows had an entrance tunnel about 15 centimeters wide and 6 cm high that gently sloped downward toward the interior, says MacEachern. The center of the tunnel floor had a slightly raised, flat-topped ridge marked by scratches and flanked by two smooth grooves, each a few centimeters wide. At deeper levels in each burrow, the tunnel became more curved and progressively smaller in diameter. In some places, it branched at right angles. Many of these small tunnels ended in smooth-floored chambers.

The animals trapped in the second tunnel system were of several different ages. Two adults and one juvenile died together in one chamber, which suggests that some portions of the burrows were places for rearing young. A few chambers contained fecal pellets, indicating that these dead ends were latrines. Other larger chambers could have been for food storage, MacEachern notes.

The ridge in the center of the entrance tunnel was too wide for most of these reptiles to straddle when walking, but the groove on each side was approximately the width of the animal. This two-lane traffic pattern and the smooth grooves are evidence of frequent travel to and from the surface, say the researchers.

Why did the animals leave the burrows? Probably to collect food. The animal's body type and the structure of its teeth and jaw, as determined from the fossilized remains, suggest that it fed on plants above ground.

Digging such a complex system of tunnels required a big investment. Therefore, these animals probably didn't create burrows for one season's hibernation or for infrequent use, says MacEachern. As herbivores, the reptiles were unlikely to temporarily abandon their home and then regain it from fiercer squatters. Instead, Trirachodon probably lived in colonies in these burrows over extended periods, possibly for generations.

The Trirachodon could have inhabited burrows for several reasons, including protection from predators while it was rearing its young. Living underground also would have helped the animal escape seasonal and daily temperature extremes. MacEachern notes that the climate in South Africa at the time was similar to that in the southwestern United States today, with generally arid conditions and big swings in temperature between day and night. The dry soils and low water tables might have encouraged the animals to dig their burrows in riverbanks or other areas that flooded only occasionally.

Boreholes and parasites

Trace fossils indicating one animal's behavior can show up on another animal's fossil. Many fossils of marine mollusks carry boreholes that reveal that predators or parasites attacked them. Modern-day drilling snails, for example, attach themselves to a variety of mollusks. The snails secrete a chemical that they repeatedly dab on the shell of their prey. A rasp on the snail's tongue removes material from the shell as it dissolves. The completed hole enables the snail to get at the fleshy part of its prey.

Some species of snails make small, cylindrical holes, while others create cavities that are beveled around the edges. The size and shape of the hole correspond to the dimensions of the snail's chemical-secreting gland and can indicate the type of snail that made a particular hole.

To learn about the drilling techniques of ancient snails, paleontologists study boreholes in fossilized seashells. They can't study the snails' rasps directly because, even though it's rough, the rasp is soft tissue and doesn't readily fossilize.

Although some marine fossils more than 500 million years old sport holes, many paleontologists have been hesitant to say these are signs of predators, says Audrey Aronowsky, a paleoecologist at University of California, Berkeley. That's because the modern-day snails that drill similar holes didn't evolve until about 110 million years ago.

"It's tough to tell who's doing the drilling [more than 150 million years ago] because the organisms were so very different," says Aronowsky.

By studying the holes in prey shells over the past 500 million years or so, Aronowsky says, scientists might be able to pinpoint when chemical-secreting glands first evolved among ancient predators. Changes in drilling techniques through the eons may have stimulated a biological arms race between predator and prey, she adds.

Some of the holes drilled in fossils of the marine echinoderms called blastoids and crinoids seem to have been made by parasites, not predators, says Tomasz K. Baumiller, a paleontologist at the University of Michigan in Ann Arbor. He points to the trace-fossil evidence.

Blastoids and crinoids are related to modern starfish and sea urchins. These animals attached themselves to the sea floor on stalks, and their feathery arms gathered food by filtering the currents through an open, tubelike gut. All blastoids died off at the end of the Permian period about 245 million years ago, and only a few species of crinoids survive today.

When Baumiller looked at more than 4,000 fossils of the blastoid Heteroschisma, he found that 139 had conical or cylindrical holes. Of those specimens, 5 percent had multiple drill holes–a proportion high enough to suggest that the drilling wasn't fatal, Baumiller says. Other researchers have found multiple holes, as well as some incomplete and healed holes, in specimens of other blastoids.

Many fossilized blastoids and crinoids have platyceratid-snail fossils attached near their anus, a location that matches most of the drill holes. Baumiller points out that the chances of fossilization catching large numbers of predators in the act of killing their prey are very slim, so the snails were probably parasites.

Modern crinoids continue to feed even when their gut is full, and their ancient relatives probably fed the same way. The waste would have been a rich source of food for parasites. For example, snails attached near the anus could consume this waste or reach directly into the gut to snatch undigested food.

A new analysis by Baumiller backs up his conjecture that snails of more than 150 million years ago were parasites. He's compared the nutritional value of the typical modern crinoid with the calorie content of the excess food that the animal collects. He concludes that if the ancient echinoderms had similar characteristics, snails would have been better off to treat blastoid and crinoids as long-term hosts rather than as prey.

Mystery remnants

Sometimes, scientists have no idea what kind of animal left an intriguing trace fossil. To make scientific discussion easier, paleontologists give even the remnant a name of its own.

Some of the commonest, best preserved, and most mysterious remnants of marine organisms are the trace fossils known as Zoophycos. First described in the 19th century, some of these ribbed, spiral, or fan-shaped structures measure up to 1.5 meters across. Although scientists recognize that Zoophycos are the remains of burrows, researchers have a hard time imagining the animal behaviors that generated these fossils.

The animals that produced Zoophycos fossils inhabited the deep-ocean floor, says William Miller III, a paleontologist at Humboldt State University in Arcata, Calif. Many examples of Zoophycos suggest that the animals making them lived beneath the surface of the ocean floor's ooze, constructed layers of the burrow along food-rich zones, and stored fecal pellets. Other Zoophycos seem to have been dug by animals that fed atop the surface of the ooze. Still other specimens have tunnels through areas of stored fecal pellets.

The complex set of behaviors recorded in these fossils makes sense if the animal that produced Zoophycos was long-lived and engineered its environment, says Miller. The radial tunnels suggest that the organism either stored fecal pellets for reconsumption or used them as fertilizer for a garden of microbes. Either way, Miller contends, the Zoophycos fossils record animal behavior that suggests the organism was modifying its ecosystem in an attempt to deal with a sporadic food supply.

"Most scientists think that organisms, in response to adverse environmental conditions, either have to migrate, evolve, or die," says Miller. Zoophycos fossils suggest that the simple animals that produced them explored another option. In an ancient forerunner of technology, they took control of their surroundings.

Information about fossil burrows, boreholes, and footprints is beginning to transform long-dead creatures, some of which left no other traces, into animate members of ancient ecosystems.

"Fossils of animals aren't always the most interesting thing in the world," says Aronowsky. "Trace fossils are a much more interesting way of looking at life through time."

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