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Social Behavior in 240 Million Year Old Reptile



Not quite dinosaurian but interesting all the same.

>From the current issue of Science News (6/9/01) in an article about
going beyond bones (and looking at trace fossils).

 http://www.sciencenews.org/20010609/bob9.asp

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.