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DINOSAUR BABIES AND BONE ISOTOPES
The evidence that dinosaurs had high metabolic rates and good
thermoregulatory abilities continues to mount up.
In the Dinosaur Eggs & Babies book (1994, ed. K. Carpenter) I published a
paper explaining that baby hadrosaurs probably had high metabolic rates and
good thermoregulatory abilities for two reasons. First, they grew much faster
than reptiles. Second, they were altricial and therefore lived in open pit
nests exposed to the elements, as do some endothermic bird nestlings. No
ectothermic baby reptile lives in an open nest because they would die from
the extremes of solar heating and chilly nights and storms.
At SVP in NY J. Horner confirmed that hadrosaur babies were probably
nestbound. The evidence is multi-faceted. Hatchlings weighed at most 4 kg,
but juveniles weighing ~20 kg have been found in or near the nests. Eggs
shells in the nests are trampled into small bits, suggesting the nests were
inhabited for an extended period. The limb bones of the nestlings were too
poorly ossified for them to have wandered far from the nests. This data
contradicts recent assertions that hadrosaur babies were precocial and
quickly left the nest.
It takes well over a year for wild reptile babies to grow 15 kg. Since
hadrosaurs were doing so while in the nest they probably grew three-fold in
a month or two, a growth rate achievable only by high metabolic rate
homeotherms (bone microstrucutre also indicates that hadrosaurs grew faster
than reptiles). Indeed, such a high growth rate suggests parental feeding of
chicks that expended no energy to get the food.
To switch gears for a moment, for years bone isotope data has been used to
tell whether fossil herbivores ate grasses or bushes. Not much of a fuss was
made. But when isotopic data indicating that dinosaurs were homeothermic came
along all sorts of questions were raised about the long term stability of
isotopes in fossil bones. It is partly politics. Worse, it was armchair
criticism. No one sampled bones of clearly nonhomeothermic fossil reptiles
and showed that the bone isotopes HAD been altered in such a manner that the
signal was rendered psuedohomeothermic. More recently Barrick et al. (1996
Palaios 11:295) showed that bone isotopes DO distinguish between
nonhomeothermic reptiles and dinosaurs from the same formation. Unless it is
shown that bone isotopes actually are altered, Barrick's method appears to be
valid. Concerning baby duckbills, Barrick & Showers (1995, Paleobiology
21:550) published bone isotope data indicating that juvenile hadrosaur
nestlings were homeothermic, and therefore had high metabolic rates.
The combination of the bone isotope data, rapid growth and exposure to the
elements strongly indicates that growing hadrosaurs had bird-like energetics
and thermoregulatory abilities. Reptilian energetics are effectively
falsified. In the DinoEgg&Bab paper I speculated that nestling hadrosaurs had
some sort of pelage to insulate them from the sun, cold and rain (this
covering would be lost in favor of scales as the hadrosaurs grew). This
notion is somewhat less speculative in view of the feathered theropod.