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Re: Turtle phylogeny based on DNA study (free pdf)

Just appeared in tomorrow's Nature Magazine.

Clair Russell Ossian, Ph.D.
Professor Emeritus, Geology
Tarrant County College
2805 Raintree Drive
Carrollton, TX 75006

Seasonal bone growth and physiology in endotherms shed light on dinosaur

Meike Köhler,  Nekane Marín-Moratalla, Xavier Jordana  & Ronny Aanes

Cyclical growth leaves marks in bone tissue that are in the forefront of
discussions about physiologies of extinct vertebrates1
. Ectotherms show pronounced annual cycles of growth arrest that correlate
with a decrease in body temperature and metabolic rate; endotherms are
assumed to grow continuously until they attain maturity because of their
constant high body temperature and sustained metabolic rate. This apparent
dichotomy has driven the argument that zonal bone denotes ectotherm-like
physiologies, thus fuelling the controversy on dinosaur thermophysiology and
the evolution of endothermy in birds and mammal-like reptiles. Here we show,
from a comprehensive global study of wild ruminants from tropical to polar
environments, that cyclical growth is a universal trait of homoeothermic
endotherms. Growth is arrested during the unfavourable season concurrently
with decreases in body temperature, metabolic rate and bone-growth-mediating
plasma insulin-like growth factor-1 levels, forming part of a plesiomorphic
thermometabolic strategy for energy conservation. Conversely, bouts of
intense tissue growth coincide with peak metabolic rates and correlated
hormonal changes at the beginning of the favourable season, indicating an
increased efficiency in acquiring and using seasonal resources. Our study
supplies the strongest evidence so far that homeothermic endotherms arrest
growth seasonally, which precludes the use of lines of arrested growth as an
argument in support of ectothermy. However, high growth rates are a
distinctive trait of mammals, suggesting the capacity for endogenous heat
generation. The ruminant annual cycle provides an extant model on which to
base inferences regarding the thermophysiology of dinosaurs and other
extinct taxa.