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NESTING TROODONTS



As promised in the 'Next issue' box of Jan 9th's _Nature_, the new one has an
important article about nesting dinosaurs. Incidentally, the issue - its text,
graphics, page layout - everything - is entirely new look.

Nest and egg clutches of the dinosaur _Troodon formosus_ and the evolution of
avian reproductive traits. VARRICHIO, D.J., JACKSON, F., BORKOWSKI, J.J. and
HORNER, J.R. 1997. _Nature_ 385: 247-250

Reporting on the more than three _Troodon_ nests now know, all from the Two
Medicine Fm (Campanian), the bottom line of the whole thing is that these
theropods used a nesting strategy intermediate between that of crocodiles and
that of birds, with both substrate and body warmth used to incubate eggs. There
is extensive discussion of what egg size, shape and development tell us about
theropod reproductive traits. As eggs are paired (as they are in oviraptorid and
therizinosauroid clutches), it is concluded that there were most probably two
functional avian-like reproductive tracts. I think this what is known as
monoautochronic ovulation - it is both unlike, say Varrichio et al., the mass-
laying of crocodiles and the sequential laying of birds. 

A new specimen, figured in the paper, shows an adult _Troodon_ in association
with eggs for the first time (these eggs, and the nests they are in, were
initially reported to be of _Orodromeus_, the worse known best known
hypsilophodont. Horner and Weishampel reported on the troodont affinities of the
nests and eggs in _Nature_ 383: 103). There is no evidence for vegetative cover
of any kind on the nest, nor for the nest having served for rearing of post-
hatching young. Varrichio et al. say that 'the degree of embryonic ossification
and preservation of intact hatched clutches, indicate that the precocial
_Troodon_ young left the nest soon after hatching' (p. 249). Replace all those
precocial hypsilophodonts in the Doug Henderson paintings with baby troodonts.

That the troodont eggs are sediment bound, as they are in oviraptorid clutches,
shows, state Varrichio et al., that they were not rotated by a parent. This runs
counter to a comment Gauthier made for a _Science_ news article: he said that
theropods 'played with their eggs'. 

Basing their logic on the fact that crocodile embryos die if their eggs are
inverted or rotated (the reason why you draw a cross on the upper apex of a croc
egg when you take it from the nest), Varrichio et al. conclude that the troodont
and oviraptorid eggs at least lacked chalazae (those albumen chords that hold
the embryo in an upright position). They speculate that chalazae may have
evolved 'as birds or climbing coelurosaurians sought sediment-free nesting sites
such as trees and rocky cliffs' (p. 249). We know nothing definite about the
evolution of chalazae at present, but if they were absent in all pre-bird
theropods, all must have laid their eggs in sediment or litter-bound structures.

A cladogram of archosaurs (fig. 4) has nodes characterised by changing
reproductive traits. Carnosauria is called Allosauria here BTW.

And, Gareth: you're thanked in the acks.

DARREN NAISH