By now, you're probably all aware of the new study published a few weeks ago in Science magazine about how male maniraptors brooded the eggs. It's old news, I know, but I'll lay out the basics for you anyway. Three dinosaurs have been found sitting atop nests: Citipati, Oviraptor, and Troodon. Everyone assumed they were females, but not so fast, kiddies! Now there's a way to tell. Remember way back when Horner and his colleagues sliced open a T.rex femur and found gold...I mean, medullary bone inside? Well, medullary bone has since been found in Allosaurus and Tenontosaurus. Finding medullary bone in dinosaurs isn't particularly surprising, as it's present in crocodilians and birds. For the uninformed, medullary bone is laid down in the long bones of archosaurs just before, during, and after a female becomes pregnant and provides the calcium needed to produce egg shells.
So if we assume that female archosaurs produce medullary tissue in their long bones in association with egg-laying (all signs point to "yes"), we can test those "nesting" theropod fossils to see if girls, but instead of lifting up their skirts, we're sawing their femurs in half. Anyway, it turns out that all three nesting dinosaurs, Citipati, Oviraptor, and Troodon, lack medullary bone, which means they're males. That means males brood the nest, a trait shared by modern paleognathine birds (ratites + tinamous), which most likely means that male brooding predates "modern" birds. Given the clutch sizes of the three dinosaurs, it also seems likely that, again like paleognathes, nests were filled by more than one female. So that's awesome--now we know a whole lot more about maniraptoran breeding behavior and physiology. We can probably assume that chicks were precocial and left the nest soon after hatching, although in some ratites, the chicks stay with the parent for several months to a year.
But wait--there's more! The authors of the paper perhaps did not realize that if maniraptors bred like paleognathine birds, there are implications for sexual dimorphism, too. In modern paleognathine birds, sexual dimorphism is limited to small size differences between the sexes. In the ostrich, plumage in males is more "flashy" than females. In general, however, paleognathine birds have fairly drab plumage. The cassowary is the most colorful ratite, but that's because its neck skin is bright blue! Bright plumage seems to be linked with mating dances and competition between males for females...however, with the flashy performances comes maternal brooding, biparental care and altricial young.
Another possibility is that the three nesting dinosaurs are males by coincidence, and that those species bred like raptorial birds. In raptors, both parents share brooding duties. The young are born extremely altricial but develop quickly. Like ratites, raptors don't show much sexual dimorphism aside from size differences. However, the clutch size of the fossil brooders argues against a modern raptor comparison. Raptors will often lay only two or three eggs in a single large nest. Paleognathine birds lay many more eggs, and oftentimes, multiple females will deposit their eggs in a shared nest, which the male then broods. So I think the paleognathine interpretation is the right one. Now, as for sexual dimorphism and coloration, I wonder if we could carry the ratite comparison over to non-avian maniraptors, too. Food for thought!
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3 comments:
It all sounds good, but it's based off a pretty big leap of faith. We have three instances of medullary tissues in the bones of different dinosaur species. Given that there's a fair amount of difference between the three species it is fair to conclude that it might be the case for all female dinosaurs. That's probably fair. But you can't negate the inverse of that rule.
Your statement is essentially that "if the dinosaur had medullary tissues, it was female." That is NOT the same as "If the dinosaur did not have medullary tissues, it was not female". It's no different to trying to reverse the negation of the statement "It is raining so I take an umbrella" - "I did not take an umbrella so it will not rain" is a logical fallacy.
If you do find that tissue in the brooding species you've listed, then you've proven that it was female. If you do not, you have proven that it was male, OR that there is another explanation for the lack. Such as that species simply not producing the tissues.
The implication you've constructed can only work if you first establish that the only situation where medullary tissues would not be present is when the dinosaur is not female. You would need to examine fossils of each of the three species until you found those tissues present in one of them. This would prove that that specific species does produce the tissues, and so it is reasonable to assume that the lack of it in the brooding species implies that that specimen was male. To go back to the analogy before, "If I do not take an umbrella it will not rain" is a perfectly logical and factual statement if you live in the center of the Sahara desert and it will never rain regardless of your umbrella.
Well, the fact that we have medullary bone from a carnosaur (Allosaurus), coelurosaur (Tyrannosaurus), and ornithischian (Tenantosaurus) argues in favor of medullary bone being present in their common ancestor, which, because crocs have it too, is WAY down at the base of the Archosauria. So it's really not a big leap to consider medullary bone present in all dinosaurs.
Look at it this way: We have hair, and chimpanzees have hair. We've never found hair on an australopithicine, but we assume they did have it because their closest living relatives (chimps and humans) have hair. That's phylogenetic bracketing for you.
And given the timeframe for the appearance and retention of medullary bone in modern brooding birds, we would expect to see it in brooding non-avian maniraptors, too, seeing as they are avian ancestors.
Male parentage in birds usually involves polyandry, by which females choose multiple male mates, lay their eggs in a collective clutch, and move on. This leaves a single parent who is larger and fit to care while the females produce many, many young.
One problem with this is that the Elongatoolithus eggs from Mongolia (rather tha tackle the "troodon" nests) are regularly arranged as those layed, in sequence, by a single animal in a nest. The eggs are not apparently arranged after the fact, but layed in place, then covered, due to the general pairing and spaces between pairs in the AMNH/GIN nests This implies a single female layed the nest. Male/female nest sitting occurs in birds, as well, among pair-bonds, which can also explain the finding of non-medullary bone in the sectioned material.
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