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It seems that this story is already all over the internet – I would have posted on it sooner this morning but was busy with amphibians! Anyway… back in 2005 Mary Schweitzer and colleagues dropped a bombshell into the world of dinosaur palaeontology: they reported the discovery of medullary bone within a Tyrannosaurus rex specimen (Schweitzer et al. 2005). Medullary bone is a specialized, densely mineralized, highly vascularized tissue laid down on the endosteal surface of long bones: its formation is triggered by hormones that are produced during ovulation, and it functions as a calcium store used in eggshell production. It was always thought unique to birds (though, incidentally, some palaeontologists had speculated that we should at least look for it in nesting dinosaur fossils (Martill et al. 1996)). However – as with quite a few traits of behaviour and anatomy conventionally thought unique to birds – its discovery in T. rex (a non-avian coelurosaurian theropod) has changed that. A new study, published today, takes this research to the next level…
[the image used above shows a cross-section through an Allosaurus tibia and reveals growth rings and the position of medullary bone. This individual died at 10 years old. Silhouettes indicate the relative sizes of a juvenile and a fully-grown Allosaurus. From the UC Berkeley newscenter site.]
I went to a talk given by Mary on the subject of medullary bone a few years ago. Firstly, having seen giant on-screen projections of the T. rex medullary bone compared with the medullary bone of extant birds, I can’t see how the T. rex tissue can be anything other than medullary bone. Secondly, the audience at the talk included comparative endocrinologists and histologists, and not just palaeontologists. While the latter are mostly unfamiliar with medullary bone, the others aren’t, and I note that they were in strong agreement with the identification. If medullary bone is present in tyrannosaurs, then by inference it presumably must also occur in many other coelurosaurs, given that most of them are closer to birds than tyrannosaurs are. Has it been found in other theropods, or in other non-avian archosaurs? A careful and thorough search failed to find it in crocodyliforms (Schweitzer et al. 2006).
But in a new study published today, Andrew Lee and Sarah Werning at the University of California report medullary bone in a non-coelurosaurian theropod (Allosaurus) and, more excitingly, in an ornithischian (Tenontosaurus). This indicates that the production of medullary bone goes back to the Triassic common ancestor of Ornithischia and Saurischia at least [adjacent image shows - at top - medullary bone in a chicken (it's the red spongy-looking stuff) with the medullary bone of a tyrannosaur below].
But the paper does a lot more than simply report new occurrences of medullary bone. Recent work on dinosaur growth rates (primarily based on counts of growth lines seen in bone sections) have shown that dinosaurs grew surprisingly quickly, with sauropods and hadrosaurs for example growing at rates comparable to placental mammals and precocial birds (Erickson et al. 2001, 2007, Sander et al. 2004). The presence of medullary bone in three non-avian dinosaurs now allows us to demonstrate sexual maturity in these taxa, and by counting growth lines we can go better and say exactly how old they were when they reached this age: Lee & Werning (2008) show that Tenontosaurus, Allosaurus and Tyrannosaurus were respectively 8, 10 and 18 years old when they were at sexual maturity (note that the presence of medullary bone doesn’t necessarily mark the onset of sexual maturity, as the animals might have been producing it when they were even younger).
Particularly interesting is that the inferred presence of sexual maturity in these dinosaurs does not correlate with asymptotic growth – at the ages given above, the animals were about half of full adult size, so they still had a way to go before growth stopped (asymptotic growth can be determined from bone histology by the presence of an external fundamental system or by tightly spaced growth lines at the bone’s outer edges). In other words, these dinosaurs were, potentially, teenage parents [adjacent image shows cross-sections through Tenontosaurus tibia, again showing growth rings and medullary bone. This individual was 8 years old when it died. Also from the UC Berkeley newscenter site].
The study goes one stage further. By using this sexual maturity data, Lee & Werning compared the estimated growth rates of these dinosaurs to those seen in extant non-avian reptiles. We know from their bone histology that dinosaurs grew quickly: however, if non-avian dinosaurs grew like scaled-up conventional reptiles, sexual maturity would have occurred decades later than what’s indicated by the presence of medullary bone. In fact, Lee & Werning estimate that Tenontosaurus, Allosaurus and Tyrannosaurus would have reached sexual maturity at 82, 87 and 218 years respectively if they grew like conventional reptiles (I’m not going into all the details here, you’ll need to see the paper if you want that). Indeed, if big dinosaurs grew like conventional reptiles (i.e., if we scaled conventional reptiles to tenontosaur, allosaur or tyrannosaur sizes), they’d have to reach sexual maturity at something like one-tenth of full size, and would have to keep growing for five to ten decades. These predictions are exactly the opposite of what the evidence indicates.
The new data showing that tenontosaurs, allosaurs and tyrannosaurs were all able to reproduce before reaching full adult size agrees with other lines of evidence. Animals that breed before reaching asymptotic size tend to suffer from high mortality as adults – there is therefore a rush to reproduce. And indeed what we know indicates that even adult mortality was high in dinosaurs: as Tom Holtz says, life was cheap in the Mesozoic. The onset of sexual maturity when young also means that particularly big dinosaurs – like giant sauropods – could have started reproducing before they grew to full ridiculous size. Modern birds have since evolved to do things differently, as they effectively obtain asymptotic growth by the time they leave the nest (though, incidentally, data indicates that this wasn’t the case for basal birds like archaeopterygids) [adjacent image shows dinosaur growth curves: note exponential growth early in development followed by achievement of asymptotic size. These graphs are from Erickson et al. (2001) and were borrowed from here on the Nature site].
There’s a lot more in Lee & Werning (2008) than I’ve hastily summarized here, but it’s an excellent study that adds a lot to our growing view (no pun) of dinosaur biology. Of course, all this stuff about rapid growth leads on to all those awkward questions about metabolic rates and endothermy. After all, all good scientists are inherently sceptical of the silly notion that dinosaurs might have been endothermic aren’t they……. to be continued….
Thanks to Sarah for help with this, and congrats!
Refs – -
Erickson, G. M., Curry Rogers, K., Varricchio, D. J., Norell, M. A. & Xu, X. 2007. Growth patterns in brooding dinosaurs reveals the timing of sexual maturity in non-avian dinosaurs and genesis of the avian condition. Biology Letter 3, 558-561.
- ., Curry Rogers, K. & Yerby, S. A. 2001. Dinosaurian growth patterns and rapid avian growth rates. Nature 412, 429-433.
Lee, A. H. & Werning, S. 2008. Sexual maturity in growing dinosaurs does not fit reptilian growth models. Proceedings of the National Academy of Sciences 105, 582-587.
Martill, D. M., Barker, M. J. & Dacke, C. G. 1996. Dinosaur nesting or preying? Nature 379, 778.
Sander, P. M., Klein, N., Buffetaut, E., Cuny, G., Suteethorn, V. & Le Loeuff, J. 2004. Adaptive radiation in sauropod dinosaurs: bone histology indicates rapid evolution of giant body size through acceleration. Organisms, Diversity & Evolution 4, 165-173.
Schweitzer, M. H., Dacke, C., Horner, J. & Lamm, E. 2006. Do egg-laying crocodilian (Alligator mississippiensis) archosaurs form medullary bone? Bone 40, 1152-1158.
- ., Wittmeyer, J. L. & Horner, J. R. 2005. Gender-specific reproductive tissue in ratites and Tyrannosaurus rex. Science 308, 1456-1460.
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You would probably find Greg Paul’s 1998 paper “Terramegathermy and Cope’s Rule in the land of titans” (Modern Geology 23:179-217) interesting. Paul argues that endothermy is a prerequisite of large size on land, and to be fair all our empirical evidence seems to support this: the largest known terrestrial unambiguous ectotherms weighed in at a weedy 1 tonne.
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:-O 
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