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Re: attack on dinosaur--horrific video

Birds--and probably non-avian dinosaurs--are/were limited re night vision by their eye structure--sclerotic rings restrict light gathering--owls, for example, must have huge eyes to enhance night vision--crocs lack sclerotic rings, as do we.

I'm not sure this is correct; do you have a citation? Owls, granted, have huge eyes and the overall shape maximizes sensitivity. However, owls have 1) much greater sensitivity than many other nocturnal animals (so the fact that they have larger eyes than crocs, proportionately, does not mean that crocs have an advantage) and 2) owls also cannot sacrifice too much acuity (they do still sacrifice acuity to a substantial degree, but have limits). Because acuity and sensitivity have conflicting shape requirements, the eyes of animals that maximize one parameter must generally have huge eyes if they do not minimize the other.

I hate to use such fait accomli arguments such as: placentals are the dominant form of mammal; their characteristics may have had something to do with this.

Indeed; and so one might suspect that reproductive mode has made a major difference >within< mammals. Arguing across major clades with other differences, however, is more difficult. In fact, what is really striking about modern placentals (comparing them to their Mesozoic ancestors, as well as other living clades) is their major presence as medium to large-bodied, terrestrial forms. Given that dinosaurs fell into the same category in the Mesozoic, we might expect that key to being viable in that morphotype lies in the similarities between placentals and dinosaurs, rather than the differences. In any case, you and I have discussed this particular issue at length in the past. We don't seem to agree, but I do find the life history discussion lively and fun.

One point I would like to toss in, which I don't think has come up thus far, is that the nest predation lines are really rather blurred. Many mammals have rather altricial young with high mortality (often confined to burrows or dens), and quite a few squamates utilize viviparous life histories. No archosaurs do this, of course, but many birds have quite precocial offspring, while others have very short incubation periods (such that their offspring probably have similar predation rates to altricial mammal dens). To really make the point that there is a major difference in predation-based mortality for placental mammals, we need some data for these various life history strategies. Put simply, we need to first demonstrate with some confidence that being a viviparous mammal vastly increases overall juvenile production (after mortality) over oviparity. I think you make a strong case that nest protection is very important. Certainly, we would expect that oviparous species that do not protect nests effectively will have lower reproductive success than those that do. That's very different from arguing cause for a mass extinction event, however.



Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu