The Eyes Have It
William Speed Weed writes that dinosaurs' need for camouflage is used as a basis to make "unscientific assumptions about the ability of dinosaur eyes to distinguish colors and shapes" ["What Do Dinosaurs Really Look Like?" September]. I humbly submit that Mr. Weed is in error. Using extant phylogenetic bracketing (EPB), we can say that dinosaurs had color vision just as certainly as we can say that they had tongues. EPB, using the genes that code for visual pigments, indicates that color vision was well established before vertebrates colonized land. The eyes of crocodiles, turtles, birds, and lizards all attest to the likelihood that dinosaurs' ancestors never went through a period similar to that of our Mesozoic ancestors, who lost nearly all their ability to make color discriminations. Hence, dinosaurs probably retained the facility for color vision that they inherited from the first terrestrial vertebrate. Further, a ring of bones embedded in the eyes of many modern animals appears to be an osteological correlate of a suite of characters related to daytime vision. The fossil evidence suggests that all dinosaurs had these bones. Thus it is reasonable to conclude that all dinosaurs were diurnal animals. Because slit pupils are associated with nocturnal animals, it is likely that all dinosaurs had round pupils rather than slit.
Mickey P. Rowe
University of California
Santa Barbara, California
William Speed Weed responds: Mickey Rowe's assertion that dinosaurs saw in color satisfies only the first step of EPB as proposed by Lawrence Witmer. Take two living creatures whose common ancestor is also believed to be the ancestor of an extinct group, and see if they have a common characteristic. The living relatives of dinosaurs Rowe mentions do have color vision. However, step two involves finding a marker of that characteristic that can also be found on the extinct animal, such as the marks soft tissues make on bones. Finding an osteological correlate to color vision cannot be accomplished by genetic analysis, because we have no dinosaur DNA. And, according to Witmer, the ring of eye bones Rowe refers to is a poor correlate to diurnal behavior. "The function of these rings remains enigmatic," Witmer says, noting that nocturnal owls have the most developed bone rings around their eyes. Common sense does dictate that dinosaurs probably had color vision. Yet without any clear evidence from dinosaur remains, the assertion stays in the realm of best guesses by illustrators.
Taking a Dive
Having been a diver for more than 30 years, I found Pamela Grim's discussion of the bends [Vital Signs, September] not quite up to date. I haven't seen U.S. Navy dive tables used at any sport scuba-diving location for the last 15 years. Those tables have been replaced by more conservative ones developed specifically for recreational divers by several of the civilian scuba-diving training agencies. The new tables have reduced the time allowed at specified depths, decreasing the probability of the diver getting decompression illness. Moderate tables and the widespread use of dive computers programmed with even safer times are everywhere I go to dive. I also take issue with the statement that "the rule in diving is never to dive the day before you fly." Some of the training agencies and several of the dive computer programs allow flying as soon as 12 hours after surfacing from the last dive. Finally, Grim's concluding statement, "[D]octors recommend that someone who has suffered the bends never dive again," is contradicted by the Divers' Alert Network: "There are no definitive studies that have attempted to quantify the risk exposure in returning to diving after decompression illness. . . . a return to diving should be contingent upon a decompression illness consistent with the diving exposure, a good response to treatment, no sequelae, and the absence of identifiable risk factors."
Capt. John D. Thurber, USN (Ret.)
Pamela Grim responds: Thanks and a hat tip to Captain Taylor for making me blow the dust off my clinical recommendations. A note of caution, though. Using more conservative dive tables to shorten underwater time will not prevent divers' decompression sickness if that same time is returned to the diver by "pushing the tables" using diving computers. Let the diver beware.