DarylDomning
As the author of the article under discussion, I’ve been asked to weigh in on all of this. Most of the disputed points seem to have been clarified already by others, but I’ll try to clear up some of the remaining areas of doubt.
• The “Seven Rivers site” is a single, artificially-dug drainage ditch in the vicinity of the village of that name. The strata exposed there (in total, about 7 meters of section) are part of what is now called the Litchfield Formation (formerly Guys Hill or Chapelton Formation). This is part of the Yellow Limestone Group, which is widely exposed in Jamaica. The strata in the Seven Rivers site are estimated to be Middle Eocene in age (circa 41-43 million years old). This is close to being as old as any other fossil sirenians in the world, except for Prorastomus sirenoides, which came from another site in western Jamaica that is slightly older (in the early Middle Eocene Stettin Formation, circa 47 mya); it is also more primitive than Pezosiren.
• Of the 20 lithologically-distinct beds or strata that are identified in the Seven Rivers site, several have produced bones, but the vast majority of the bones have come from just 5 layers, as others have stated. Of these 5 major bonebeds, the rhinoceros Hyrachyus has only been identified in the lowest; it and other land-dwelling vertebrates are vanishingly rare throughout the section, whereas sirenians are represented by hundreds of bones. (Turtles and crocodiles are also common.)
• The sirenian Pezosiren portelli is the only sirenian found in the bottom 3 major bonebeds and makes up the vast majority of the sirenian fossils found, including associated and semi-associated partial skeletons. In the upper 2 major bonebeds, the sirenians all seem to belong to a different genus and species that has not yet been named. (I apologize to the human race for the long delay in publishing this new animal, along with an exhaustive description of the remains of P. portelli. These contributions are to form the main part of a multi-authored volume describing the entire Seven Rivers fossil fauna; most of the chapters have been completed, but several important ones are still in preparation. If you’ve ever been an editor of a similar multi-authored book, I know you can sympathize!)
• The key difference between Pezosiren and the later, more derived genus is that the former has a sacrum comprising 3 or 4 tightly-jointed vertebrae that together connect the rest of the backbone to the pelvis and hindlimbs, as in a normal land mammal. The later seacow has a sacrum consisting of only a single vertebra, also attached to the pelvis; the other sacral vertebrae had become loosened from each other, making the vertebral column as a whole more flexible in the hip region. This was a necessary evolutionary step toward the degree of flexibility of the backbone seen in all later sirenians, as well as in whales and dolphins, which all swim by undulation of the tail rather than kicking with hind limbs.
• Several sacra of Pezosiren have been collected from the lower bonebeds, but never from the two higher ones; and no sacrum like the one from the 2 higher bonebeds has been found among the scores of vertebrae in the lower beds. This supports the idea that the later species had either evolved or moved into the area – and Pezosiren had departed – by the time those higher beds were deposited.
• Concentrations of numerous animal carcasses of the same species (or mixed species) are not at all uncommon in the fossil record. As can be seen happening today, this can result from mass mortality from natural disasters, as well as from gradual accumulation of dead animals in a spot where the bones become buried by sediments with little or no disturbance. The Seven Rivers bone accumulations seem to have occurred in a brackish coastal lagoon, likely near a river delta that was supplying sediment. This is exactly the sort of place where one expects to find both land-dwelling, semiaquatic, and fully marine organisms buried together. Carcasses of the herd-dwelling seacows were mostly dismembered; no skeleton was preserved complete, but a few partial ones remained more or less associated (which helps to confirm that, e.g., pelvic bones of a certain type were associated with sirenian ribs or other bones and not some other kind of mammal). When the time comes to analyze accumulations like these, it’s usually not hard to tell whether a given isolated bone is from a seacow, a rhino, a crocodile, etc. – bones of different animals really look different, so it’s generally not a matter of guesswork!
• As stated in my Nature paper, the skeleton illustrated is a composite of bones from different bonebeds but representative of the same species, with a few parts restored that were not found at all – e.g., foot bones, which were logically among the first to fall off a rotting carcass or be nibbled off by the numerous crocodiles. (We only found 3 foot bones, as I recall, but these suffice to show that the feet were built for land and were not manatee-like flippers.)
• Also, no widespread catastrophe needs to be postulated to explain a pile of bones only a few meters across. Dozens of African wildebeests, for example, might drown when a huge herd of them crosses a single river, and the bodies could end up piled and eventually buried on the next point bar downstream. And this could happen over and over again in the same place, years apart or centuries apart.
• Finally, a personal note: As a lifelong practicing Christian, I am always dismayed to see evolution being a stumbling block that divides my fellow Christians against one another. It’s not necessary! I can testify that in my own career, not only have I personally dug plenty of “transitional fossils” out of the ground, but also found that Christian theology makes a lot more sense in the light of old-Earth, Darwinian evolution (with modern updates, of course) than otherwise. Let’s keep it respectful, sisters and brothers – and a merry and blessed Christmas to all!