Can you explain the deaths of three sloths by anything other than some kind of catastrophe? In a drought herbivores congregate around the remaining water sources, and soon exhaust all the good forage near by. If the drought continues, eventually they die of malnutrition. Predators have plenty of meat available so they leave most of the carcasses undisturbed. Even scavengers that normally consume bones switch to soft tissue.
Shipman (1975) identifies three levels of drought: mild, when seasonal watering holes dry up and juveniles tend to die; severe when even permanent water sources are affected and animals in their prime die, and; extreme drought when entire lakes and rivers dry up and the environment is permanently altered. If we had a drought, it was severe given the presence of an adult. We’ve found some turtle and frog bones, and mollusks, but there’s no evidence of an extreme die-off. The end of the Ice Age isn’t envisioned as drought prone, but we don’t have a firm radiocarbon date yet either.
Shipman offers some clues for recognizing a severe drought in a bone assemblage: Often some of the bones are still articulated. That’s because when the rain returns the dry soil erodes easily and covers them. The assemblage often includes a variety of animals from disparate habitats, forced together in their need for water. Also, the mineralogy of the sediments may show evidence of the arid conditions (i.e. hardpan).
Our bones show few signs of scavenging but they are disarticulated (How do you explain the bone arrangement?). There’s no evidence of a pulse of sediments covering the bones (no layers evident anywhere in the clay). Mineralogical analysis is awaiting the next NSF grant. We haven’t found a single bone from another herbivore–strange if water was scarce, but would the presence of two juvenile sloths make “Mom” aggressive enough to drive other animals away? The jury is still out on the question of drought—we need more data.
Haynes (1985) has studied elephant bone assemblages in Africa and notes that bones tend to accumulate near water for reasons other than catastrophic droughts: 1) animals spend more time there so they will die there more often simply from natural causes; 2) predators know that’s often the easiest place to make a kill; 3) catastrophic floods may be more frequent there, and; 4) bones are more apt to be buried and preserved there. We know there wasn’t a flood, but his other points merit some thought.
Conybeare and Haynes (1984) studied an assemblage of elephant bones after a severe drought at a place called Shabi Shabi in Zimbabwe. The assemblage seemed to have a natural age distribution reflective of one catastrophic event, but they concluded it had actually accumulated over several years and that trampling, kicking and digging by the elephants, combined with compaction and drying had preserved the assemblage as a single homogeneous bone bed. The lesson is clear—don’t jump to any conclusions that our sloths died in a single event. We have to prove it. . . . Dave
References:
Conybeare A. and Haynes, G.1984. Observations on elephant mortality and bones in water holes. Quaternary Research 22: 189-200.
Haynes G. 1985 On watering holes, mineral licks, death, and predation. In Environments and Extinctions: Man in late glacial North America, Eds. J Mead and D Mettzer. Center for the Study of Early Man. pp. 53-71.
Shipman, P. 1975. Implications of drought for vertebrate fossil assemblages, Nature 257:667-668.
Isn’t it a bit odd not to see any layers in the clay?
I may be anticipating a future blog entry, but I’d like to comment on the question “Can you explain the deaths of three sloths by anything other than some kind of catastrophe?” I suppose even a drought could be considered a natural catastrophe. If the adult sloth is the mother of the two recovered juveniles (which of course is a big assumption), maybe the mother died from either illness or injury. The juveniles may have instinctively remained close to the carcass of their mother and died over a period of days or weeks from either dehydration or starvation. Just a thought to consider in this discussion…
Travis, I wondered about the absence of layers too. Asked an expert. The assumption is whatever layers formed when this quiet backwater area was flooded, they were obliterated by animal life stirring up the bottom–e.g. tadpoles, turtles, frogs, insect larvae, wading birds, etc. There’s no variation in particle size, color, etc. that we’ve noticed so none of the flood events stands out as especially serious or distinctive.
Don, Your mother first scenario makes sense. A suckling juvenile has nowhere to go; an older sibling may not know where to go or have the strength to go any where. Conybeare and Haynes (1984) reported that it was the weaned juvenile elephants that had the most difficulty in a drought (ages 2-8). Too old to suckle and not big enough or strong enough to compete for food and water, or escape. Are elephants a good model for Megalonyx? Who knows? That’s why we keep digging. . . .