Mesozoic herds

kw: musings, dinosaurs, ecology

I am reading a book, not done yet, about dinosaurs and how they fit into the ecologies of their times. I'll make a couple observations while I am thinking of them.

When Charles Darwin visited Patagonia while voyaging on The Beagle, he observed a modern bone bed where two streams came together, and compared its seeming richness with a landscape containing scattered herbivores. He remarked about the concentrating effect of the rains washing corpses or skeletons of the animals together. We need only recall that, while a cow or llama may be able to live for ten, twenty, even thirty years, most live three or four years before disease or predation does them in. On a particular square mile of the prairie, it only takes one or two of the few dozen animals that die every year to wash into the stream bed, to produce a mighty bone pile in just a thousand years.

I have visited two semi-recent fossil bone beds, La Brea Tar Pit in Los Angeles, California, and the Mammoth Site near Hot Springs, South Dakota. Both of these were active as animal traps for thousands of years. I don't find it surprising that each has a few hundred mammoth skeletons; I find it surprising that there are not more of them. Most of the mammoths that died in the environs of these "traps" did not wind up in them. Yet they are considered marvels of fossil richness.

Consider an area such as Dinosaur National Monument (or, indeed, the entire Four Corners region), where animal remains were protected and fossilized over a span of a few million years. We need to remember that the animals found in them did not die all at once, but over the whole time span. There are really very, very few places of "mass killings". Where they do occur, we still find evidence that there was after-death concentration of remains, such as by fluvial gathering into stream beds or pools.

The really giant animals of the Mesozoic needed large ranges to support their feeding. In the grass-rich Badlands of South Dakota and Nebraska today, it takes fifty acres to support one 300 kg (650 lb) cow. If 50-ton sauropods were warm blooded, how many acres did each require? Several thousand? Even in richer grasslands such as in Iowa (pre-Corn), one would need ten acres per cow, and perhaps 1,500 per adult sauropod (that's over two square miles, or about ten square km).

But did sauropods have a similar energy requirement per pound of weight as a cow, even assuming they were endotherms? Probably less, so let us posit a food need of the growth of one square mile of Iowa prairie per animal.

Of course, sauropods didn't eat grass. Grasses hadn't evolved yet. Perhaps their fare was richer than modern grasses. I think it may have been, because with grasses came a big change in the atmosphere: ultra-low carbon dioxide.

Prior to the evolution of grasses and other plants that use C4 photosynthesis, the amount of CO2 was never less than 1000ppm. C3 (and CAM) photosystems can't draw down CO2 below that point. But an atmosphere like that would facilitate heavier cover by shrubs and other small plants.

Today's CO2 level of 380ppm, while well above the 260ppm of the 1820s, is much less than the optimal level for the growth of leafy plants. Experiments with boosting CO2 to 580ppm have shown that most plants grow faster and more succulent; they'd be better eating!

I conclude that, while Mesozoic environments may have been more productive than modern ones, due to increased CO2, they were probably no more than twice as productive. Of course, that is comparing any area to itself under the different conditions, and with a different array of herbivore food.

So if it took a few hundred acres to feed each large sauropod, how big could their herds have been? It all depends on how far such a super-elephant can walk in a day. A dozen cattle on a square mile of Nebraska prairie need to keep on the move. After cropping the grass in one area, they can't return for a couple of weeks (let's ignore the winter situation for this exercise). So if they form a single herd, and I have observed that a herd of a dozen is quite common, they need to cover the whole square mile in two weeks, walking 6-10 miles daily (While we might model a cow's motions as a plowman's row-turn-row, they operate more randomly and thus cover even more ground).

Now if each sauropod needs a square mile or more, a herd of a dozen might need 15-20 square miles. Their long necks are a benefit here, as they reduce the walking needed to browse the trees and shrubs (remember there's no grass). It is possible that they need walk no further than a cow does, each day. It also means, however, that the herd spends most of its time spaced two neck-lengths apart, for more efficient "mowing". I consider that reasonable. Let us also remember that their mode of feeding was more like an elephant's, stripping the trees to neck-height in addition to slurping up all the shrubbery.

With all this in mind, I imagine this kind of scene, as a Mesozoic rancher on horseback. I can see a sauropod from ten miles away, so I can survey all the herds in an area of 300 square miles (1,200 sq km). That land area supports a few hundred of them, but in loose clumps here and there, perhaps in 20-30 "small" herds. They'd need to take turns going over to the river for a drink. Taking large herds of cattle to a river creates an awful, muddy mess. When left to themselves, they go at more scattered times. Dinosaurs ought to have developed/evolved similar habits.

If I am instead on foot, so I cannot see more than three miles (though I might see a sauropod's up-stretched neck from six or eight or ten miles away), I can still see as many as 30 animals, though the nearest is probably half a mile away, and for safety's sake, I hope so! The more immediate risk would be falling into a dino-sized "pie", which could weigh several times as much as me…but monster feces is better left to another riff.