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Dinosaurs
Dinosaurs
The Cretaceous dinosaurs reach their peak in size. The giant sauropod]Argentinosaurus, is believed to be the largest land animal in Earth’s history, lived around this time. with length estimates ranging from 30 to 40 metres and weight estimates from 50 to 100 tonnes or 80,000kg. This is an Argentine treasure since it was discovered in late 1900s, and is 50 my later than the main dinosaur era. It is a heavy beast to go trampling over soils, so imagine the strength of soils to withstand these sorts of weights.
Argentinosaurus,
Although sauropods (long-necked, plant-eating dinosaurs) produced up to a ton of poop every day, carnivore dung was more likely to become fossilized because it contained calcium-rich bones and teeth that decomposed more slowly than plant matter. Coprolites tell scientists a lot about what an animal ate and how it live and often referred to as “trace fossils” because they reveal information about animal behaviour. Bloodsuckers, scavengers, dung-eaters and parasites would have been particularly interested in the resources in and emanating from the dinosaurs' large bodies.
Ichnites
Ichnites
A dinosaur could leave only one skeleton but many tracks, and a lot of dung. Rear footprints of sauropods, living about 120mya in a valley near Fort Worth are as much as 3 feet in diameter and 18 inches deep. While most people will be looking at the fossil tracks (or ichnites) to tell us about the behaviour of dinosaurs, I wonder what they can tell us about the soil. But the trouble is most of the tracks found are on mudflats or shorelines. For tracks to form and preserve, conditions must be just right, where the ground can't be too hard or too soft. If the ground is too hard, the resulting print would be very shallow or not form at all. If the ground is too soft, the track could collapse in on itself. Unlike bones, which needed to be covered quickly once a dinosaur died to preserve as much of the animal as possible, tracks first needed to be baked hard by the sun, taking days to months depending on the conditions. Then a layer of mud or ash would help to preserve the tracks.
Underprints getting deeper
Underprints
Underprints
The ground below the foot is compressed, causing a displacement which form features below the true track; these are known as undertracks, underprints, or ghost prints Very heavy dinosaurs would leave underprints, stepping so forcefully that they compressed deeper layers of soil-like, essentially leaving their tracks protected underground. To me, it is telling me that the soil was firm, and resilient enough to deal with dinosaurs treads such that they did not always leave an ichnite (trace fossil).
"Many of the Cretaceous formations of southern Korean peninsula formed on emergent lake margins. ..The available evidence is that the unusual, ridged track are undertracks of tridactyl bipedal dinosaurs made on a subaerially exposed tuffaceous sand that penetrated through to an underlying, previously tracked black mud." (Hwang et al 2008)
Tractor Tread
Tractor Tread
"The similarity in mass and contact area between modern farm vehicles and sauropods raises the question: What was the mechanical impact of these prehistoric animals on land productivity? (Keller & Or 2022) ) Farm vehicles have grown over the past few decades, to the point where they may be compacting the subsurface soil where roots of crops extend.
Today, that average weight (of a combine harvester) has grown to over 35,000 kg. To avoid crushing the soil beneath this bulk, tires have got larger, and they're operated at lower pressures, allowing the tyre to spread over a larger area to limit the compaction of the surface soil. The stresses (that go deeper down) largely depend on the mass per wheel”
Sauropods only had four legs, and all their weight would have shifted as they walked – rather than six for a combine or four wheels for a slurry tanker. The authors wondered how – on earth – could the soil withstand these sorts of pressures, when today’s soil is being compacted so badly. The researchers concluded that "The potential for significant soil compaction by foraging sauropods seems incompatible with productive land that supported renewable vegetation for feeding these prehistoric herbivores" . This is saying that the answer to this apparent contradiction was beyond their study - although they did hint that aquatics may be something to do with it.
So, I’ll have a go. Firstly, the Sauropods perhaps crushed the same circle of soil rarely, rather than driving over the land several times a year. Also, it may be telling us that the state of the soil then was much better than it is now. We know that for every group/unit of small soil animals in forest soils (=90) like when the Sauropods walked about, there are about half these numbers under grassland today (=40), where the slurry tankers go. In arable land, where the massive ploughs go, there is only about a third (=30) small soil animals in arable fields compared with forest floors (p175). We have seen how those small soil animals construct the soils, so the soils with less will have less well-structured soils. The structure of soils today – especially in vast tracts of arable land - is half wrecked before the machines come in, whereas the soil in Sauropod times could recover – clearly successfully.
The existence of dinosaurs 'roaming the land' implies that the soil must have been strong enough to take their weights. While most of their tracks are in mud, it is hard to see how they could have lived in that mud, as they needed lots of leaves to live.
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