Box The Triassic townet

For a century or more, fossil hunters had been aware of some astonishing fossils from the Jurassic of Germany that showed long, slender crinoids (see p. 395) attached to driftwood. In life, these cri-noids must have dangled beneath the driftwood, and their mode of life was a mystery. Driftwood crinoids have now been identified in many parts of the world, from the Devonian onwards.

Crinoids today can live attached to the seabed, as most of their fossil ancestors did, filtering food particles from the bottom waters. Most living crinoids are free-swimmers, but they do not seem to attach to driftwood. So why did the fossil forms do it, and how did they live?

New discoveries from China (Hagdorn et al. 2007) give some clues. Numerous pieces of driftwood have been identified in the Late Triassic Xiaowa Formation of Guizhou, southwest China, each carrying 10 or more beautiful specimens of the crinoid Traumatocrinus (Fig. 6.10a). The juveniles were presumably free-swimming microscopic plankton, as with other echinoderms, and they settled on driftwood logs. Many juveniles have been found on the logs. The crinoids then matured and became very long. Their feeding arms were longer than in seabed crinoids, perhaps to capture more food. This floating mode of life has been termed pseudoplanktonic, meaning that the crinoids are living like "fake plankton". They probably fared better up in the oxygenated surface waters than in the black anoxic seabed ooze.

The functional interpretation of a Traumatocrinus colony (Fig. 6.10b) is that it worked like a tow-net (Fig. 6.10c), a standard kind of fishing net towed in the open sea. As the boat moves forward, the tow-net hangs passively behind and billows outward. Any fishes encountered are caught. The Traumatocrinus colony similarly spread its feeding arms passively as the log moved forward in the gentle Triassic sea currents. Any food particle encountered by the crinoid net would be captured and eaten. Paleontologists have to use their imaginations and intellects in finding plausible functional models for some ancient organisms!

Figure 6.10 The use of a modern analog to interpret a mysterious fossil. (a) A colony of the pseudoplanktonic crinoid Traumatocrinus attached to a fossil piece of driftwood, from the Late Triassic of China. (b) Reconstruction of the crinoids in life, showing how the wind pulled the log to the left, and the dangling crinoids captured plankton like a net. (c) A tow-net used to maximize catches of fish, a possible modern analog that explains the feeding mode of the fossil colony. (Courtesy of Wang Xiaofeng.)

Figure 6.10 The use of a modern analog to interpret a mysterious fossil. (a) A colony of the pseudoplanktonic crinoid Traumatocrinus attached to a fossil piece of driftwood, from the Late Triassic of China. (b) Reconstruction of the crinoids in life, showing how the wind pulled the log to the left, and the dangling crinoids captured plankton like a net. (c) A tow-net used to maximize catches of fish, a possible modern analog that explains the feeding mode of the fossil colony. (Courtesy of Wang Xiaofeng.)

tures, while other modeling methods seek to establish how ancient organisms moved.

A number of attempts have been made to understand how dinosaurs ran, and of course everyone focuses on Tyrannosaurus rex. At one level, we all know how T. rex ran - we have seen it on Jurassic Park and Walking with Dinosaurs, so what is the problem? The locomotion in those movies was based on study of the limb bones, calculation of their ranges of movement, observation of modern ostriches at speed, and computer animations that rendered a reasonable swing of the leg, and that prevented the animal from falling over. But Hutchinson and Gatesy (2006) have urged caution. They argue that the style of locomotion shown in those films is probably near enough right, but that the animators chose only one out of many possible positions for the limbs.

In the most likely running gait (Fig. 6.13a) the backbone is horizontal and the legs relatively straight and long. Whatever happens, the animal must not fall over, so the first thing in reconstructing locomotion is to determine the center of mass, the central point in the core of the body. This can be found crudely by dangling a plastic model from a string and finding the three-dimensional central point of balance - or a more elaborate set of calculations can be done in the computer. In T. rex the center of mass lay just in front of the hips, and the tail balanced the body over the hips that acted as a fulcrum.

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Figure 6.11 Basic mechanical models for biological structures. There are different kinds of levers in use in everyday appliances, and these styles may be seen in biological structures. (a) In a class 1 lever the effort and load are on opposite sides of the fulcrum. (b, c) In class 2 and 3 levers the effort and load are on the same side of the fulcrum, with the effort furthest away in a class 2 lever (b), and closest in a class 3 lever (c).

The running cycle of any animal can be divided into the stance phase, when the foot touches the ground, and the swing phase, when the foot is off the ground (Fig. 6.13a). The limb swings through three extreme postures during the stance phase, from the point at which the foot touches the ground, through mid-stance as the body moves forwards to late stance just before the foot leaves the ground (Fig. 6.13b-d). An animal in contact with the ground produces a ground reaction force (GRF) that is the reaction to its body mass and the force of the limb hitting the ground during movement. The GRF swings its line of action as the limb shifts its position, and the point of maximum stress on the knee is at the mid-stance position (Fig. 6.13c) when the knee is bent, the knee moment arm is longest, and the muscle moment about the knee acting against gravity is at its highest.

Hutchinson and Gatesy (2006) showed that this is only one of many other possible poses for the limbs. Could T. rex have run in a high ballet-dancer pose or an extreme crouch (Fig. 6.13e-g)? The ballet-dancer pose is ruled out because the line of the GRF is in front of the knee at mid-stance and this would require the muscles at the back of the leg to act in order to balance the force in front. Living animals do not do this, so there is no reason to assume that extinct ones did. Crouched poses are ruled out too because the knee moment arm would have been too long and the knee muscle moment too high: T. rex would have had to have muscles relatively much larger than those of a chicken to cope. So the real T. rex probably stood and moved somewhere between these columnar and crouched extremes (Fig. 6.13g), which still leaves a large area of possibilities that cannot be excluded.

Circumstantial evidence_

Paleontologists are inquisitive by nature and they gather evidence of all kinds to test their hypotheses. Clues about the lifestyle of an ancient plant or animal may come from the enclosing rocks, associated fossil remains, associated trace fossils and particular features of the body fossils themselves. These can be grouped as circumstantial evidence.

1 Fossils are generally preserved in sedimentary rocks, and these record all kinds of features about the conditions of deposition. Fossil plants may be found at certain levels in a cyclical succession that tells a story of the repeated buildup of an ancient delta as it fingers into the sea, the development of soils and forests on top, and its eventual flooding by a particularly high sea level. Marine invertebrates may be found in rocks that indicate deposition in a shallow lagoon, offshore from a reef, on the deep abyssal plain or many other

Box 6.5 Finite element analysis of the skull of Tyrannosaurus rex

Emily Rayfield of the University of Bristol (England) had a dream PhD project, to work out how the skulls of the theropod dinosaurs worked, using finite element analysis (FEA). In FEA, the structure is modeled in the computer and its strength characteristics entered. Then the whole three-dimensional shape, however complex, is converted into a network of small triangular or cuboid cells, or elements. When forces are applied (a side wind on a skyscraper, a bite force on a skull or jaw bone) the elements respond and the effect can be seen. In Rayfield's FEA model of a dinosaur skull, as the bite force increases, the zone of element distortion increases and it becomes clear why the skull is shaped the way it is.

In one of her studies, Rayfield (2004) attacked the skull of T. rex (Fig. 6.12a). She tried to resolve a paradox that had been noted before: while T. rex is assumed to have been capable of producing extremely powerful bite forces, the skull bones are quite loosely articulated. Rayfield applied FEA to assess whether the T. rex skull is optimized for the resistance of large biting forces, and how the mobile joints between the skull bones functioned. She studied all the available skulls and constructed a mesh of triangular elements (Fig. 6.12b). Bite forces of 31,000 to 78,060 newtons were applied to individual teeth, and the distortion of the element mesh observed (Fig. 6.12c). The bite forces had been taken from calculations by other paleobiologists, and from observations of tooth puncture marks (a piece of bone bitten by T. rex showed the tooth had penetrated the bone to a depth of 11.5 mm, equivalent to a force of 13,400 newtons or about 1.5 tons).

Rayfield's results show that the skull is equally adapted to resist biting or tearing forces and therefore the classic "puncture-pull" feeding hypothesis, in which T. rex bites into flesh and tears back, is well supported. Major stresses of biting acted through the pillar-like parts of the skull and the nasal bones on top of the snout, and the loose connections between the bones in the cheek region allowed small movements during the bite, acting as "shock absorbers" to protect other skull structures.

Read about dinosaur feeding behavior in Barrett and Rayfield (2006) and about finite element analysis in Rayfield (2007) and at http://www.blackwellpublishing.com/paleobiology/.

Figure 6.12 Finite element analysis of the skull of Tyrannosaurus rex. The skull (a) was converted into a cell mesh (b), and biting forces applied (c). In the stress visualization (c), high stresses are indicated by pale colors, low stresses by black. Each bite, depending on its strength and location, sends stress patterns through the skull mesh and these allow the paleobiologist to understand the construction of the skull, but also the maximum forces possible before the structure fails. (Courtesy of Emily Rayfield.)

Figure 6.12 Finite element analysis of the skull of Tyrannosaurus rex. The skull (a) was converted into a cell mesh (b), and biting forces applied (c). In the stress visualization (c), high stresses are indicated by pale colors, low stresses by black. Each bite, depending on its strength and location, sends stress patterns through the skull mesh and these allow the paleobiologist to understand the construction of the skull, but also the maximum forces possible before the structure fails. (Courtesy of Emily Rayfield.)

situations. Moreover if they had a widespread geographic distribution, perhaps they were planktonic. Dinosaurs or fossil mammals may be found in sandstones deposited in an ancient river or desert. All these clues from sedimentary rocks guide the paleontologist in interpreting the environment of deposition, and in turn can reveal clues about climates and other physical conditions.

2 Associated fossils also give clues. They can show where the organism of interest sits in a food web (see p. 88) - who ate it, and what did it eat? Sometimes groups of fossils may be associated in death in such a way that they indicate life habits. For

Figure 6.13 The running stride of Tyrannosaurus rex. (a) The main components of a stride, showing the stance phase when the foot touches the ground, and the swing phase. (b-d) Three positions of the limb in early stance, mid-stance and late stance, as the body moves forward, and showing the main forces, including the ground reaction force (GRF). (e-g) Three alternative postures for the limb, with the body held high or low. Read more, and see the movies at http://www.rvc.ac.uk/AboutUs/Staff/ jhutchinson/Researchlnterests/beyond/Index.cfm. (Courtesy of John Hutchinson.)

Figure 6.13 The running stride of Tyrannosaurus rex. (a) The main components of a stride, showing the stance phase when the foot touches the ground, and the swing phase. (b-d) Three positions of the limb in early stance, mid-stance and late stance, as the body moves forward, and showing the main forces, including the ground reaction force (GRF). (e-g) Three alternative postures for the limb, with the body held high or low. Read more, and see the movies at http://www.rvc.ac.uk/AboutUs/Staff/ jhutchinson/Researchlnterests/beyond/Index.cfm. (Courtesy of John Hutchinson.)

example, fossil reefs may be killed off in a particular crisis, and all the organisms that lived together are found in life position; some corals, bryozoans and crinoids may be fixed to the substrate in their normal growth position, and mobile organisms like gastropods or trilobites may be preserved among the thickets of benthic sessile organisms. Similarly, a paleosol (see p. 518) may preserve roots and stems of dozens of plants in life position, together with burrows of insects and worms that lived among them. Associations of fossils can also be more intimate, where for example parasites may be found attached to their hosts, or fossils of one species may be found in the stomach region of another.

3 Associated trace fossils can sometimes be linked to their producers, but not always.

There are some rare examples of arthropods preserved at the ends of their trails. The link between trace fossil and producer is usually a little less clear: dinosaur footprints may be found at certain levels within a particular geological formation, and the skeletons of likely producers at other levels. The bones of fishes and marine reptiles may be found associated with phos-phatic coprolites (fossil dung) in certain marine beds - it is likely that the copro-lites were dropped by one or other of the associated animals. If a link can be made between a trace fossil and its maker, then a great deal of additional paleobiologi-cal information can be established (see Chapter 19).

4 Close study of the body fossils themselves is also warranted. Skeletal fossils regularly preserve evidence of soft tissues and other unpreserved components. Fossil plant stems may be stripped of leaves, but the leaf bases are still there. A fossil trilobite may preserve limbs and other structures under the carapace. Bones often show muscle scars. In conditions of exceptional preservation, of course, skin outlines, muscles, sensory organs and internal organs may also be preserved. For example, the spectacular fossils from Liaoning in China (see p. 463) have confirmed that the fossil birds had feathers and the mammals had hair, as had been expected, but other fossils showed that all the carnivorous dinosaurs had feathers too. That dramatically changes all previous paleobiological interpretations of those dinosaurs because they must have been warm-blooded in some way.

These four kinds of circumstantial evidence have been useful in understanding how ancient rodents fed on nuts (Box 6.6), and also how T. rex fed. The biomechanical models of feeding in T. rex (see Box 6.5) tell us a great deal. The rocks in which T. rex bones are found confirm it lived in hot, lowland, forested areas. Associated fossils include numerous species of plant-eating dinosaurs, and some of these even carry tooth marks likely made by T. rex. Tracks of footprints made by T. rex, or a relative, show that it trotted along steadily, but not fast. A famous 1 m long cop-rolite dropped by T. rex contained pulverized bone of ornithischian dinosaurs that had been corroded to some extent by stomach acids, but not entirely destroyed. This suggests a relatively rapid transit of food material through the gut. The bones themselves have

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