Box Who ate my nuts

Animal-plant interactions are often beautifully documented. Paleobotanists have identified marks of chewing, tunneling and munching in leaves, stems and seeds from the Devonian Rhynie Chert (see p. 489) onwards. Margaret Collinson and Jerry Hooker, experts on fossil plants and mammals, respectively, from London, spotted possible feeding damage in small nuts they collected in the Eocene of southern England, and reported in 2000. Some of the tiny seeds of the water plant Stratiotes had round holes on one side and the internal contents had been removed, leaving a husk (Fig. 6.14a). Stratiotes, sometimes called the water soldier or water aloe, still grows today in the fens and waterways of eastern England, as well as elsewhere in Europe, where it is rooted in the mud or floats on the surface of shallow pools and sends spiky, sword-like leaves up out of the water.

Close study of the holes in the seeds showed that some animal had cut the hole vertical to the outer surface, and that the cut edges of the hole bore numerous parallel grooves. There were more grooves around the hole, as if some creature had been grabbing at the seed to hold it firm while cutting the hole. Collinson and Hooker immediately thought of rodents as the seed eaters - rodents cut straight-sided holes into seeds, and leave parallel grooves formed by their long incisor teeth. The size of the seeds (4-5 mm long) and the size of the holes and grooves suggested a small rodent with incisors at most 1 mm wide, clearly smaller than a squirrel. Today, bankvole, woodmouse and dormouse use different gnawing actions for opening nuts so the authors used modern gnawed nuts for comparison. Of these three, the woodmouse makes the most similar feeding marks (Fig. 6.14b). After gnawing through the surface to make a small hole it grips the outer surface with its upper incisors and vertically chisels the walls of the hole with its lower incisors. It then inserts its lower incisors inside the hole to dig out the kernel.

The Eocene beds of southern England have yielded a variety of rodents, and the two that are the right size to have gnawed the Stratiotes seeds are two species of the early dormouse Glamys. Eocene Glamys may have swum to retrieve the seeds, or patrolled the shores of small ponds looking for any that had been washed up.

Figure 6.14 Evidence for a rodent-plant interaction from the Eocene. (a) Seed of the water plant Stratiotes carrying a neat hole gnawed by a rodent, from the Eocene Bembridge Limestone Formation of the Isle of Wight, southern England. (b) A hole gnawed by a modern woodmouse, showing the same kind of perpendicular narrow grooves made by the tips of the upper incisors. Scale bars, 1 mm. (Courtesy of Margaret Collinson.)

Figure 6.14 Evidence for a rodent-plant interaction from the Eocene. (a) Seed of the water plant Stratiotes carrying a neat hole gnawed by a rodent, from the Eocene Bembridge Limestone Formation of the Isle of Wight, southern England. (b) A hole gnawed by a modern woodmouse, showing the same kind of perpendicular narrow grooves made by the tips of the upper incisors. Scale bars, 1 mm. (Courtesy of Margaret Collinson.)

been very revealing. The teeth are sharp and curved, and the edges carry serrations like a steak knife - clear evidence of meat eating. Close study of the teeth also reveals minute scratches that were produced by the bones and other tough food material in the diet (Barrett & Rayfield 2006). Bones of the prey offer clues too: some examples show that T. rex could penetrate deep into the bones of its victims, but also that it chomped and tore at the flesh in such a way that it sometimes left dozens of tooth marks as it stripped the bones. All these circumstantial discoveries add to a rich picture of how one fossil animal fed.

Review questions

1 How are fossil species told apart? Look up information on any pair of species within a single genus (such as the human species Homo erectus and Homo sapiens; the dinosaurs Saurolophus osborni from North America and Saurolophus angu-stirostris from Mongolia; or any of the 10 or more species of the trilobite Paradoxides), and write down as many distinguishing characters as you can track down. How easy are these morphological characters to observe in the specimens?

2 Make a study of allometry in humans. Select a real baby, several children and an adult (all male or all female), and measure total body length (top of head to base of foot) as a baseline measurement, and then height of head (top of crown to base of chin), length of chin (bottom of lower lip to bottom of chin), arm length (tip of longest finger to armpit) and hand length (tip of longest finger to the line of hinging at the wrist). Which of these show isometric growth, and which are allometric? Are they positively or negatively allometric? If you do not have access to real people of different sizes, use images from books or the web.

3 Read around some recent papers on Hox genes, and find out how many are involved in determining the development of the vertebrate hindlimb. What does each gene do?

4 You want to understand how some fossil organisms moved and fed. What would be good modern analogs for trilobites, ich-thyosaurs and crinoids? Compare images and descriptions of the fossil and modern groups, and indicate how confident you would be in using each of the modern analogs.

5 Find an image of the skull of the dinosaur Plateosaurus. Why is the jaw joint lower than the tooth row? Think of modern analogs, perhaps among common domestic items, and think how the dropped jaw joint might affect the lever performance of the jaw.

Barrett, P.M. & Rayfield, E.J. 2006. Ecological and evolutionary implications of dinosaur feeding behaviour. Trends in Ecology and Evolution 21, 217-24.

Briggs, D.E.G. & Crowther, P.R. 2000. Palaeobiology, A Synthesis, 2nd edn. Blackwell Publishing, Oxford, UK.

Carroll, S.B., Grenier, J. & Weatherbee, S. 2004. From DNA to Diversity, 2nd edn. Blackwell Publishing, Oxford, UK.

Carroll, S.B. 2005. Endless Forms Most Beautiful: The New Science of Evo devo and the Making of the Animal Kingdom. W.W. Norton & Co., New York.

Futuyma, D. 1998. Evolutionary Biology, 3rd edn. Sinauer, Sunderland, MA.

Gould, S.J. 1974. The origin and function of "bizarre" structures: antler size and skull size in the "Irish Elk," Megaloceros giganteus. Evolution 28, 191-220.

Kodric-Brown, A., Sibly, R.M. & Brown, J.H. 2006. The allometry of ornaments and weapons. Proceedings of the National Academy of Sciences, USA 103, 8733-8.

Rayfield, E.J. 2007. Finite element analysis and understanding the biomechanics and evolution of living and fossil organisms. Annual Review of Earth and Planetary Sciences 35, 541-76.

Ridley, M. 2004. Evolution, 3rd edn. Blackwell, Oxford, UK.

Shubin, N. 2008. Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. Pantheon, New York.

References

Barrett, P.M. & Rayfield, E.J. 2006. Ecological and evolutionary implications of dinosaur feeding behaviour. Trends in Ecology and Evolution 21, 217-24.

Benton, M.J. & Kirkpatrick, R. 1989. Heterochrony in a fossil reptile: juveniles of the rhynchosaur Scaph-onyx fischeri from the late Triassic of Brazil. Palaeontology 32, 335-53.

Coates, M.I., Jeffery, J.E. & Ruta, M. 2002. Fins to limbs: what the fossils say. Evolution and Development 4, 390-401.

Collinson, M.E. & Hooker, J.J. 2000. Gnaw marks on Eocene seeds: evidence for early rodent behaviour. Palaeogeography, Palaeoclimatology, Palaeoecology 157, 127-49.

Gould, S.J. 1974. The origin and function of "bizarre" structures: antler size and skull size in the "Irish Elk," Megaloceros giganteus. Evolution 28, 191220.

Hagdorn, H., Wang, X.F. & Wang, C.S. 2007. Palaeo-ecology of the pseudoplanktonic crinoid Traumato-crinus from southwest China. Palaeogeography, Palaeoclimatology, Palaeoecology 247, 181-96.

Hutchinson, J.R. & Gatesy, S.M. 2006. Dinosaur locomotion: beyond the bones. Nature 440, 292-4.

McNamara, K.J. 1976. The earliest Tegulorhynchia (Brachiopoda: Rhynchonellida) and its evolutionary significance. Journal of Paleontology 57, 46173.

Molyneux T. 1697. A discourse concerning the large horns frequently found under ground in Ireland, concluding from them that the great American deer, call'd a moose, was formerly common in that island: with remarks on some other things natural to the country. Philosophical Transactions of the Royal Soaety 19, 489-512.

Rayfield E.J. 2004. Cranial mechanics and feeding in Tyrannosaurus rex. Proceedings of the Royal Society of London B 271, 1451-9.

Further reading

Shubin, N., Tabin, C. & Carroll, S. 1997. Fossils, genes and the evolution of animal limbs. Nature 388, 639-48.

Tickle, C. 2006. Making digit patterns in the vertebrate limb. Nature Reviews: Molecular Cell Biology 7, 45-53.

Witmer, L.M. 1997. The evolution of the antorbital cavity of archosaurs: a study in soft-tissue recon struction in the fossil record with an analysis of the function of pneumaticity. Journal of Vertebrate Paleontology 17 (Suppl.), 1-73.

Zakany, J. & Duboule, D. 2007. The role of Hox genes during vertebrate limb development. Current Opinion in Genetics and Development 17, 35966.

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