Ancient Animals Have Stories to Tell

Fossils can illustrate evolutionary themes: speciation, evolutionary radiation, invasion, sexual selection, gigantism and dwarfism. Despite important modern advances in genetics and molecular biology, much of what we know about how life on Earth has evolved still comes from fossil evidence. Fossils are witnesses to the past, albeit imperfect witnesses demanding close scrutiny if the story they tell is to be understood properly. Here are a few extraordinary fossils that have helped chart the story of life’s evolution.

Sharks' Teeth

The idea that fossils are the buried remains of once living animals and plants only became widely accepted about 300 years ago. Before this time, the origin of fossils was a matter of considerable dispute. Many naturalists considered that a mysterious ‘plastic force’ caused fossil shells and bones to grow within rocks like minerals. Indeed, repeat visits to cliffs and pits seemed to show this growth actually taking place as particular fossils came to protrude more and more from the rock face with time. We now know that such observations are due to the progressive erosion of the soft rocks around the hard fossils, rather than growth of the fossils themselves. Even when fossils were correctly interpreted as the ancient remains of animals and plants, their specific identity and mode of formation caused difficulties for early naturalists. For instance, how could the shells of animals that lived in the sea be found on the tops of mountains hundreds of miles away from the coast?

One man is accredited with doing most to establish the true nature of fossils. Niels Stensen (1638–1686), or Steno as he is better known, was a Danish physician and naturalist who spent the latter part of his life in the service of Grand Duke Ferdinand II de’ Medici in Florence. He was familiar with the curious fossils called ‘glossopetrae’ or ‘tonguestones’. These had long been exported from Malta for their supposed medicinal powers. The story goes that in 1666 Steno dissected the head of a large shark landed at the port of Livorno on the coast of Tuscany. Noticing the close similarity between its teeth and glossopetrae, he came to the conclusion that the fossils must be the teeth of primeval sharks. Thus, these fossils at least represented the buried remains of animals that swam in ancient oceans at some earlier time, before dying and becoming entombed in sediment on the seabed.

Steno and contemporary naturalists would have possessed examples of the largest shark’s teeth – those belonging to a species called Otodus megalodon, which lived between about 23 and 2.6 million years ago. Reaching up to almost 60 feet in length, this huge animal, which has been found fossilized in rocks from many parts of the world, dwarfed even the modern great white shark. As with other sharks, new teeth grew constantly to replace older ones in a conveyor-belt-like manner. Each individual shark could produce a very large number of teeth with the potential of becoming fossils. Megalodon teeth embedded in fossilized bones demonstrate that this fearsome predator included large whales in its diet.

Edrioasteroids

You could be forgiven for thinking that this object is a Celtic stone carving. However, the circular motifs with their swirling patterns are, in fact, fossils of animals called edrioasteroids that in this case lived attached to the surface of another extinct marine animal known as a conulariid. Edrioasteroids are cousins of modern starfishes and sea urchins, whereas the conulariid is the sea-floor-dwelling stage of a kind of jellyfish. In both cases the original mineralized skeletons of the animals have been dissolved by water passing through the rock. This has left the fossil as a mould in the fine sandstone rock, reinforcing the erroneous impression that it is a carving.

The clue that edrioasteroids are echinoderms lies in their five-fold symmetry. Edrioasteroids have been described as looking like small starfishes sitting on top of a cushion composed of numerous small plates. The five arm-like structures, known as ambulacra, contain plates that could apparently be opened. This allowed the animal to feed on plankton, possibly using tube-feet similar to those found in living starfish. Beneath each ambulacrum is a groove that conveyed food to the mouth at the centre of the animal. In some species of edrioasteroids the ambulacra are straight, but in others they are curved – as in the species shown, which was collected from Ordovician rocks in Morocco. Curved ambulacra can have an anticlockwise or a clockwise curvature, and sometimes one of the five is curved in the opposite direction to the other four.

How did edrioasteroids live? The majority of edrioasteroid fossils are found attached to the surfaces of other fossils, such as the conulariid that hosted the Moroccan examples, or to brachiopod shells. In many instances evidence implies that the edrioasteroids colonized living animals, possibly to the detriment of the host. Other edrioasteroids occur on rocky submarine platforms, often in very dense populations with neighbouring animals touching one another. There is debate about whether edrioasteroids could move around or were permanently cemented in one place throughout their life. Regardless, they may be viewed as the Palaeozoic ecological equivalents of the acorn barnacles (p. 188) that also cover hard surfaces and which are particularly common on rocky shorelines today.

The mouse goat of Minorca

Normally it helps to be big if you are an animal: you are less likely be eaten, more capable of subduing animals you wish to eat, and – like the giraffe – you will be able to reach foliage and fruit on high branches. But large size brings with it one major disadvantage: big animals require a lot of food. So when species of a large size colonize small islands they often exhibit dwarfism. In these settings, natural selection may favour small body size, because food resources can be limited. Also, because predators tend to be fewer on islands, one of the main benefits of being big vanishes.

In addition, the numbers of animals that can live on a small island is limited. This socalled ‘carrying capacity’ of the environment is lower for large animals: there are far fewer elephants than mice. And small populations are more vulnerable to extinction – a fact well known to conservation biologists. Dwarf species are able to maintain larger population sizes, meaning that they are less likely to become extinct.

A natural stage for the evolution of dwarfism was provided by some Mediterranean islands during the Pleistocene epoch. Fossils of pygmy elephants and deer have been found on Cyprus, Crete, Malta and Sicily. Further to the west, the Balearic islands of Minorca and Majorca were home to a pygmy goat until about 4,500 years ago. Bones of this animal – the so-called ‘mouse-goat’, Myotragus balearicus – were first discovered by the pioneering fossil hunter Dorothea Bate (1878–1951). Adults stood about 20 inches tall at the shoulder and, for their size, had a proportionally small brain and short legs – it seems there was little need to be especially intelligent or fleet of foot in a natural environment containing few predators. Fossil faeces (coprolites) found in caves occupied by mouse-goats indicate that the diet of the animals included the toxic Balearic box, Buxus balearica. But eating this shrub was not the reason for their extinction; rather, the culprits were probably humans, perhaps in combination with climate change. Before they had been overhunted, however, unsuccessful attempts had apparently been made to domesticate mouse-goats. This is indicated by the occurrence of fossil mouse-goats with trimmed horns showing regrowth – such trimming could only have been accomplished by humans, and it clearly happened during the lifetimes of the animals.

Mesolimulus

The idea of a ‘living fossil’ was introduced by Charles Darwin (1809–1882) in On the Origin of Species in 1859. He used the term to refer to two primitive animals – the duck-billed platypus and the South American lungfish. Both have endured to the present-day by inhabiting a confined area in which they are exposed to less severe competition. Exactly what constitutes a living fossil has been disputed, but still certain animals and plants are branded with the title. The most widely cited are the ginkgo tree, the coelacanth Latimeria, and the horseshoe crab Limulus. They share the attributes of being the last surviving and relatively unchanged representatives of groups that in the distant geological past were more common, diverse and widespread. Living fossils are the tapering ends of formerly thick branches on the evolutionary tree of life.

This is the Jurassic fossil Mesolimulus, named for its strong similarity to the modern horseshoe crab Limulus polyphemus. The shield-like carapace has a pair of small compound eyes and covers the segmented part of the body bearing the limbs used in walking, swimming and respiration. A long tail projects from beneath the carapace, and in living animals acts as a lever to help animals on their back regain their correct orientation. These arthropods are not true crabs but are more closely related to spiders and scorpions. They belong to a group called the xiphosurans, which first appeared in the Ordovician period, nearly 480 million years ago; they were moderately diverse during the Palaeozoic, some inhabiting the sea but others living in freshwater or brackish environments. Like so many other animals and plants, xiphosurans were hit very hard by the mass extinction at the end of the Permian. Indeed, just one lineage survived into the Mesozoic, and only four species of xiphosurans live today.

What evolutionary significance can be accorded to the living fossil Limulus? Unlike some living fossils, Limulus is widespread geographically, occurring along the Atlantic seaboard of North America and in Southeast Asia, and is very abundant where it does occur. Lacking a mineralized hard skeleton, the fossil record of xiphosurans is insufficient to know whether the group was ever represented by more species at any one moment in the geological past than the four that are living today. What is clear, however, is that xiphosuran morphology has changed little over time. Perhaps the horseshoe crab is an evolutionary example of the old idiom ‘if it ain’t broken, don’t fix it’ – an animal owing its extreme longevity to a ‘design’ that has been good enough to cope with the multifarious biological and physical changes that have occurred through more than 400 million years of Earth’s history.
 

A History of Life in 100 Fossils is newly available in paperback from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from A History of Life in 100 Fossils © 2014 by The Trustees of the Natural History Museum, London