In The Blink Of An Eye

by Andrew Parker

  Figure 8.3 Odaraia and Sidneyia from the Burgess Shale.

  Using crustaceans as their modern-day representatives, it would seem that most trilobites were predator-scavengers. That is, they fed on the bodies of other multicelled animals, either living or dead. Their heavily spined, robust limbs could have had no other purpose but to grasp and tear apart whole animals. As will be examined in more detail at the end of this chapter, the majority of early trilobites were active predators - they moved rapidly to hunt their prey. Further evidence to support this view is sealed within fossils of the Naraoids.

  Naraoids were a sister group of trilobites - they were their closest relatives and bore a physical resemblance. Naraoids too possessed very spiny and formidable-looking limbs, in addition to fang-bearing mouthparts. They probably fed on worms and other soft-bodied creatures. But Naraoids were different from trilobites in one respect - they had relatively soft bodies. The upper surface of their exoskeleton was not calcified like that of a trilobite, but was only organically strengthened. For this reason, to support the massive spiny limbs the upper surface of the body could not have been jointed like that of a trilobite - it would have been too weak. The upper body surface is a point for muscle attachments and is comparable to the supporting walls of a house. So the predatory limbs of Naraoids came at quite a cost to their bodies.

  Figure 8.4 Naraoia, a Naraoid from the Burgess Shale.

  Naraoids evolved from the ancestor of the trilobites before the exoskeleton became strengthened with calcium carbonate, or calcite. Naraoids are often found distorted in their burial grounds as a result of their weak exoskeletons, and they also missed out on eyes. The important evolutionary history of Naraoids and trilobites will be considered further in this chapter, but first we should examine the remaining evidence for predation in the Cambrian. So far we have looked at eyes, feeding apparatus and digestive systems. Now we should look for teeth marks in the prey.

  The display of Burgess Shale fossils at the information centre in the Canadian town of Field contains an interesting trilobite. Although most fossils here are remarkably complete and favourably orientated in the rock, and include some of the best examples of the Burgess species, a specimen of the trilobite Olenoides is particularly noteworthy. A large part of its body is missing, but the regularity of the semicircular omission suggests this was not an artefact of preservation. It could be only one thing - a bite mark. A large Cambrian predator bit this trilobite - it was Cambrian prey.

  Many other Cambrian trilobites have been found with scars, signs of attack by a predator sustained while still alive. These wounds proved not to be fatal because of the animals’ ability to heal. This is an interesting concept in itself. Cambrian trilobites were well prepared for attack not only in their protective armour but also in their ability quickly to seal the newly exposed body sections - they could form calluses. Human skin is thin and can be easily cut. For this reason, our blood has the ability to coagulate and seal up broken blood vessels, preventing blood loss and infection. Arthropod exoskeletons, on the other hand, are tough and designed to withstand the rigours of their hosts’ lifestyles . . . except when they are heavily attacked. The self-healing of Cambrian trilobites indicates they were so prone to attack that predation had certainly been a selection pressure during their evolution. Today animals can be found with hard shells that have functions other than to protect them against predators, such as providing support for tissues. But not only had Cambrian trilobites evolved armour, they had also evolved a self-healing mechanism to function in the event of attack by predators. Their hard shells had a role in protection against predators from the beginning.

  There have been so many Cambrian trilobites found with bite marks that a theory of ‘handedness’ has been suggested. In a large sample of trilobites, seventy-seven specimens had sustained injuries of unknown origin, perhaps caused by accidents during moulting or mating, but eighty-one specimens revealed injuries caused by predatory attacks. Researchers at Ohio State University found that 70 per cent of all scars left by predators were on the right side of the trilobites. It is thought that trilobites, their predators, or more likely both, tended to favour one side. Trilobites probably veered to one side in an attempt to evade an attacker. Also, predators probably tended to attack from the same side. Such asymmetrical behaviour is commonly seen today - a horse tends to turn its head to the left and 90 per cent of humans are right handed. But of most relevance to this chapter is the shape of the trilobite scars, whether on the left or the right side of the body. Many were W-shaped, conspicuously matching the size and shape of the iris-forming, triangular mouth plates of Anomalocaris.

  With the exception of the trilobite-like Naraoia, all the Burgess arthropods were protected within armour. They possessed head shields that were sometimes further protected by solid bumps or spines. Many trilobites possessed large spines - the defensive role of these becomes obvious when trilobites are considered in their curled-up posture. Here the animal is transformed into a hard ball with projecting spines. Furthermore, the spines are sometimes quite elaborate, with serrations and spikes.

  Long, sharp spines can be found on many hard exoskeletons, but also protecting softer, more fragile bodies like that of the Burgess lace ‘crab’, Marrella. In fact the use of armoured spines to protect soft bodies was employed by animals from a range of diverse phyla. There was the velvet worm Hallucigenia with its soft body protected by long, upwardly projecting spines. The bristle worms are more classical cases of this phenomenon; Canadia, for instance, wore a coat of spines projecting upwards and sideways. It is thought that bristle worms and their relatives in freshwater today independently evolved spines for defence purposes. Such convergence suggests that this means of protection is a good one. The epitome of protection within the Burgess bristle worms, however, was to be found in Wiwaxia, an oval-shaped animal not only completely covered by overlapping shields but also with long swords projecting outwards that made even Hallucigenia look like easy pickings. Halkieriids are possible ancestors of, and bore a close resemblance to, Wiwaxia - they possessed a similar means of protection in their chain-mail coat of shields.

  Figure 8.5 Photograph of a trilobite when rolled up - ‘head’ spines can be seen projecting from the body. When the trilobite is flat, as we usually view trilobites, these spines lie flush with the body.

  The sponges Choia, Halichondrites, Pirania and Wapkia of the Burgess Shale contained spicules that not only provided a supporting lattice, but also projected into the environment as deadly blades. Burgess priapulid worms had spines in the region of their mouths for feeding, but also on other parts of their bodies, where they took their most fearsome forms. Like most lamp shells, the Burgess hyolith Haplophrentis completely closed shop by surrounding its entire body with exceptionally hard armour. The Burgess echinoderms, relatives of starfish today, similarly revealed no soft parts to a passing predator. And finally we can consider Micromitra, where a shell as hard as a mussel’s was obviously not enough to escape predation - it further evolved long spines around its edge.

  An earlier lamp shell, Mickwitzia, may have taken protection a stage further. Mickwitzia possibly employed chemical defences - it squirted toxins through holes in its shell. The evidence for this derives from the other shelly fossils found with Mickwitzia, which all exhibited boreholes made by predators. Mickwitzia, on the other hand, was always borehole free. To conclude, all of this evidence can mean only one thing - that animals possessed protection against predators in the Cambrian.

  Figure 8.6 Pirania, Micromitra and Haplophrentis from the Burgess Shale.

  The hard parts described so far all evolved at one point in time. This evolution was the Cambrian explosion - all animal phyla suddenly evolved their hard parts simultaneously between 543 and 538 million years ago. As mentioned already, hard parts can have functions other than to provide protection against predators, but it would appear extremely coincidental for all phyla to evolve hard parts at precisely the same time to provide strength o
r as a barrier against osmotic stress. Multicelled animals from different phyla had been around, in soft-bodied form, for 100 million years or so beforehand. And as established in Chapter 1, physical environmental conditions that could have demanded hard parts were not the cause of the Cambrian explosion. Now it becomes important to chart the original appearance of predators, particularly the highly active forms. This will be investigated as soon as all the clues from the Cambrian have been gathered.

  The fact that all Burgess arthropods possessed protective spines, or some form of protection against attack, means they were not only predators, they were also prey. With the exception of the top predator Anomalocaris (which lacks protective spines) it is not surprising we could not deduce whether most Burgess animals with eyes were predators or prey based on their optics. In fact the ambiguity in the optical data supports the idea that most Cambrian animals in the open water were looking out for both prey and predators. With Anomalocaris a common menace in the Cambrian, the first rule was to stay alive, which meant keeping a lookout for the big-eyed giant. Cambrian eyes must have been adapted for scanning the complete environment, and any modification to this must have been slight due to the wrath of Anomalocaris and other highly mobile predators. That, as it happens, is exactly what we found in the Burgess eyes - adaptation for 360° of vision, with minor directional qualities.

  As for who ate who exactly, we can assume that the larger swimming forms preyed upon both the smaller swimming forms and the soft-bodied bottom-dwellers. But in addition to the telltale scars of Anomalocaris, there are other signs of predation in action where we can solve this problem more precisely. Fragments of the Burgess hyolith Haplophrentis have been discovered in the guts of thirty individuals of Ottoia, a priapulid worm, and also in the gut of the large arthropod Sidneyia. In that same gut of Sidneyia have been found seed-shrimps and trilobites - Sidneyia could feed on hard-shelled animals. And a closer inspection of the gut of one Ottoia revealed part of another Ottoia - so this priapulid worm was a cannibal.

  One fossil I picked up from the display table at the Burgess quarry was also interesting from this respect. This was the shrimp-like crustacean Canadaspis . . . and the tiny trilobite Ptychagnostus. The trilobite lay within the rounded head shield of Canadaspis and was probably its dinner. Other small trilobites have been found within the head shields of other Burgess arthropods, and it is possible that they were parasites. Since Ptychagnostus is found commonly in isolation, and probably lived in mid-water, maybe it was both parasite and prey.

  It is worth a closer look at this situation from another perspective. Canadaspis has eyes whereas Ptychagnostus does not. Canadaspis and Ptychagnostus evolved to gamble on different aspects of The Laws of Life. Canadaspis placed its chips on ‘eating’, Ptychagnostus on breeding. Ptychagnostus was extremely common in the Cambrian, whereas Canadaspis, and all other large Cambrian predators, was far less numerous. Ptychagnostus as a species was obviously prepared for predation - its survival was dependent on numbers. In other words, Ptychagnostus must have evolved a successful breeding strategy, so there were more individuals living in the water than could be consumed by predators - the strategy taken by krill in response to baleen whales. But this bottomless-pit-of-food scenario was not so accommodating to Canadaspis and its fellow predators. There remained the simple matter of ‘search and destroy’.

  The open water is three-dimensional. One is less likely to bump into an animal in the open water than on the sea floor, which is a two-dimensional environment. The ocean is vast, and Ptychagnostus mobile. To catch Ptychagnostus one must have the ability to find it and swim after it. Thanks to the Cambrian explosion, the strong, skeletonised limbs with internal muscles achieved the mobility required. And like dragonflies in the air today, the eyes of Canadaspis gave it the means to find. Here we are building a picture of how life functioned in the Cambrian, and the rules are similar to those today. In Chapter 3 it was revealed that eyes and armour are not directly related in that species with eyes do not always have armour, and vice versa. However, there may be a link in behaviour . . . and evolution.

  Chapter 1 introduced an old idea for the cause of the Cambrian explosion, one reworked recently by Mark McMenamin and Dianna Schulte McMenamin - that a food web was developed at the beginning of the Cambrian, where every species had its own predators and food. But can entire food webs simply spring up out of nowhere? Or is there a factor that triggers a chain reaction, ending in the formation of a full-blown food web? The range of animals living just after the Cambrian explosion, with their diversity of shapes and sizes, suggests a mature food web was in place at that time. But did this food web mature quickly, or instantaneously as the Cambrian enigma demands? Or did it assemble gradually, beginning in the Precambrian? These questions indicate that our Cambrian jigsaw puzzle is nearly complete.

  The McMenamins resurrected a century-old idea that animals developed shells as shields against predators, a fact we too have established. In this chapter, predators are emerging as hugely important factors in the way life works today and how it did in the past. But when did the remaining feeding modes within food webs first appear? This question may be irrelevant to our overall quest if food webs, and consequently The Laws of Life, were established before the Cambrian. It would seem appropriate for this chapter to end in the manner of the previous one, in a search for the beginning of predators on Earth.

  In the original line of fire

  Journeying beyond the Cambrian explosion and into the ‘relative unknown’ that is the Precambrian, the first port of call is the age of Ediacara. The Ediacaran suite of life forms is best represented by the original finds from South Australia, around 565 million years old, although the same organisms existed right up until the Cambrian explosion itself, when they disappeared without trace. But while they existed, they did exhibit a variety of lifestyles. We know this from the shapes of the life forms themselves and from their trace fossils - footprints and their equivalents.

  The Cambrian was once described as a peaceful time, but we have now established this is not true. In fact we know now that predators existed even before. In the Precambrian there were jellyfish pulsating through mid-water, and relatives of the Portuguese man-of-war floating on the surface. Any creature that accidentally encountered the stinging tentacles of these animals would have instantly become their prey. On the sea floor were anemone-like creatures with their stinging tentacles waving expectedly upwards. And then there was Precambrian prey. In some cases the Ediacaran predators would have preyed upon each other. But there were also flat, worm-shaped animals that probably undulated their bodies to propel them through the water - and sometimes into the lions’ den. Occasionally they would have propelled themselves into the nets of stinging tentacles, cast hopefully into the water.

  Although the word ‘hopefully’ infers personality in these primitive forms, it is appropriate in that Precambrian predation was a comparatively random process. There was no Anomalocaris with its advanced detection system and search-and-destroy capabilities. All that patrolled the Precambrian water were the stinging nets of jellies. But in the jelly’s favour, the prey could not sense them coming either.

  This last statement may not be strictly true. Although too small to be recorded as fossils, Ediacaran organisms surely possessed sense organs of some kind. They may have sensed vibrations in the water, based on movements of tiny hairs on their skin, which could signal the advance of a barrage of stinging cells. Indeed, the probable relatives of some Ediacarans today are endowed with hairs of this type. But detection in the Precambrian would have been possible only in close encounters. And selection for a more advanced sensing system would have been minimised by the generally slow speed of the advancing predators. This was a kitten-and-mouse game, in comparison with the cat-and-mouse Cambrian.

  On the sea floor the threat of predation was no more severe . . . but did exist all the same. A worm-like animal called Claudina lived in the sediment just prior to the Cambrian, about 550 million ye
ars ago. It is known from precisely 524 fossils from Shaanxi province in China. The fossils are not of the animal itself, but of its tube - this is the first animal known to possess hard parts. It appeared to have jumped the gun before the start of the Cambrian, at the same time demonstrating that environmental conditions were not completely restrictive for making hard parts before the great explosion.

  Fourteen tubes of Claudina revealed boreholes - holes made by a predator on the sea floor in a successful attempt to consume the soft animal within. Stefan Bengtson of Uppsala University and Yue Zhao of the Chinese Academy of Geological Sciences, who found these fossils, believe the predator to have been a mollusc, possibly a relative of snails today. But in the Precambrian, molluscs, like most other animal phyla, looked like ‘worms’, or rather had completely soft bodies. There was not even a hint that one day its descendants would carry around huge shells.