Colours of Wildlife: Trilobites of the Cambrian and Ordovician
Created | Updated Apr 6, 2024
Trilobites of the Cambrian and Ordovician
Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"
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Today we're not going to see colour, but we're looking at wildlife � indeed, extremely ancient wildlife. If you're a long-time reader of this column, you'll know that I like to look back in time a lot, but most of what I've done here thus far was comparatively not that far back. I have done lots of mammals from the Pleistocene, Pliocene and Miocene, occasionally even more ancient. But today we're going back, way back, to the very start of complex multicellular life. The Earth has existed for about 4 500 million years, and life began almost as soon as conditions were reasonably stable. Fossil bacteria were found aged 3 500 million years – interestingly, here in South Africa! There are potentially similarly-aged fossils from Australia, and potential ones from Greenland from about 3 800 million years ago. The Earth's crust formed about 4 000 million years ago, which means life originated almost as soon as conditions were favourable. But it took billions of additional years before life 'advanced' beyond the bacterium stage. Eukaryotic single-celled organisms evolved when some bacteria somehow ingested others without killing them! These ingested organisms became components of the cells, most notably the mitochondria (the powerhouses of eukaryotic cells) and chloroplasts (the organelles with which plants and algae photosynthesize to produce sugars from carbon dioxide and water).
About 2 000 million years ago, there was a crisis among the living things of Earth! The result of the photosynthesis carried out by many algae caused oxygen to increase in the atmosphere, and this gas was poisonous to most life. There was a great dying off of oxygen-sensitive species but new kinds evolved that could actually use the poisonous gas for breathing and getting energy! That included our own forebears! But they were still just single-celled things at this stage.
The step to multi-celled life likely happened over 1 000 million years ago. The first multicelled organisms were seaweeds and sponges. For a long time, these represented the apex of complexity of life on Earth. But, likely slowly, things kept diversifying, and by about 600 million years ago a strange suite of things were around called the Ediacaran Biota, or the Vendian Biota. These things were sometimes quite large, reaching over a metre in length, but we have no idea which modern things, if any, they were related to. This ancient fauna was not well-preserved, since the rocks of the Earth have been so much jumbled and reworked over time, but there are traces of it in Britain, Australia, Canada, Mexico, Russia, and Southern Africa. These enigmatic animals are among the greatest puzzles in palaeontology, since we don't know what they evolved from, what they evolved into, or how they lived. They appear to have lived in ecosystems based around 'microbial mats', thin layers of algae and other tiny organisms that covered the seafloors in those times.
The very origin of these first macroscopic organisms doomed the microbial mat habitats to destruction. Their feeding destroyed the integrity of these fragile mats, and their activity disturbed the seabed, making it difficult for any kind of stable structure to persist. But again this chaos gave the opportunity for new ways and forms of life to arise.
So this brings us to the critters we look at today. About 538 million years ago, something happened we still don't quite understand. Suddenly, geologically speaking, a host of new lifeforms emerge in the fossil record, and for the first time, we can understand them! Most of these ancient Cambrians belonged to groups that still survive to this day. These were indeed the days in which all the modern phyla, the big groupings into which living things are classified, first established themselves. This event is called the Cambrian Explosion!
We still don't know what caused the Cambrian Explosion to happen. We know that, around that time, many critters 'suddenly' started having hard parts: shells, skeletons, legs, mouthparts. This made their preservation as fossils much more likely. But why so many kinds, and why just then? We hardly have any trace of the long stretch of evolution these things must have gone through prior to the Cambrian, in order to have the complexity they did at the start of the explosion. We cannot trace their ancestry back to the Ediacarns. But here we find them in the Cambrian for the first time, and they're already extremely complex and sophisticated! We don't know how it happened!
So let's today look at a group that demonstrate what I mean. These things emerged in the Cambrian at full complexity. They're called trilobites! That's because their bodies have three lengthwise 'lobes', a central section flanked by a left and right section. They're also divided into three front-to-back, with a head shield or cephalon, a body or thorax, and a tail section or pygidium. Their bodies were protected by a thick and very hard shell, which preserved easily, meaning that we today know a wealth of different kinds of trilobites.
You can see from my illustrations how complex these critters were. They had numerous specialized parts. In addition to the hard, segmented body, they had complex legs on most segments. The legs typically had two branches, a typically clawed, segmented 'walking' leg, and a feathery branch that was used either as a gill or a swimming organ. They had a pair of long, sensory antennae at the front of their heads. Most trilobites had eyes, which were compound and very complex, with varied lenses (in some species more than 15 000 per eye!) made of calcite crystals of precise arrangements. Some trilobites had pits on their head shields that likely housed sensory organs. They had complex digestive systems also, with mouths on the underside of the head shield, leading to a stomach more to the front of the head shield, and an intestine that led from there back to the pygidium at the rear. Food items were passed forward to the mouth, while also being shredded, by the legs. It appears that they could also secrete enzymes to help them digest hard-shelled prey. Trilobites had varied diets.
They also seem to have had other internal organs, such as hearts, brains and livers. They had muscles of different types, such as those needed to move their legs, and also some that passed through their bodies and enabled them to move their segments relative to each other. Many trilobites were able to roll themselves up into balls, protected on the outside by their hard shells. Trilobites also went through a process of differentiation or metamorphosis over the course of their lives, from simple larva-like infants, adding segments and parts in stages until they were fully developed adults.
So these were clearly very sophisticated little things, with very well-differentiated parts, and yet we have no fossils to show how their parts and systems arose and diversified before this point. Can you guess to which modern group trilobites belong? They are Arthropods – jointed-legged animals. Modern arthropods include insects, spiders, scorpions, mites and ticks, crabs, lobsters, prawns, centipedes, millipedes, and many others – indeed arthropods constitute the majority of species of living things today. And here they were at the very start of complex life in the Cambrian, in the form of the trilobites, which were quite as complex and sophisticated as any arthropod alive today!
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What you see here is a selection of species from first the Cambrian, and then the Ordovician, the period that followed the Cambrian. The Ordovician was from 485 to 444 million years ago. Olenellus was a typical trilobite, here showing the antennae and the jointed legs. Olenellus averaged about 5 cm in length. Peronopsis was an Agnostid trilobite, rather different in shape from typical trilobites. We're not quite sure whether it was bottom-dwelling or might have been able to swim. It was only 4-5 mm long. Pemphigaspis, Olenelloides and Peachella show additional variations in the trilobite form, with bulges or spiny parts. I don't show the antennae or legs here, just the main carapaces. These were all fairly small trilobites.
The next picture shows a selection of trilobites from the Ordovician. Isotelus was one of the largest of the trilobites, a specimen of Isotelus rex measuring 72 cm in length and 40 cm in width! Megistaspidella included various species; long-snouted isvosica was about 8 cm long. Asaphus, also, included various species, kowaleskii having long-stalked eyes! It was about 7 cm long, the eye stalks reaching 2.5 cm. Onnia superba was a small trilobite, its body reaching about 2.5 cm in length, but its 'horns' were longer. It had no eyes, but many pits, likely as sensory organs, on its head shield. Ceraurus had long spines on its hindmost body section, the pygidium. It grew to about 12 cm in length. Ampyx priscus had a long horn on its 'nose' as well as long horns on its head shield. Amazing fossils of them have been recently found in Morocco, where lots of them line up nose-to-tail as if in a conga line! For some reason they appear to have crawled over the ocean bottom in queues like this, we don't yet know why. They were similar in size to Onnia except for the longer horns. Then there are two small swimming trilobites. If you remember, trilobites had two-branched legs, one branch being a typical segmented leg with a clawed tip, the other branch being feathery and used either as a gill or, as in these, as a swimming organ. Pricyclopyge appears to have swum upside down! Carolinites swam in a more typical position. Both had unusually large eyes. They were actually not closely related! Pricyclopyge was apparently a deep-water (about 175 m) swimmer, its huge eyes helping it see in the dim light. Its eyes had a large number of lenses, looking a bit like the eyes of a dragonfly.
These are just a few trilobites; we have today fossils of over 22 000 species! They remained a very diverse and successful group over the next periods, the Silurian and Devonian, but then suffered a great reduction in diversity. They hung on into the Carboniferous but were wiped out at the end of the Permian. So this immensely successful and diverse group of arthropods are no longer with us. But they left us so, so many beautiful fossils! I have one myself, of a little Devonian species in the genus Phacops.
But mainly I just want you to see and appreciate how amazing and intricate these little things were, that lived around half a billion years ago. There were a great many other types that also originated in the Cambrian, including our own ancestors, the first Chordates!
