Given the scale of its meaning, era is a small word. Description of Earth’s history has to cover its 4.5 billion year existence, and in geological science that gigantic amount of time is divided into periods within which significant things happened to and on the planet. Eras apply to periods of several hundreds of millions of years. For modern humans, who have existed for only about 300,000 years and who usually live for less than 100, these time spans defy comprehension. Importantly, eras are a useful division of time when considering the evolutionary history of groups of animals.
Ammonites were molluscs, now extinct but widely recognized in the beautiful fossils formed from their spiral shells. These extinct cephalopods, a class of molluscs that includes modern octopus, squid, cuttlefish and nautilus (the latter of which they superficially resembled), lived from 409 million years ago (Mya) to 66 Mya. That 343 million year period spanned two geological eras, starting about half way through the Paleozoic and ending at the close of the Mesozoic. To put that into context, the dinosaurs evolved, lived and became extinct entirely within the Mesozoic era. So, in terms of how long they existed, ammonites were very successful, and that success is demonstrated by their diversity.
Image credit: Partonez, CC BY-SA 4.0 , via Wikimedia Commons
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Around 10,000 species of ammonites have been identified from their fossils. Probably, more than 10,000 actually existed, though not all at the same time; individual species arose and became extinct as the class continually evolved over its 343 million years. As they evolved, they developed great variety in their distinctively coiled shells. The shells varied in size between a few centimetres to several metres in diameter. They also differed in their features and shapes, with variation in external lumps, ridges and spikes, and in patterns of invagination on their septa (the walls separating the series of chambers of which the shells were constructed). Some ammonites weren’t even helical, having straight shells or ones that were only slightly coiled.
Ammonites as a group, then, clearly had an effective way of living which made them extremely successful. Despite this, they suffered disaster several times . Conditions on Earth alter profoundly during eras. Titanic processes such as vulcanicity cause changes to atmospheric oxygen concentration, sea levels and other environmental conditions that have huge, sometimes devastating, effects on living things. This happened to the ammonites which, during their time, experienced no fewer than four mass extinctions. How they survived and prospered after three of them is striking.
After each of the first three extinctions there appeared new ammonite species with notably small and simple shells1. Resembling the shells of long extinct ammonites, they were, at least morphologically, primitive. Prior to the extinction, each period of evolution had progressively generated ammonite species with increasingly complex shells. That elaborate morphology probably indicated specialisation in how individual species lived, the ecological niches they occupied, what they fed on, their defences against predation, and how they behaved. Conversely, the simplicity of the new species that replaced them after mass extinction may well suggest that these were generalists rather than specialists, possessing flexibility that enabled them to live, for example, on varied types of food during times of scarcity. As evolution continued, these simple replacement animals gave rise to progressively more complex and specialised species as ammonite populations expanded and re-diversified into new and more niche environments.
“… there appeared new ammonite species with notably small and simple shells. Resembling the shells of long extinct ammonites, they were, at least morphologically, primitive.”
Animals that are simple and relatively non-specialised, and that maximise their chances of successful reproduction by producing large numbers of progeny, are often good survivors in difficult or changing environments. So the availability of simple forms of an animal group is therefore an important resource upon which evolution can act. Some groups may have small and simpler species already in existence, contemporary with evolutionarily more advanced and complex relatives, prior to any challenge from serious changes in their environment. That was not the case for ammonites. The paper argues that the fossil record and other evidence show that the newly evolved primitive forms only appeared after the major ammonite extinctions had occurred; they were new species, not expanded populations of species that existed before the extinction events. The reappearance of primitive forms of biological characteristics, resembling those present earlier in organisms’ evolutionary history, is called atavism. It appears that, several times, atavism resulted in these simple and versatile replacement ammonite forms, enabling them to adapt to and thrive in new environments, becoming new starting points for evolution to continue1. If we consider these atavistic events with reference to the MES we encounter a discontinuity.
The core of the current orthodox version of evolution is that random changes (mutations) in organisms’ genetic material (their genomes) cause variations to the forms and functions (i.e. the phenotypes) of organisms. Those phenotypic variations (remember that in the MES model they have a random cause) may be beneficial or deleterious to individuals that have them in populations of species, consequently affecting their relative ability to survive and reproduce successfully, and for their progeny subsequently to survive and reproduce. Biologically, this comparative ability is known as “fitness”. Natural selection, then, is the process in which variations endowing higher fitness in comparison to others lead to their bearers dominating and eventually replacing compatriots lacking those variations. Progressively, repeated and cumulative iterations of that process generate substantially altered forms of organisms, and eventually their evolution into new species. Thus, variation in phenotypes is the raw material of natural selection and, (consequently) evolution. In the MES, the randomness of genetic variation is critical in generating multifariousness in the phenotypic variation that occurs.
But the repeated ammonite atavism is a phenomenon of similarity, not disparity, suggesting that there is more to the story than random processes. Evolution generated strikingly similar new ammonite species in response to three dissimilar, extremely severe environmental crises. This suggests that the atavistic processes behind those evolutionary similarities did not have an entirely random basis. Instead, they seem somehow to have been likely to occur; rather than being generated entirely from random variation, the capability to evolve in an atavistic direction appears to have been innate. The next section describes how this might have happened.
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