Relict lineages and late surviving species

We all know how the story of a species goes; At some point it evolves, arising from a previous ancestor. Maybe it thrives and spreads far and wide, or maybe it remains restricted to a small area, persisting in relatively low numbers. If it does gain a wide distribution, it will probably split into multiple subspecies, and eventually, some of these may themselves evolve into what we would term a new species. Eventually however, animals recognizable as members of the original species will go extinct, descendants or no descendants. They disappear from the world, their role in the ecosystem taken by other creatures, or perhaps left vacant if said niches no longer exist. Whatever the case, they are gone. But the borders between extinction and survival are not as solid as many people think. First off, we have the aforementioned situation in which a parent species gives rise to further descendants. If the wild Grey wolf went extinct, Canis lupus would still persist in the form of the Domestic dog and Dingo, and any descendants that they may have. In this way, species with descendants never really go "extinct" in the technical sense, their lineage is carried on, but the distinct morphology and ecology is still lost. Another form of not-quite extinction is called Functional Extinction, when individuals of a species are still present, but persist at so low numbers that they will never be able to recover, and are doomed to eventually die out. An example of this would be the Thylacine, which while officially extinct by 1933, probably continued to persist in low numbers until at least the 50s. Despite this, the species itself was probably functionally extinct as early as the beginning of the 1930s, and by the time the last captive individual, Benjamin, died, the species had already been doomed to extinction for a while.

The type of not-quite extinction I will be talking about today is the concept of relict lineages, groups which, while long past their glory days, still exist in the form of at least a single species. For me to consider a lineage "relict", it must be represented by only a few species, typically only one, and must be in constant but slow decline. Relict lineages can persist for a relatively long time after the rest of their relatives die out, and these few remaining species are termed "late survivors", but if they continue for too long, say more than 5 million years, they are clearly capable of adapting and diversifying, almost certainly possessing quite a large population, and thus not properly considered "relict". In this post, I will be going over some hypothetical late survivors, how and when they may have lived, and why we don't know about them.

Life and death in the Wessex Formation

The Wessex Formation is a geological formation dating back to the early Cretaceous, 130 million years ago. It is a part of the Wealden Group, which in turn is part of the Wealden Supergroup. This Supergroup also houses the Weald Clay Group and Hastings Beds Group, which, along with the Wessex Formation, were home to some of the first dinosaurs discovered, such as Baryonyx and Iguanodon. During the early Cretaceous, the Wessex Formation, and Wealden Supergroup in general, was part of a large island in the Tethys, where Britain now is. It had a dry, fairly arid climate, with distinct wet and dry seasons, and where temperature differences between summer and winter were stark. Much of the Wessex lowlands during this period would have been mostly treeless, covered in shrubs and other low-lying plants. The uplands and floodplains on the other hand would have been much more verdant, at least for some parts of the year, and plausibly housed a greater diversity of species. Due to these differences in habitats, animals would probably have migrated across the island depending on the seasons, spending the fairly cool and wet winters in the lowlands, and the warm and dry summers in the uplands. In this post I will be covering several of the major groups present in this formation, from the smallest Crocodyliforms to the largest Sauropods.

Polacanthus at a creek, by John Sibbick