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.
The Thylacine Benjamin, last known individual
Studying relict lineages in the modern world is very difficult, and this is down to two primary reasons. First off, it is almost impossible to detect if a lineage is in slow decline, as they will still appear relatively abundant in the present. Even late survivors are quite well adapted to the environments in which they live, after all, otherwise they would already be extinct. For this reason, studying relict lineages is essentially only possible with the benefit of hindsight, as slow declines are easy to detect on a geological timescale, though the late surviving species themselves, as we will get to, often are not. The second reason why studying them in the modern world is difficult is much more simple: We probably killed most of them. Relict lineages are per definition only barely scraping by, capable of persisting in their current environment, but only at low numbers, ill equipped for change. Because of this, humans are essentially the worst possible enemy for a late surviving species, almost perfectly adapted to extinguish them. It is quite probable that most "modern" relict lineages are now extinct, before we could get a chance to study them.
Thus, to properly study relict lineages, looking into the fossil record is the easiest way, right? Well yes, but only in the sense that it is pretty much the only way. Relict lineages are not well suited for preservation. They persist at low numbers, live in times when we would not except to look for them, and exist for a relatively short timescale geologically. All of these factors combined make it extremely unlikely that we would find the fossils of late surviving species, and indeed we very rarely do. Relict lineages are pretty much a statistical certainty, they must have existed. Considering the amount of extinction events in Earth's history, local and global, there must have been at least a few cases where a few members of a lineage managed to survive, but not properly diversify in the aftermath. Thus relict lineages and late survivors fall more into the camp of thought experiments, ideas that can be considered, but never truly verified. We can assert that they may have existed, but never confirm it.
The giant millipede Arthropleura. A rare case of a confirmed late survivor, as at least one species survived into the early Permian
As an example, I will focus much of the rest of this post on a particularly famous concept for a relict lineage, late surviving non-avian Dinosaurs. Now to be completely clear, I am not talking about the Mokele-mbembe, or any other alleged modern day animals. When I say late surviving non-avian dinosaur, I mean one that persisted into the Paleogene, anything further than that is highly improbable. I have already covered the Paleogene in a previous post, and I would advise you to read that one first to get some context. To briefly describe the environment of the early Paleocene however, it was a world mostly devoid of any large animals. Most animals even slightly specialized had died out, including the majority of pure herbivores and carnivores, leaving mostly small omnivores, and a fairly large amount of insectivores. The biggest creatures at the time were probably mammals about the size of a cat, but even these were rare. The planet was covered nearly from pole to pole in rainforest, and both the temperatures and sea-levels were much higher than today. The seas were populated almost exclusively by fish, with the exception of small Crocodiles and Champsosaurs which had survived the extinction.
Taking these factors into account, we can try and see what non-avian dinosaurs possessed these traits. First off, with small size, adaptability, and a non-specialized diet being the most important factors determining what survived the extinction, it is obvious why Birds were the only Dinosaurs to make it. They were pretty much the single lineage most perfectly prepared for the situation, if anything was going to survive, it was them. But if we hypothesize that at least a single other lineage survived, maybe only for a million years, what would it be? Well, of the late Cretaceous Dinosaurs, it must be said that aside from Birds, the other small Maniraptorans probably seem the most likely. A diminutive Troodontid or Dromaeosaurid would probably be the most likely options, perhaps slightly disappointing for anybody hoping for late surviving Sauropods. Already pretty much identical to Birds, the difference here between a small Troodontid and early Avian is tiny, and they would both have looked and acted almost exactly the same. But this fact may actually help make Troodontids and Dromaeosaurids slightly less likely, as birds may already have filled all of the niches they could have taken.
The tiny Troodontid Jinfengopteryx, by Matt Martinyiuk
So what group of small, non-specialized Dinosaurs not similar in ecology to birds existed? The obvious answer here seems to be small Ornithischians. While the larger Hadrosaurs and Ankylosaurs were far too large to have survived, small genera such as Leptoceratops potentially could have. Leptoceratops in particular may be unlikely, as the animal was still very big compared to any other animals known from the early Paleogene. A smaller relative would have been an ideal match however, and may have been one of the most likely candidates. Another possibility is a small species of Pachycephalosaurid or perhaps an undiscovered miniscule Hadrosauriform, we simply do not know. What we do know for certain however is that even if any non-avian dinosaurs did survive, they would not have persisted for very long. Being the blank slate that the early Paleogene was, any species of dinosaur that managed to gain a foothold would almost certainly have rapidly diversified, thus becoming a successful part of the time's fauna, and probably spawning a long line of descendants. If a small Ceratopsian had become established in the early Paleogene, it is quite plausible that Mammals would never have become as dominant as they are today.
Unescoceratops and Gryphoceratops by Julius Cstotonyi
At the end of the day, we do not know if any non-avian dinosaurs survived the k-pg extinction, just as we cannot know whether any other hypothetical late survivors actually existed. What we can however be sure of is that animals like them have existed in the past, small groups of practically defunkt creatures, clinging onto existence for just a little while longer than their relatives. Whether they be late surviving non-avian dinosaurs, Cretaceous Dicynodonts, or early Jurassic Rausuchians, relict lineages have existed, and some may still exist today, without us even realizing it. There is often something odd about them, like a Roman in Renaissance France, they seem temporally displaced, out of time and space.