How many extinction events have there been?

Reasons Why Living Fossils Exist?

TL; DR: Living fossils are the children of long-dead fossilized creatures who were good at resembling each other; other animals are the children who were good at being different.

The boring answer is that there are living fossils because nothing has been done to make them become extinct.

The more exciting answer is a story of intrigue and lies dating back hundreds of millions of years:

The lie that the "species" is

The concept of a species is, of course, a lie. It's a useful lie, and one without which the equally useful field of taxonomy would be impossible, but a lie nonetheless. Every living being is ultimately a unique individual for itself. We group these people and call them things, after more or less arbitrary things, and we put these groups into larger groups and fill books on books using this method because it is super easy to get a good overview of how life is developed.

However, if we are to understand the details, we need to see previous species and look at the lineages.

The shark, our ancestor 1

One such line that is of interest is that of the shark. Sharks are ancient : They are about as old as plants and older than almost all land fauna. They are vicious, relentless killing machines, and with their slender shape, conveyor belts of teeth, and rough skin, they are far from perfect. Eventually, however, when the shark lineage split into two parts: one branch that leads to fish, amphibians and eventually dinosaurs, squirrels and dolphins, and one that leads to sharks. The ancestor of all these things was the early cartilaginous fish of the Cambrian Ocean.

To answer the question asked, we need to compare the success factors of the two lines.

Why sharks still exist as living fossils

The real answer here is that they literally aren't. Modern sharks are no more similar to their long-dead ancestors than squirrels. However, the biological niche that sharks (apex predators) live in is successful, and sharks that already exist in that niche have had successful children who have become more successful by superficially and behaviorally looking after their parents and thus the modern shark resemble presented as a living fossil.

Why shark contemporaries Not still exist as living fossils

The real answer here is that they literally are. Aside from the morphological differences, squirrels are no more different from their long-dead ancestors than sharks. Rather, overcrowded niches have made squirrels more successful generation after generation by differentiating themselves from their parents in terms of surface and behavior, and this is how modern squirrels came into being.


To edit: Footnotes:

  1. In fact, the shark is not our ancestor, but Cambrian early-jaw fish. However, the image of a shark is far more impressive and easier to discuss.

Jayachandran

You can use "the shark, our ancestor" as a reference for your claim that fish evolved from sharks. There is also no reference for your claims. How can I accept your answer?

Williham Totland

@ Jay Sure, let me get started right away.

Williham Totland

@Jay And that's where I specified our relationship with sharks in more detail.

Jayachandran

"The shark lineage is split in two: one branch that leads to fish, amphibians and eventually dinosaurs, squirrels and dolphins, and one that leads to sharks." Here you mean the shark instead of "Cambrian early jaw fish".

Williham Totland

@ Jay The statement "The line from X is divided in two: a branch that leads to Y and a branch that leads to Z." applies equally to literally all known living things X, Y and Z, which is largely the point of the answer. The details of what creatures they are descend, are for this answer really not important. That being said, whether we are descended from sharks is really a subjective question: if you follow the shark lineage backwards, you will find it damned hard is to put a hard line where "early jaw fish" end and "sharks" begin.