How do fossils show change over time?

Answer 1

Fossils show change over time through transition. The term "transitional fossil" is used at least two different ways, often leading to muddled and stalemated arguments. For reference purposes, this article will refer to these these two meanings the "general lineage" and the "species-to-species transition". ;

; "Species-to-species transition" :

: This is a set of numerous individual fossils that show a change between one species and another. It's a very fine-grained sequence documenting the actual speciation event, usually covering less than a million years. These species-to-species transitions are unmistakable when they are found. Throughout successive strata you see the population averages of teeth, feet, vertebrae, etc., changing from what is typical of the first species to what is typical of the next species. Sometimes, these sequences occur only in a limited geographic area (the place where the speciation actually occurred), with analysis from any other area showing an apparently "sudden" change. Other times, though, the transition can be seen over a very wide geological area. Many "species-to-species transitions" are known, mostly for marine invertebrates and recent mammals (both those groups tend to have good fossil records), though they are not as abundant as the general lineages (see below for why this is so).

;

; Transitions to New Higher Taxa :

: Both types of transitions often result in a new "higher taxon" (a new genus, family, order, etc.) from a species belonging to a different, older taxon. There is nothing magical about this. The first members of the new group are not bizarre, chimeric animals; they are simply a new, slightly different species, barely different from the parent species. Eventually they give rise to a more different species, which in turn gives rise to a still more different species, and so on, until the descendants are radically different from the original parent stock. For example, the Order Perissodactyla (horses, etc.) and the Order Cetacea (whales) can both be traced back to early Eocene animals that looked only marginally different from each other, and didn't look at alllike horses or whales. (They looked rather like small, dumb foxes with raccoon-like feet and simple teeth.) But over the following tens of millions of years, the descendants of those animals became more and more different, and now we call them two different orders. There are now several known cases of species-to-species transitions that resulted in the first members of new higher taxa.

Answer 2

The fossil record demonstrates clearly that changes have occurred in animals and plants across the millennia. Further, the changes can be "followed" in the fossil record, and this demonstrates evolution in action. By looking at what was alive in an earlier block of time and what similar living thing was alive in a later block of time, we can see what changes occurred over that period. The organism got larger, smaller, or went through some other changes as revealed in the fossil record.