Organisms that have hardly changed since their earliest known fossil

Cockroaches

The earliest cockroach-like fossils ("blattopterans" or "roachids") are from the Carboniferous period 354–295 million years ago,^[4] as are fossil roachoid nymphs.^[11] However, these fossils differ from modern cockroaches in having long external ovipositors and are the ancestors of mantises, as well as modern blattodeans. The cockroach, is still around and still easily identifiable with these 311 million year old fossils.

The siderite nodule (311 million years old, from the UK) with the roachoid fossil insect Archimylacris eggintoni inside. Visible on the surface are the headshield and wings, but micro-CT also revealed both limbs, mouthparts and a portion of the body.

Coelacanth

Until 1938, coelacanths were known only as an order of peculiar lobe-finned fishes which appeared in the fossil record almost 400 million years ago and then seemed to go extinct about 80 million years ago. So the discovery of a live coelacanth off the coast of South Africa in 1938 was understandably met with great excitement. The fascination scientists and the general public have with coelacanths is likely caused by their unusual appearance, their evolutionary importance, and the fact that they have remained virtually unchanged morphologically for millions of years.