The Burgess Shale in British Columbia contains very old (~550 million years ago), very rare fossils of very early organisms. This is Haplophrentis. The function of the pair of short spines is unknown but the creature was tiny, just a few centimeters in length (hint 2.5 cm = 1 inch, more or less).
Dinosaur Teeth: Dental and dietary diversity in multituberculate mammals. From Adaptive radiation of multituberculate mammals before the extinction of dinosaurs, Gregory P. Wilson et. al. Nature Vol 483, Issue 7390 (22 March 2012) #dinosaurs Nature Publishing Group
Devonian: geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period (about 419 may) to the beginning of the Carboniferous Period (about 358 mya); named after Devon, England, where the rocks from this period were first studied; 1st significant adaptive radiation of terrestrial life; no large vertebrate terrestrial herbivores yet, but free-sporing vascular plants spread across dry land, extensive forests
This Petrolacosaurus (Science magazine), found in late Carboniferous deposits from Kansas, is one of the earliest known reptiles. Significant adaptive radiation of reptiles, like the conifers, was delayed until the aftermath of the Permo-Triassic extinction.
An Adaptive Radiation Has Led to a Dramatic Diversification of the Drosophilids in Hawaii | Evolution in Hawaii: A Supplement to 'Teaching About Evolution and the Nature of Science' | The National Academies Press