Final answer:
One of the earliest true mammals, from the genus Morganucodon, lived about 200 million years ago. It was discovered by paleontologists in the early 20th century, and had distinguishing mammalian features like differentiated teeth. The fossil record's incompleteness poses challenges, but paleontologists use reconstruction techniques to study early organisms.
Step-by-step explanation:
The identification of the species that comes first after the presumed ancestor of all mammals is a complex task, as the fossil record is incomplete and subject to interpretation. However, one of the earliest known true mammals is from the genus Morganucodon, which lived approximately 200 million years ago in the late Triassic and early Jurassic periods. Paleontologists discovered it in the early 20th century. These small mammal-like creatures had characteristics such as differentiated teeth and likely a warm-blooded metabolism, which are traits distinctly associated with mammals. When paleontologists search for evidence of early organisms, they look for fossilized bones, tracks, or other signs of life. Challenges arise due to most fossils found being incomplete, but scientists utilize various reconstruction techniques and comparisons with modern organisms to fill in the gaps. In consideration of the evolution of traits, it's important to note that mammalian traits such as hair and mammary glands are unique to the group, but when discussing the emergence of advanced characteristics like the amniotic egg, these predate the earliest true mammals and are found in earlier reptilian ancestors.