rhamphotheca:

dailyfossil:   Morganucodon

When: Late Triassic to Early Jurassic (~205 to 190 million years ago) 

Where: Laurasia (the super continent of Europe, Asia, and North America) - best fossils in England and China. 

What: Morganucodon has often been called the ‘first mammal’.  It was roughly 5 inches (~13 cm) long and most likely insectivorous.  It is the classic ‘running between the feet of dinosaurs’ mezosoic ‘mammal’.  Using proper scientific terminology, based upon cladistic trees, it is not a member of Mammalia, but rather one of the oldest members of Mammaliaformes. The label of first mammal is commonly applied to Morganucodon because it was one of the first Mesozoic mammal relatives found - it was first published in 1949.

A suite of features clearly place Morganucodon closer to extant mammals than its contemporaries. Those which are easiest to see and explain concern the teeth and lower jaw. Morganucodon has teeth clearly defined into different teeth loci (incisors, canine, premolars, molars) with distinct cusps on each - unlike the identical uni-cusped tooth of reptiles.  The jaw of this animal also has a ‘twinned’ jaw articulation. It still retains an articulation between the articular and quadrate bones (these bones later go on to become the malleus and the incus, respectively), but also has a direct connection between the dentary and the squamosal, as in all living mammals. This was a transitional point in the development of the ‘true’ mammal like ear. The amazing record we have of this, and other, transitions highlights how hard it is to pin the label of ‘mammal’ in any one place. Where a label is placed today, tomorrow a fossil might be found that just had one less feature of ‘mammals’. Thus, it is more consistant to use terms like mammaliform rather than mammal for animals such as Morganucodon - these terms are well defined and will not need to be totally reassessed as more and more fossils are discovered. 

(via scientificillustration)