Late Triassic (Norian)
Fissure Fill
Pant Quarry, near Bridgend, South Glamorgan, Wales.
Excellent 1mm tooth of one of the very earliest mammals ever found.
Mammal,

Kuehneotherium is a genus considered to be one of the earliest-known mammals, if not the earliest.  It is found in the same site in Wales as Morganucodon though Kuehneotherium is represented by far fewer specimens.  It’s possible the two belong to separate families.

In a study of the tooth wear of these two genera (see Gill et al., 2014) it was revealed that Kuehneotherium fed on softer-bodied invertebrates than Morganucodon.

Both genera were about the size of modern shrews and likely had a very similar lifestyle.

https://www.nature.com/articles/nature13622