When: Late Triassic (~210 million years ago)
What: Vallesaurus is the most complete drepanosaur (monkey lizard) ever found. This tiny lizard is only 6 inches (~16 cm) long, making it much smaller than most of its relatives. Like the rest of the clade it was an arboreal lizard, with not only long fingers and toes well adapted for gripping branches, but also a prehensile tail. Prehensile tails have developed several times in both reptiles and mammals, with chameleons an example of a living prehensile lizard. In many ways Vallesaurus and its close relatives can be thought of as triassic chameleons, occupying the same tree-dwelling and insect -eating reptile niche. More derived drepanosaurs also have opposed fingers, as in chameleons, and evidence of the same mechanism that allows chameleons to quickly dart their head forwards to capture prey. Little Vallesaurus itself does not have these adaptations, as it is a fairly primitive drepanasaur, and it is likely that it moved though the trees much faster than the rest of its clade and modern chameleons. Thus, it is shown that this group was already well adapted at climbing trees before converging upon a modern chameleon body plan. The vast majority (if not all) fossils of Vallesaurus and its kin are found in lake deposits, showing that these animals were at home high in the tree tops, with branches that reached out far over the waters. And that despite the large suite of climbing adaptations, they sometimes misstepped and took quite the plunge.
Vallesaurus is not closely related to any living lizard. Its clade is yet another group of ancient reptiles that do not have a very well understood position in the tree of life. However, it is universally agreed that they are diapsid reptiles, and more closely related to archosaurs than to lizards.
Reconstruction by Matt Celeskey at the New Mexico Museum of Natural History & Science
Mounted specimen at the American Museum of Natural History, NYC
When: Early to Middle Eocene (50-46 million years ago)
Where: Best fossils from Western North America, some European material referred, but its very fragmentary.
What: Vulpavus is a basal relative of the order Carnivora. Living species in this order include dogs, cats, bears, seals, hyenas, civets, mongooses, and meerkats, to name but a few members of this diverse clade. As these animals are all in the same taxonomic group, they have a common ancestor. Vulpavus falls on the lineage before this common ancestor, thus it is equally closely related to all living carnivorans. There are a slew of fossil taxa that fall outside of Carnivora, but are more closely related to this order than any other group, ranging from the early Paleocene to the late Eocene. Vulpavus is one of the most well studied, as complete skeletons have been known since the early 1900s.
Functional studies of Vulpavus show this animal was well adapted for an arboreal life style, climbing though the sub-tropical forests of ancient Wyoming. Previously it was thought this was the ancestral condition of Carnivora, and that later fossils that are less arboreal adapted reflected a ‘decent from the trees’ leading to Carnivora. Current studies have shown instead that Vulpavus represents an early offshoot from this lineage, which became extremely well suited for life in the trees. Not only could it climb well, but its teeth were less blade-like than its relatives, as most fossils related to Carnivora were well adapted for eating meat. This meant that Vulpavus would have eaten much more plant material than its ancestors, which had a meat based diet much like most living carnivorans. The forests of Wyoming vanished in the mid-Eocene as the area became cooler and drier, and with this loss Vulpavus also became extinct.
I like to refer to this animal and its relatives outside Carnivora as kitty-dogs, as they are a mix of features from both dog-line and cat-line modern carnivorans.