Synophalos xynos, a shrimp-like arthropod from the Early Cambrian of China (~515 mya). Thought to be closely related to stem-crustaceans like Waptia, it was about 2cm long (0.75″) and had a bivalved carapace with a segmented body ending in a forked tail.
Unlike any other known arthropods, however, it formed long “conga line” chains of up to twenty individuals, with the tail of each animal locking securely into the shell of the next. The function of the these chains is unknown, although suggestions include some sort of mating behavior, migration, or defense against predators.
Only one specimen was found completely on its own, and its slightly longer carapace suggests it may represent a different solitary life stage of these strange little creatures.
The turtle-jawed moa-nalo (Chelychelynechen quassus) was a large flightless goose-like duck from the Hawaiian island of Kaua‘i. About 90cm tall (3′) and weighing around 7kg (15lbs), these birds and their relatives were descended from dabbling ducks and existed on most of the larger Hawaiian islands for the last 3 million years or so – before going extinct around 1000 years ago following the arrival of Polynesian settlers.
Chelychelynechen had an unusually-shaped bill, tall and broad with vertically-oriented nostrils, convergently similar to the beak of a turtle. It would have occupied the same sort of ecological niche as giant tortoises on other islands, filling the role of large herbivore in the absence of mammals.
Mauriciosaurus fernandezi, a polycotylid plesiosaur from the Late Cretaceous of Mexico (~94-89 mya). About 1.9m long (6′3″) with a flipper-span of 1.5m (4′11″), it’s known from a near-complete skeleton with preserved soft tissue impressions. The fossil shows evidence of rows of very tiny scales, the skin outlines of the flippers, and also a thick layer of insulating blubbery fat.
Its body shape in life would have been similar to modern leatherback turtles, roughly teardrop-shaped and hydrodynamic – much chubbier than most plesiosaur reconstructions had been previously depicting!
The biggest known rodent of all time, Josephoartigasia monesi from the Pliocene and early Pleistocene of Uruguay, South America (~4-2 mya). Similar in size to a modern bison, it stood about 1.5m tall at the shoulder (4′11″) and weighed around 900kg (~2000lbs).
Despite looking like an extra-large capybara, it was only distantly related to the modern giant rodents. Its closest living relative is actually the much smaller pacarana.
Its 30cm long (12″) incisors could produce a large amount of bite force, and it may have used them in a similar manner to elephant tusks – rooting in the ground for food, stripping trees and branches, or defending itself from predators.
One of the earliest known dinosaurs, Herrerasaurus lived during the Late Triassic (~231 mya) in what is now Argentina, South America. It was a fairly lightly-built bipedal carnivore, with the largest specimens reaching sizes of just over 5m long (16′4″).
The exact classification of herrerasaurids is still somewhat unclear, with different analyses putting them in different positions on the early dinosaurian family tree. They’re generally considered to at least be closely related to basal theropods – but a recent analysis that reshuffles dinosaur relationships suggests their resemblance to the theropods might be a result of convergent evolution, with them being the sister group to sauropods instead.