THE AMBER WINDOW AND THE AMPHIPOD FOSSIL RECORD

The amber window for fossil preservation opens for biological reasons—plants begin producing resin in sufficient quantities such that organisms can become trapped and fossilized within it. This window of preservation does not exist during the Paleozoic, and begins to open during the Mesozoic. Our first fossils preserved in amber are Triassic in age, but abundant fossil preservation via amber does not occur until the late Jurassic and Cretaceous. The amber window allows many groups of organisms, insects especially, to be preserved that previously had no fossils record. As a result, there is a clustering of first appearances at the first opening of the amber window.

Amphipods do not fit this model. Though some have placed pre-Cenozoic fossils into the Amphipoda, none stand up to close scrutiny. Thus, the first reliable occurrence of amphipods is that of the three senticaudatan families known from the Eocene Baltic Amber. No amphipods are known from Jurassic or Cretaceous amber deposits, suggesting that the appearance of amber-preserved amphipods in the fossil record reflects the real origin of the group, not just the opening of the amber window. Though higher-level amphipod phylogeny is not well understood, the three Baltic Amber families (plus those known from the North American amber deposits) have a wide phylogenetic distribution across the Senticaudata—suggesting that the Senticaudata branch of the amphipod tree evolved relatively rapidly in the early Cenozoic.