The past week has been spent in Agadez, a culturally rich historic city that has served as a key component of trans-Saharan trade routes for over 500 years. It is fitting then that this be the site of one of the previously mentioned, planned museums to showcase the paleontological, archaeological and cultural heritage of Niger. We’ve been working tirelessly to pack and catalog with meticulous detail all of the tools, camping gear, food, water, fuel, and supplies that we will need to survive the next three weeks in the extreme environment of the Sahara. We’ve also had to do our best to patch up three land rovers from the 1990’s that will be our only lifeline out of the desert. Before I leave for this 1st of 3 legs of the expedition, I wanted to write more extensively about the fish fossils that brought me here, especially given the tetrapod and specifically dinosaur laden contents of my two previous posts. The first site we are visiting is Gadoufaoua, which dates back to the Early Cretaceous (a little over a hundred million years ago) and is where the fish fossils that I have been studying were originally found. To appreciate the significance of these fossils, it’s first important to understand the importance of fish broadly in the story of animal evolution and biodiversity today.
To start, fish predate dinosaurs by some 300 million years and without them, dinosaurs could never have existed. This is because dinosaurs (and all tetrapods including humans for that matter) evolved from a lineage of fish with lobed fins known as Sarcopterygii. While there are only a few species generally considered fish (coelacanths and lungfishes) that exist in this group today, the ray-finned fish (Actinopterygii) that diverged from their lobe finned relatives some 450 million years ago make up almost all of today’s recognizable fish, which comprise nearly half of all living vertebrate species. Thus to study ancient fish evolution is to study the history of some of the most diverse and dominant lineages of animals known today. It is in this second group, the ray-finned fishes that the fossils from Gadoufaoua belong. Specifically, they belong to two orders that split early from the ancestors of modern fish, Polypteriformes and Ichthyodectiformes. Polypteriformes are actually the group to first diverge from all other ray-finned fish. It is for this reason that scientists, including Darwin, have long considered the 10 or so living species of this group, which inhabit the rivers of Africa, “living fossils”–species that exhibit characteristics similar to their ancient ancestors. Of course, this is a large assumption, and hinges on hundreds of millions of years of missing data. In light of this, the discovery of one of the oldest and most complete polypterid skulls will allow us to test the assumption that living polypterids are anything like their ancient ancestors, giving us additional clues as to what the ancestors of most living fish were like. The second fossil belongs to the order Ichthyodectiformes. This group is known for its members’ large size and unusual jaw, often with oversized teeth. These mostly marine predators lived with the dinosaurs and, when the asteroid came, seem to have died with them too. However, the fossil found in Gadoufaoua differs from traditional conceptions of Ichthyodectiformes in many ways. It was found in a freshwater deposit, lacks teeth on its lower jaw entirely, and is relatively small in size. Thus this fossil has the potential to elucidate a new lineage of freshwater Ichthyodectiformes, giving us a better understanding of the ecological diversity of the group. It is my hope that in the next three weeks we find additional, more complete specimens of these incredible fish, or even new fishy discoveries that will require separate attention before their secrets can be unraveled. In the past, Gadoufaoua has been the site of some lobe-finned fish discoveries as well, so the potential for discovery feels truly unlimited. Below are pictures of one of our rovers beginning to be packed, the skull of the Ichthyodectiforme fossil, and a drawing of a living polypterid (the fossil is still being prepared). Updates to come!
All of Robert's research blogs can be found here: https://robertaslaroche.com/blog/.
