EARLY COLONIZATION OF NON-MARINE HABITATS BY CLAM SHRIMP—A NEW PERSPECTIVE FROM THE DEVONIAN (EMSIAN) KLERF FORMATION IN GERMANY

Excavations over the last few decades within the Klerf Formation (Emsian, Devonian) at Willwerath and Waxweiler in southwest Germany yielded more than thirty reasonably well-preserved and previously unstudied clam shrimp fossils. These fossils have provided the impetus to re-evaluate the early Devonian origin of clam shrimp. Our new collections support the presence of at least seven and possibly up to ten clam shrimp species in the Klerf Formation—though not all necessarily extant at the same time in the same locality. Multiple species do co-occur on the same slab, suggesting an amount of ecological differentiation that is unusual today. The paleoenvironment is a deltaic brackish to freshwater body of water. An interesting clam shrimp-leperditicopid mass assemblage was recovered from Waxweiler. The fact that each crustacean group present in the assemblage has bivalved carapaces in the ‘butterfly position’ suggests decay of the adductor muscles, which control closing of the carapace, prior to burial.

The Klerf Formation is early Devonian (lower to middle Emsian) in age. As such, it is essentially coeval with two other early Devonian clam shrimp localities: Yuankou Formation in Hunan, China and the Matringhem Sandstone in France. Between these three units, there is a significant familial level diversity—all apparently freshwater or brackish. There are no earlier clam shrimp—freshwater or otherwise—to help us understand their origin. Shared features amongst modern branchiopods suggest that these early clam shrimp had already pioneered desiccation-resistant eggs that could survive passage through a predator’s digestive tract. The evolutionary pressures driving the origin of those features are unknown. A new genus of ‘leaid’ clam shrimp displays nodes on the carapace arrayed in linear, carina-like features. This suggests a possible mode of origin for the distinctive carina of ‘leaids.’