Muscles and muscle scars in fossil malacostracan crustaceans

Abstract

Exceptionally preserved specimens yield critical information about the soft-part anatomy and the evolution of organisms through time. We compiled the first global dataset of exceptionally preserved muscles in malacostracans consisting of 47 occurrences, including 18 new records, predominantly preserved in Mesozoic Konservat-Lagerstätten (>70% of occurrences). Early diagenetic mineralization through phosphatization is the dominant process for exceptional preservation of muscles in malacostracans. Over 70% of taxon occurrences with muscles preserved are compressed. Rarer, three-dimensionally preserved specimens allow more detailed study of muscles. One example are specimens of the mid-Holocene ghost shrimp Sergio sp. from Panama, showing exquisitely preserved strings of muscle fibers attached to the shell interior, resembling the muscle arrangement of modern ghost shrimps. Other fossil malacostracans, including the oldest known fossil shrimp, also show musculature similar to modern taxa. We hypothesize that this muscle conservatism may be related to the confined space within the malacostracan shell in conjunction with the relatively stable body plan of several clades. We also assembled the first dataset on muscle attachment scars in malacostracans. Unlike muscles, muscle scars are more common. Approximately 24% of the 357 articles analyzed yielded evidence of muscle scars, but such scars were only recognized for 19% of the 162 taxon occurrences that showed muscle scars. Muscle scars are common from the Late Jurassic onwards, are found primarily in Brachyura and Axiidea, and do not suffer from a Lagerstätten-effect. Rocks with well-preserved specimens should yield an additional wealth of information on the soft part anatomy of malacostracans. Similarly, muscle scars represent an almost untapped, complementary source of information on muscle evolution.