New information on the feeding habits of the percomorph Rhenanoperca minuta, together with a short look at other fish species from the Eocene Messel Formation of Germany

Abstract

appler Forage remains were studied in the digestive tracts of four Messel fish species ( Rhenanoperca minuta , Thaumaturus intermedius , Cyclurus kehreri , Atractosteus messelensis ). They were found in only 4% of all samples. Particular attention was paid to R. minuta . Herein, depending on the investigation method, between 0.7% and 13% of the samples contained conspecific prey fish and/or prey fish remains. In total 1.6% contained remains of amphipod shrimps. Concerning T. intermedius , prey (arthropod) remains could be found only in one sample (3.4%). Similarly, only one (6.6%) of the bowfins ( C. kehreri ) and none of the gars ( A. messelensis ) contained such remains. The pharyngeal jaws of R. minuta exhibit two basic types of dentition. One is characterized by strong, flattened (“molariform”) pharyngeal teeth, and the other by more delicate and slender (“papilliform”) ones. This polymorphism may be indicative of a beginning or advancing speciation. The different morphotypes probably originated in adjacent water bodies (allopatric) rather than in Lake Messel itself (sympatric). The results were discussed with particular attention to extant comparable species. The high rate of evacuated digestive tracts in R. minuta very probably results from a shortage of suitable prey, and possibly also from environmental restrictions. For the other species, different factors, like diurnal or seasonal fluctuations may have played a more important role. For R. minuta , a diet switch from predominantly arthropods to fish, especially a switch to T. intermedius as a main prey, can be discarded. Rather there appears to have been a gradual transition from soft­bodied arthropods to gastropods, as known from comparable recent species, even actual though direct evidence (gastric or intestinal contents, or even co­occurrence with abundant gastropods) could not observed among the investigated