Abstract
BackgroundThe timescale of the origins of Daphnia O. F. Mueller (Crustacea: Cladocera) remains controversial. The origin of the two main subgenera has been associated with the breakup of the supercontinent Pangaea. This vicariance hypothesis is supported by reciprocal monophyly, present day associations with the former Gondwanaland and Laurasia regions, and mitochondrial DNA divergence estimates. However, previous multilocus nuclear DNA sequence divergence estimates at < 10 Million years are inconsistent with the breakup of Pangaea. We examined new and existing cladoceran fossils from a Mesozoic Mongolian site, in hopes of gaining insights into the timescale of the evolution of Daphnia.ResultsWe describe new fossils of ephippia from the Khotont site in Mongolia associated with the Jurassic-Cretaceous boundary (about 145 MYA) that are morphologically similar to several modern genera of the family Daphniidae, including the two major subgenera of Daphnia, i.e., Daphnia s. str. and Ctenodaphnia. The daphniid fossils co-occurred with fossils of the predaceous phantom midge (Chaoboridae).ConclusionsOur findings indicate that the main subgenera of Daphnia are likely much older than previously known from fossils (at least 100 MY older) or from nuclear DNA estimates of divergence. The results showing co-occurrence of the main subgenera far from the presumed Laurasia/Gondwanaland dispersal barrier shortly after formation suggests that vicariance from the breakup of Pangaea is an unlikely explanation for the origin of the main subgenera. The fossil impressions also reveal that the coevolution of a dipteran predator (Chaoboridae) with the subgenus Daphnia is much older than previously known -- since the Mesozoic.
Highlights
The timescale of the origins of Daphnia O
The north-south hemispheric dichotomy of the subgenera in extant Daphnia is unapparent in the sparse fossil record – every Cenozoic record with fossilized ephippia from the northern hemisphere reports Ctenodaphnia-like fossils (China, Germany, Spain, and USA) [4,20,21]
Inspired by the relatively undamaged impression fossils of ephippia reported by Smirnov [11] from Mesozoic Mongolian sediments, we examined existing and new samples from this site
Summary
The timescale of the origins of Daphnia O. This vicariance hypothesis is supported by reciprocal monophyly, present day associations with the former Gondwanaland and Laurasia regions, and mitochondrial DNA divergence estimates. We examined new and existing cladoceran fossils from a Mesozoic Mongolian site, in hopes of gaining insights into the timescale of the evolution of Daphnia. The timescale of the evolution of some of the most successful freshwater microcrustacean groups such as copepods and cladocerans is poorly known or controversial. Both groups are comprised of small and often fragile species, whose fossilized body parts might be overlooked. Smirnov [11] assigned impression fossils of Mesozoic ephippia to the genus Daphnia (or “Daphnia-type”), but the subgenus was unassigned
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