Morphological stasis in the first myxomycete from the Mesozoic, and the likely role of cryptobiosis

Abstract

Myxomycetes constitute a group within the Amoebozoa well known for their motile plasmodia and morphologically complex fruiting bodies. One obstacle hindering studies of myxomycete evolution is that their fossils are exceedingly rare, so evolutionary analyses of this supposedly ancient lineage of amoebozoans are restricted to extant taxa. Molecular data have significantly advanced myxomycete systematics, but the evolutionary history of individual lineages and their ecological adaptations remain unknown. Here, we report exquisitely preserved myxomycete sporocarps in amber from Myanmar, ca. 100 million years old, one of the few fossil myxomycetes, and the only definitive Mesozoic one. Six densely-arranged stalked sporocarps were engulfed in tree resin while young, with almost the entire spore mass still inside the sporotheca. All morphological features are indistinguishable from those of the modern, cosmopolitan genus Stemonitis, demonstrating that sporocarp morphology has been static since at least the mid-Cretaceous. The ability of myxomycetes to develop into dormant stages, which can last years, may account for the phenotypic stasis between living Stemonitis species and this fossil one, similar to the situation found in other organisms that have cryptobiosis. We also interpret Stemonitis morphological stasis as evidence of strong environmental selection favouring the maintenance of adaptations that promote wind dispersal.

The Volyn biota (Ukraine) – indications of 1.5 Gyr old eukaryotes in 3D preservation, a spotlight on the “boring billion”

“…Filamentous millimeter-long sporocaps, such as shown in Fig. 3a in Rikkinen et al (2019), are similar to what we see in Fig. 4b.…”

New mid-Cretaceous cryptic slime mold beetles and the early evolution of Sphindidae (Coleoptera: Cucujoidea)

“…The fossil record of slime molds is exceedingly scare, owing to the fragile nature of their fruiting bodies. Nonetheless, recent discoveries of exceptionally preserved slime molds in Burmese amber (Poinar and Vega 2019;Rikkinen et al 2019) suggest that the group was diverse in saproxylic habitats by the mid-Cretaceous. The growing fossil record of Cretaceous sphindids provides corroborating evidence that slime molds were important players in terrestrial ecosystems in the Mesozoic.…”

Past and Ongoing Field-Based Studies of Myxomycetes

“…However, due to the very fragile nature of the fruiting body, fossils of myxomycetes are exceedingly rare. Domke [ 3 ] described a species of Stemonitis, and Dorfelt et al [ 4 ] a species of Arcyria from Baltic amber dating approximately 50 million years ago, but the earliest known record of a myxomycete is a fossil of Stemonitis [ 5 ] from the mid-Cretaceous, about 100 million years ago ( Figure 3 ). Interestingly, except for their age, all three fossils could easily have been assigned to still extant species.…”