First Fertile Pinnule Fossils of the Extant Southern American Tree Fern Genus Loxsomopsis (Loxomataceae) in Mid-Cretaceous Myanmar Amber

Mesozoic tree ferns related to the extant Loxsomopsis, a relict and taxonomically isolated genus endemic to tropical South America, are exceedingly rare, making it difficult to trace its origin and evolution. Here, we describe a mid-Cretaceous fossil attributable to Loxsomopsis, Loxsomopsis minor sp. nov., based on two fertile pinnules preserved in Myanmar amber. The new species is assigned to the extant genus based on a suite of diagnostic features, including pinnate fertile segments with short lobes, marginal paraphysate sori, narrowly cyathiform to urceolate indusia, columnar and exserted receptacles, oblique sporangial annuli, and trilete spores. This discovery extends the fossil record of Loxsomopsis back to the Albian–Cenomanian and provides new evidence that the lineage—now restricted to tropical South America—was once more widespread. The occurrence of Loxsomopsis in the Myanmar amber biota is consistent with a Gondwanan affinity for this mid-Cretaceous forest ecosystem and suggests that the genus may have likely originated prior to the breakup of Pangaea.