Revisiting the enigmatic Cheirolepidiaceae: origins, phylogenetic relationships, and a new whole-plant concept

The extinct conifer family Cheirolepidiaceae spanned from the Late Triassic to early Palaeocene and was distributed worldwide. While it is known from multiple organs, there are no Cheirolepidiaceae whole-plant reconstructions to date. The relationships of Cheirolepidiaceae with different plant groups have been extensively discussed in the literature, but their phylogenetic position has not been thoroughly explored. To fill these gaps, we studied seed cones associated with other Cheirolepidiaceae organ remains from the Early Cretaceous of Arkansas and investigated their phylogenetic position.

We studied previously collected fossil remains from the Holly Creek Formation and included them in total evidence phylogenetic analyses with other Cheirolepidiaceae species and representatives of all major living and extinct conifer groups. We performed a time-calibrated Bayesian analysis under the fossilized birth-death model and 60 maximum parsimony analyses with varying search conditions.

We describe Arkansia axsmithii sp. nov. based on seed cones and isolated ovuliferous complexes associated with previously described wood, leafy shoots, pollen cones and pollen. We separated the associated vegetative material into Pseudofrenelopsis axsmithii sp. nov. Our phylogenetic analyses most frequently show the Cheirolepidiaceae as sister to the crown group of conifers, and more rarely as sister to Araucariales, Cupressales + Araucariales, or Araucaria. In some cases, Pararaucaria is recovered outside of Cheirolepidiaceae. We estimated ages of Carboniferous for the divergence of conifers and Cordaitales, and Late Triassic for the initial diversifications of Cheirolepidiaceae and the conifer crown group.

The Arkansia plant constitutes the most completely known Cheirolepidiaceae plant to date. Our phylogenetic analyses support Cheirolepidiaceae as a transitional lineage between voltzialean and living conifers, reflected in their ovuliferous complex morphology, which shows intermediate traits between voltziacean voltzialeans and extant taxa. Age estimates of deep nodes of conifer phylogeny are broadly congruent with the fossil record, suggesting a long-lasting stem group and an origin of crown-group conifers in the Late Triassic.