A fossil-informed pattern of body size increase and local extinction in Basiceros dirt ants (Hymenoptera: Formicidae)

Basiceros dirt ants are morphologically distinct and widely distributed members of Neotropical communities. These ants possess features that aid in leaf litter camouflage and are larger than other closely related lineages with similar cryptic adaptations. Here, we report the first fossil of this genus-group, Basiceros enana sp. nov. from Miocene-aged Dominican amber, which reveals a pattern of body size evolution and disjunct biogeography. The fossil evidences the local extinction of Basiceros in the Caribbean, even as living Basiceros are known from Honduras to Southern Brazil. Using combined morphological and molecular datasets of all closely related lineages, we recover the evolutionary trajectory for body size within the group, demonstrating that body size was initially small in these ants and followed by a rapid expansion of body size in the common ancestor of all living species. Results reflect the capacity for early morphological evolution to influence perceived patterns of body size increase through a mosaic of ancestral legacy and continuous enlargement.

Methods
Our dataset is composed of data for 3 analyses: comparative morphospace, combined morphological-molecular phylogenetic reconstruction, and ancestral state reconstruction and body size evolutionary rates.

For the comparative morphospace, ant trait measurements were taken of all species within the genus Basiceros, as well as representatives from each genus within the Rhopalothrix genus group. Morphometric sampling included linear measurements of 19 morphological traits: 11 cephalic, three mesosomal, and five metasomal. We used a subset of measurements taken above for morphospace analysis to exclude measurements that cannot be obtained through photographs: HL1, HL2, HW1, MDL, SL1, SL2, PDL, AFL, FUL, EL, EW, ML, MFL, MTL, PTH, PTL, PTW, PPL, and PPW. We employed principal component analyses (PCA) on linear measurement data to reduce the dimensionality of the morphological dataset. All measurements can be found in the file Basiceros_enana_complete_database.xlsx.

For the combined morphological-molecular phylogenetic reconstruction, we compiled molecular data from Probst et al. 2019 as well as those available from GenBank, along with morphological characters from 17 species in the Rhopalothrix genus group. We then performed a series of phylogenetic reconstructions based on molecular and morphological data to assess the placement of Basiceros enana sp. nov. among extant congeners. All analyses were run on Mr. Bayes. The morphological matrix can be found in the file Basiceros_enana_complete_database.xlsx.

Finally, for the ancestral state reconstruction and body size evolutionary rates, we performed an ancestral state reconstruction of PCA scores. We extracted principal component 1 (PC1) scores from a PCA of 17 linear measurements taken from the smallest specimen for each species included as a terminal in our combined phylogenetic analysis. We reconstructed ancestral PC1 scores using a single rate method with fastAnc in “phytools” v.2.3-0 (Revell 2012).