Macroevolution & Evolutionary Morphology

Stingrays are an underappreciated study system for analyzing ecomorphological diversity.  Batoids make up more than half the total species diversity of the elasmobranch fishes (sharks and rays), while encompassing the majority of the functional, dietary, and ecological disparity within the lineage.  Elasmobranch cranial anatomy is a deceptively simple system on the surface, with comparatively few skeletal elements compared to teleosts.  However, the paucity of skeletal elements belies the intrinsically dynamic nature of these systems due to skeletal decoupling and resulting cranial kinesis, where much of the structural and functional stability of the system are filled by muscles (Kolmann et al., 2014).  South American freshwater  stingrays (Family Potamotrygonidae) are particularly variable even for stingrays, in having evolved what appears to be, significant variation in diet and foraging strategies.  This pattern is especially interesting given potamotrygonid rays’ use of some novel prey items (insects), a presumably young clade age, and occupation of a unique habitat (for elasmobranchs): inland freshwater tributaries of the Amazon, Orinoco, and other Guiana Shield drainages.

Character displacement is thought to drive diversification in regions where competitors overlap in range. Competition then promotes disparity in body size and shape, leading to the wide diversity of animals present in different habitats. One clade in particular, the anchovies (Family Engraulidae), have a global distribution and occupy an array of habitats, from freshwater rivers to coastal estuaries. The wide variety of habitats in which anchovies occur has presumably influenced their diversification, especially where multiple, related species co-occur.     Using a molecular phylogeny coupled with shape morphometrics, this study investigated whether New World, marine anchovy species with overlapping ranges experience greater body shape diversification, presumably a result of local interspecific competition. Museum specimens were photographed and then analyzed using geometric morphometric techniques.   Co-occurring species pairs were tested against allopatric pairs to determine if their mean shape disparity was greater than expected by chance.  Results suggest that despite greater species richness in the Caribbean anchovies from the Pacific Ocean occupy a greater area of morphospace than do Caribbean taxa. Differences in body shape disparity are primarily in the head region, particularly the mouth, suggesting that feeding morphology may explain differences between taxa.

The common ancestor of sharks and stingrays diverged early in the evolution of elasmobranch fishes.  The batoid fishes now make up over half of the morphological diversity/disparity of extant elasmobranch lineages.  Shared in common by all batoid fishes is a euhyostylic jaw suspension, which enable the jaws to be completely decoupled from the cranium except for a connection via the hyomandibular cartilage.  In addition, almost all batoids have ventrally-oriented mouths.  Broadly, how have these modifications to the primitive gnathostome feeding apparatus contributed to the modern myliobatiform cranial bauplan? 

Objectives:  1) Can we distill testable hypotheses of functional ability from muscle configuration, architecture and attachment?  2) From an anatomical standpoint, what differentiates stingrays from their skate cousins?  3) What commonalities (if any) are there between durophagous and non-durophagous stingrays?

Kolmann, M.A., Huber, D.R., Dean, M.N. and Grubbs, R.D. (2014). Myological variability in a decoupled skeletal system: batoid cranial anatomy. Journal of Morphology.