DISPERSAL ECOLOGY OF A LATE CAMPANIAN FLORA FROM SOUTH-CENTRAL NEW MEXICO; IMPLICATIONS FOR THE BREADTH AND TIMING OF ANGIOSPERM REPRODUCTIVE STRATEGIES

Seed dispersal shapes ecological and evolutionary patterns by controlling the geographic distribution of taxa, the gene flow and genetic structure of populations, and the biotic interactions of ecosystems. Morphological aspects of plant diaspores (the unit of dispersal consisting of the seed and accessory structures) reflect important aspects of the life history strategies of seed plants. In particular, seed mass in angiosperms has been shown to correlate with plant size and the likelihood of being biotically dispersed, and inversely correlated with growth rate and the number of seeds produced by a plant. During their radiation in the Cretaceous, abiotic dispersal and “weedy” reproductive strategies are thought to have been important aspects of angiosperm biology, reflected by their small average diaspore sizes and restricted within-flora size ranges. Values similar to modern floras are not seen until the Eocene. We tested these patterns using new collections from a diverse in-situ Late Campanian flora from the Jose Creek Member, McRae Fm. of south-central New Mexico. The flora is preserved in a single layer of volcanic ash spanning over 1.2 km, and preserves seeds as adpressions, casts, and molds. From a study of over 300 diaspores, we described over 30 morphotypes and binned them in broad taxonomic groups based on morphological characteristics. Mode of dispersal (biotic, wind-dispersed, or unassisted) was assigned to each morphotype based on the presence/absence of wings or a fleshy layer. We calculated their volume (a strong predictor of mass) following established protocols using measured length and width. Seed thickness was estimated as 2/3 of width when it could not be confidently measured directly from the specimens. Mean angiosperm diaspore volume was ~45 mm³, and the total volume range spanned 3.5 orders of magnitude. Approximately 40% of diaspore morphotypes are fleshy, suggestive of biotic dispersal, whereas only ~5% are winged. Average volume and size range were both greater than those of other Cretaceous floras examined with this methodology. These findings reinforce evidence for the prevalence of fleshy diaspores and biotic dispersal in the Cretaceous, and show that, in some environments, angiosperms attained a large breadth of reproductive strategies by the Late Campanian.