BODY SIZE IN BARNACLES FROM THE MIDDLE TO LATE MIOCENE, CALVERT CLIFFS, MARYLAND: SUBSTRATE, BIOTIC DENSITY, AND CLIMATE CHANGE MATTERS

The Middle and Late Miocene were times of major climatic change, which can be an analog to today’s changing climate. The fossil record provides an excellent means for analyzing how organisms respond to climate change. Here, we focus on how barnacle body size responds to not only their substrate and density, but also climate change. We examined Balanus concavus (cf. Concavus chesapeakensis) from the Calvert Cliffs, Maryland Shattuck Zones 16–23. Results indicate that B. concavus lived on mostly mollusks (scallops, whelks, mussels) and sand dollars. The largest barnacles were found on sand dollars (Abertalla: height (mm)/width (mm): 80.44/71.91), followed by scallops (Chesapecten: 31.17/36.02), whelks (Buccinofusus: 29.48/24.68), and mussels (Modiolus: 27.74/29.54). Most barnacles occurred in clusters (average 23 barnacles to a cluster) on Chesapecten and from one to three barnacles on the other species. Barnacles living in clusters were significantly smaller (average height 30.79 mm, width 28.13 mm) than the other biotic substrates with few barnacles (average height 51.04 mm, width 48.00 mm; t-test of unequal variance, p < 0.0001). Widths were larger when barnacles were not clustered, especially on sand dollars where barnacles achieved their largest size. Finally, barnacles were largest during the Middle Miocene Climatic Transition (SZ 16–18; average height 40.61 mm, width 43.11 mm) when the East Antarctic Ice Sheet began to regrow, cooling the Earth compared to the Late Miocene (SZ 21–23) where they were significantly smaller (average height 22.91, width 28.03 mm; t-test of unequal variance, p < 0.0001). The Late Miocene is characterized by cooler and drier (less rainfall) climates, possibly affecting nutrient delivery to the coastal oceans leading to smaller barnacle body size. Overall, to understand body size patterns in relation to climate change, we must consider substrate and density of organisms prior to analysis of temporal climatic trends for encrusting organisms like barnacles.