Abstract Detail

Paleobotany

Bonacorsi, Nikole [1], Leslie, Andrew B [2].

Spore size, sporangium size, plant architecture, and the evolution of plant reproductive allocation over the Devonian.

The Middle Devonian was a pivotal time in plant evolutionary history, which saw the rise of the first large trees and diverse forest ecosystems. Plant reproductive strategies also diversified over the Middle Devonian, as reflected in the evolution of larger spore sizes and the first appearance of heterospory. The increase in reproductive strategies may generally reflect the ecological diversification of plants, but the exact reasons for these changes and their evolutionary consequences remain unclear. Here we explore the consequences of increasing spore size on the allocation of reproductive resources within specific sporangia, in order to better understand the consequences of increasing spore size on the reproductive strategies of Devonian plants. We compiled a dataset of microsporangium and megasporangium sizes, based on linear measurements and estimated volumes from published literature sources, for the Late Silurian through the Late Devonian. We particularly focused on taxa containing in situ spores, which we also coupled with information on supporting branch diameter where available. We find that Early Devonian sporangium sizes were relatively large, but sporangia show a decrease in average size over the Middle Devonian. Sporangia sizes then increase in average size and variance in the Late Devonian. The Middle Devonian decrease occurs at the same time that large spores (both large meiospores and early megaspores) become more abundant, meaning that the number of spores produced per sporangium declines significantly over this interval. In situ data suggests this pattern is mainly driven by early euphyllophytes, as lycophytes show consistently larger sporangia. These patterns are consistent with differences in branching architecture between these groups; euphyllophytes evolved finely dissected branching systems that terminated in small sporangia, while lycophytes generally had larger branches bearing larger lateral sporangia. Our results suggest that sporangium size and spore size were largely decoupled across the Devonian. They also demonstrate that the evolution of different plant architectures has a pervasive influence in structuring plant reproductive allocation, not only in terms of the aggregation of sporangia into stobili as noted in previous studies, but also in terms of the specific reproductive output of the sporangia themselves. With its combination of increasing spore size and decreasing sporangium volume, the Middle Devonian is then a unique time period in the history of plant reproductive allocation and the evolution of reproductive strategies.

1 - Brown University, Ecology and Evolutionary Biology, 40 Taber Ave, Providence, RI, 02906, USA
2 - Brown University, 80 Waterman St., Providence, RI, 02912, United States

Keywords:
Devonian
heterospory
sporangium
euphyllophytes
lycophytes.

Presentation Type: Oral Paper
Session: 16, Cookson/Moseley and Paleozoic paleobotany
Location: Sundance 4/Omni Hotel
Date: Monday, June 26th, 2017
Time: 4:00 PM
Number: 16002
Abstract ID:370
Candidate for Awards:Isabel Cookson Award