Rare leaf fossils of Monimiaceae and Atherospermataceae (Laurales) from Eocene Patagonian rainforests and their biogeographic significance.
Open Access
- Author:
- Knight, Cassandra
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 05, 2012
- Committee Members:
- Peter Daniel Wilf, Thesis Advisor/Co-Advisor
- Keywords:
- Eocene
Patagonia
Monimiaceae
Atherospermataceae
Paleobiogeography - Abstract:
- Two Eocene fossil sites, Laguna del Hunco (LH, ca. 52.2 Ma) and Río Pichileufú (RP, ca. 47.7 Ma), in Patagonia, Argentina have some of the most diverse fossil floras known. They represent angiosperm-dominated Gondwanan rainforests and have been the focus of many paleontological and geological studies. Here, I focus on rare, toothed fossil leaves that have received little attention. These show affinities to Laurales, a magnoliid order in the basal angiosperms, representing the families Atherospermataceae and Monimiaceae. These ancient families currently exhibit broad, often disjunct southern distributions, and thus hold much interest for Gondwanan biogeography. Their fossil records are exceptionally scarce, in that only ca. 17 fossil species, 11 based on fossil wood and six on leaf adpressions, are reported between Atherospermataceae and Monimiaceae. The fossil wood records have low taxonomic precision but are more recently published. However, the leaf macrofossil record, prior to the single Neogene example, is in need of critical revision because most identifications are poorly supported. Thus, any new foliar material referable to these families is of great interest. My purposes are to: 1) evaluate new fossil material for one fossil-leaf compression species of Atherospermataceae and one of Monimiaceae from Eocene Patagonia; 2) determine modern genera showing the closest affinities to these fossil species; and 3) improve current understanding of Laurales paleobiogeography. Historically, the RP and LH floras were thought to include several fossil species with affinities to living forests in southern South America. Many of these have recently been revised to genera with Australasian affinities, indicating that the floras may not include many connections to living Patagonian floras. Furthermore, the Australasian elements show that biotic exchange was occurring between South America and Australasia, via Antarctica, before the final separation of South America and Antarctica during the middle Eocene. The first fossil species evaluated is Laurelia guinazui Berry 1935, long considered a probable Patagonian element of the RP flora. Laurelia guinazui was originally described as Monimiaceae, but Laurelia (South America and New Zealand) is now assigned to Atherospermataceae. I report 16 new fossil specimens of L. guinazui from LH and RP that have greater detail preserved than the syntypes (RP) and confirm their placement to Atherospermataceae. Morphological character evaluation suggests an extinct taxon with closest affinity (in decreasing order) to the extant genera Daphnandra, Doryphora (both Australia), Laureliopsis (Chile and Argentina), and Laurelia (New Zealand and Chile), and I propose the new combination Atherospermophyllum guinazui (Berry). The second fossil species, Monimiophyllum callidentatum sp. nov., is new and represented by a single specimen from LH. It exhibits distinct ‘Monimioid’ teeth and other features that allow confident placement in Monimiaceae. Among extant genera, the greatest morphological similarity is to Wilkiea (Australia), which appears to contrast with molecular analyses that place the divergence of the clade containing Wilkiea in Australasia between 16 and 38 Ma (Renner et al., 2010. J. Biogeography 37:1227). This result implies that the lineage may be much older and have a much broader biogeographic history across Gondwana. This study significantly improves the fossil record of two early-diverging families in Laurales, by establishing what are probably their most reliable pre-Neogene occurrences, in Eocene Patagonia. The fossils were found at great modern distance from many of their closest living relatives, further weakening the signal of extant Patagonian forests in these exceptional Eocene fossil floras and increasing their links to Australasia.