Serpent Research: Reptile Evolution and Rainforest Collapse

Growing-up in northeastern Illinois I became familiar with Coal Swamp Forest fossils at an early age. Roaming the slag piles of strip mines in southern Will County fossil ferns, the occasional horsetail, crustaceans, or the highly sought after Tullymonster (Tullimonstrum gregarium) could be found. The pits were dug by the Peabody Coal Company, and abandoned after they would no longer produce. Today, the slag is covered with dense vegetation making it more difficult to find ancient remains and the pits are filled with water and home to hybrid trout.

In the Middle Pennsylvanian (311-306 millions of years ago -MYA), Coal Swamp Forests were widespread and covered tropical Euramerica, an area that includes what is now Europe, eastern North America, and extreme northwestern Africa. By the Middle Pennsylvanian much of the Euramerican Coal Swamp Forests disappeared. Howard Falcon-Lang of the University of London and William Dimichele of the Smithsonian have proposed that the Coal Swamp Forests responded to alternating glacial and inter-glacial periods (cyclotherms) with the Swamp Forests expanding and thriving during the interglacial periods and Seasonally Dry Forests replacing them during the glacial periods when much of the water was tied up in ice.

About 305.4 MYA, about the boundary of the upper to middle Pennsylvanian there was what Falcon-Land and Dimichele call a hyperconstriction event. This was an abrupt change from coal forests dominated by lycopsids (club mosses) to those dominated by tree ferns that ultimately resulted from an extreme glacial period that produced equatorial refugia.

In a more recent, related paper Sarda Sahney and colleagues (2010) suggest that the evolution of reptiles was stimulated by the cyclic expansion and contraction of the Coal Swamp Forests – habitat fragmentation. In a press release Howard Falcon-Lang explained, "Climate change caused rainforests to fragment into small 'islands' of forest. This isolated populations of reptiles and each community evolved in separate directions, leading to an increase in diversity." And, the other co-author, Mike Benton added, "This is a classic ecological response to habitat fragmentation. You see the same process happening today whenever a group of animals becomes isolated from its parent population. It's been studied on traffic islands between major road systems or, as Charles Darwin famously observed in the Galapagos, on oceanic islands."

The authors discovered changes in tetrapod diversity across the Moscovian-Kasimovian interval, by constructing two late Paleozoic tetrapod databases, comprising records of global and alpha diversity (family diversity) over nine global stages ranging from 346 to 270 MYA. They restrict the analysis to this time span because bracketing Tournaisian and Kungurian (359.2-306.5 MYA) stages were times of very low diversity, which had previously been interpreted as mass extinctions or gaps in the record. They tabulated 67 families from 163 tetrapod sites worldwide into a global database. Stratigraphic ranges were assigned to each family and the associated dates were correlated with the time scale. Each family was given an ecological assignment based on size and diet resulting in 12 ecological niches. Diet was inferred from jaw and tooth structure, patterns of tooth wear, body size, and whether the animal was adapted for a predominantly aquatic or terrestrial lifestyle.

Several patterns emerge from the analysis. They found global diversity steadily increased through the study interval from 6 or 7 families to 39 families. Alpha diversity (the number of families) closely tracked global diversity until the late Moscovian (about 306 MYA), the two curves dramatically diverged across the Moscovian-Kasimovian boundary as alpha diversity collapsed from 20 families to 7 families. The authors saw only one way to reconcile the rise in global diversity at a time when alpha diversity was falling – the number of endemics greatly increased between the Moscovian and Kasimovian-Gzhelian intervals. Confirmation came from the calculations. The ecological data confirmed the diversification, the number of ecological niches occupied by tetrapods increased from 4 to 9.

The authors concluded that the abrupt collapse of the tropical rainforest biome (the Coal Forests) drove rapid diversification of Carboniferous tetrapods (amphibians and reptiles) in Euramerica. Amphibians were devastated because of their dependence on wet environments – their eggs need wet environments for development, while the amniotes ('reptiles') fared better, because they were ecologically adapted to the drier conditions. Amniotes had evolved the “land egg,” an egg where the embryo is protected from desiccation by a series of membranes. The paper demonstrates, for the first time, that Coal Forest fragmentation had a tremendous influence on the ecology and evolution of the terrestrial fauna. And, it illustrates the tight coupling that existed and still exists between vegetation, climate, and trophic webs.