The demise of the dinosaur and reptile dominance at the K-T boundary has been the subject of so much speculation, that it can be hard to filter out the rampant storytelling from the underlying observations. In a fairly recent Pearl piece in PLoS Pathogens, Arturo Casadevall summarizes and advances his idea for a fungal role in the rise of mammals and the end of reptilian dominance.
Many of the key bits in the argument focus on mammalian resistance to fungal infection due to endothermy and the interaction between body temperature and vertebrate immunity. Warmer body temperatures can exclude many fungi, and optimal body temperature is a key regulator of successful immune response. Large-bodied warm blooded dinosaurs would have depended on high food intake to maintain body temperatures, and the disruption of the ecosystem, including the near complete lack of phtosynthesis, would have limited their ability to mount successful defences against disease. Meanwhile, fungi could take advantage of the saprophytic paradise created by the bloom of available dead plant material. Such a bounce in fungal biomass could have produced conditions favouring disease outcomes for animals that would have normally resisted lower fungal doses with relatively more effective immune responses.
This is an interesting theory, particularly given the huge numbers and diversity of invisible fungal propagules that are regularly encountered in the air, and the recent emergence of fungal and fungal-like diseases threatening animals with extirpation and extinction. However, I wonder how much fungi mattered as a selective force compared the to other factors at the time. Would body size and food inflexibility have been a bigger more influential filter? Something Casadevall did not discuss much was the relative success of smaller-bodied adaptable reptiles (Squamates) or the amphibians across the Tertiary, although amphibians are now under serious threat of a fungal disease driven mass extinction. I also wondered why Casadevall didn't explore fungal adaptation during the period of mass upheaval near the K-T boundary. Fungi are certainly not universally saprotrophs. What are the limits and influences on fungal diversity. Was there a fungal crash or expansion at the K-T boundary? Obviously, another good Pearl since not only have I learned about the core about this particular theory, but I have had my curiosity piqued enough to start digging into diversification literature I have not scoured in years.
As will now be normal for these posts until further notice, Research Blogging citations will be proceeded with the icon, but links to other works may be embedded in the text of the post.
Arturo Casadevall (2012). Fungi and the Rise of Mammals PLoS Pathogens DOI: 10.1371/journal.ppat.1002808