Transitional fossils are important findings that always lead to a better understanding of life here on Earth. These intermediate stages seen within a group of organisms provide us with the sequential steps evolution has taken over broad timescales. Pairing the intermediate morphology with carbon dating, we can gauge when these changes happened, and can predict when ecological revolutions ensued. Transitional fossils are quite awesome, but living transitional organisms are in a league of their own. The species featured in today’s edition of Fungi Friday is an eye-catching, purplish-blue, puffball-looking fungus that is only found in New Zealand. It represents a gilled mushroom, transitioning towards a puffball fungus. Everyone, I’d like you to meet Clavogaster virescens, also known as the blue pouch fungus.
Not too long ago, this species was wrongly placed in the genus Weraroa. The handful of species within this polyphyletic genus have slowly but surely been dispersed to other genera, thanks to modern genetic sequencing. For example, a similar looking, hallucinogenic fungus was once known as Weraroa novaezelandiae but is now called Psilocybe weraroa. And just a few years back, Leratiomyces erythrocephalus was wrongly titled Weraroa erythrocephala. What would we do without next generation sequencing?? Even if we didn’t have these technologies, we would still be able to understand that Clavogaster virescens represents an intermediate stage between a gilled mushroom and a puffball.
Cross section of Clavogaster virescens. Photo by Michael Wallace.
The most famous transitional organisms are the iconic transitional fossils called Archaeopteryx. Found in 1861, just two years after Darwin published his famous book, “On the Origin of Species,” the fossil was concrete evidence supporting modification with descent. The fine sediments deposited at this slate quarry in Bavaria, Germany allowed for fine details to be preserved for 150 million years. I’ve seen replications of Archaeopteryx fossils in the Royal Ontario Museum in Toronto, and you can clearly make out individual feathers. Yet unlike modern birds, when I looked at the Archaeopteryx skull, it was robust and lizard-like, fashioned with homodont teeth. It had a thicker, heavier, skeleton that wasn’t the best for flying, leading some researchers to suggest that this organism was more of a glider than an active flyer.
The Archaeopteryx fossil and an artist’s depiction of the ancient lizard bird.
Birdlike reptile fossils that date back before Archaeopteryx was gliding throughout ancient forests reveal even more evolutionary intricacies. The 160-million-year-oldCaudipteryx too had feathers, but these feathers were symmetrical instead of asymmetrical. Asymmetrical feathers provide the proper aerodynamics for flight, so this specific transitional fossil suggests that feathers predated flight. Feathers evolved first to help keep these ancient birds warm, since they also evolved endothermy like us mammals. Finding these transitional organisms always provides us with so much ecological and evolutionary insight.
Seeing a living transitional organism fills me with awe. Clavogaster virescens lies in-between a gilled Agaric and spore contained puffball state. The cap of this mushroom never opens up, yet if you slice this fungus open, you can see vestigial honeycomb-like remnants of the gills that used to be functional in its ancestors. This is not the only species that represents this type of intermediate stage. The term secotioid fungi was actually conceived to label this not-so-rare transitional phase. Clavogaster virescens along with others have not fully progressed into a gasteroid form. This evolutionary process of shifting into a fully gasteroid, or puffball form is called gasteromycetation.
Clavogaster virescens and its vestigial honeycomb-like gills within.
Transitional fossils have helped us understand evolution better. We weren’t here 150 million years ago, yet we know reptiles took to the air, and started interacting with a wide range of species differently as soon as they started to glide and eventually fly. Living transitional organisms like Clavogaster virescens represent the intriguing secotioid fungi. These forest floor inhabitants have not fully transitioned, and still show characteristics from their gilled ancestry. Fully moving towards a puffball ecology takes time, but with Clavogaster virescens never opening up its cap, it is nearly there. Given enough time, this blue pouch fungus should lose the vestigial gill tissue within its fruiting body. When this tissue is completely gone, and its stipe becomes reduced, the fungus will have even more space to form spores, and one would say it has fully completed gasteromycetation.