Collection location: White Oak Mountain, Haywood Co., North Carolina, USA [Click for map]
old open oak-maple forest on west-sloping side-ridge
thin white scurfy crust, small irregular convex erumpent apothecia, epihymenium dadrk brown, hymenium hyaline, asci broadly clavate to balloon-shaped I+ red-brown strong, spores 4-5 per ascus muriform hyaline
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I’d rather suspect that NY folks just don’t believe Arthothelium is a “good” genus. I personally would be very surprised if muriform spores would be enough to separate genera. There are far too many genera which have both 1-septate and muriform spores: Rinodina, Buellia, Rhizocarpon to name three off the top of my head. I suspect NY has good molecular/sequence data supporting the merging of Arthonia and Arthothelium.
… But this is all pure speculation based on hunch. I’ve not discussed it with James at all.
But lichenologists of eastern North America choose to call this Arthonia despite the muriform spores. I’m sure they have their reasons. Since this was collected in eastern North America, I’ll follow their lead. It’s all the same to me!
I don’t have my literature with me, so I couldn’t say what the differences between A. susa and A. spectabile are, sorry. :( But I’ve had at least one or two specimens of this species verified by J. Lendemer at NY. I believe A. susa is the only arthonioid species with muriform spores known to occur in the Smokies(?) Maybe the coccoid alga is important, too. I mention it in my notes for all my specimens.
BTW I thought that the alikes of Arthonia with muriform spores were grouped in the genus Arthothelium (according to the British Flora). There is one species in this genus with similar features I think, Arthothelium spetabile, which according to the British also exist in N.A:. Do you know what are the differences between the two species?