WATER TRANSPORT AND BIOGEOGRAPHY OF THE LATE DEVONIAN TREE, ARCHAEOPTERIS (CALLIXYLON)

The progymnosperm Archaeopteris is a paleobotanical icon of the middle Paleozoic Era, and the Late Devonian in particular. During the Frasnian and Famennian, giant archaeopteridalean trees formed large forests across the globe. Easily-recognized organs, including trunks, branches and roots (collectively assigned to the genus Callixylon), and leaves (genus Archaeopteris), are found from Canada to Australia. Several species of Callixylon have been defined based upon wood anatomical characters, but it is not known whether these anatomical differences result in distinct physiological differences between archaeopterid morphospecies.

We used mathematical models to explore the physiology (water transport rates) of permineralized Callixylon trunks and roots, employing anatomical measurements of several thousand tracheids from Callixylon specimens, including permineralized roots and trunks, type specimens, and permineralized specimens of uncertain affinity. These included material from nearly all of the defined Callixylon species: C. brownii, C. erianum, C. newberryi, C. petryi, C. trifilevii, and C. zalesskyi.

In general, the hydraulic efficiency of archaeopterid wood is comparable to those of extinct and extant conifers and occupies a broad hydraulic range. We also find evidence for functional differences among the different Callixylon species that may translate into ecophysiological differences between species. For example, co-existing Famennian-age fossils from the Mader Basin of Morocco with different wood anatomical properties translate into hydraulic and, potentially, ecophysiological differences. Roots with Callixylon erianum-type anatomy contain xylem with a higher hydraulic conductivity than roots with a Callixylon trifilevii-type anatomy, suggesting the possibility of niche partitioning in the archaeopterid forests surrounding the Mader Basin. These results provide physiological support for the Orlova and Jurina Callixylon species taxonomy and, in short, imply that the diversification and expansion of archaeopterid forests in the Late Devonian is a functional and adaptive radiation comparable to that of the conifers later in the Phanerozoic.