Monday, January 26, 2009

Whatever Happened to the Sphenopsida?

This and the previous post are on Edition #13 of Berry-Go-Round, the plant carnival, hosted this month over at Watching the World Wake Up. Watcher has put on a great show, so go check out the rest!

Not long ago, I was shocked to discover that some of my old college pals had disappeared. Not just the Sphenopsida, but the Psilopsida were gone, and the Lycopsida had become virtual strangers. (If you’re not conversant with Old Style Plant Classification, you may want to check the review at the previous post. Also note: The Nostalgia Alert invoked in the previous post is extended in full force to this one.)

Way back in the Dawn of Time, between the Dark Ages and the Enlightenment, back when the atom had been split but not yet fractured, between the advent and the breakup of the Beatles, I wrote a college paper called A Description and Discussion of Early Vascular Land Plants. At the time, it seemed no one was quite sure what to do with these guys, but I fell in love with them. They were the psilophytes, and a more rowdy, diverse, and exciting bunch was hard to find in 1969. Young and blinded by infatuation, I failed to see the clues... they wouldn't stick around long.

Here's Psilophyton, the ringleader (aka type genus) of the group. Look at the dichotomous branching, the cute pairs of terminal sporangia... Psilophyton princeps here represents the genus described by Sir William Dawson in 1859, but not accepted at the time. From the Lower Devonian, it was among the earliest vascular land plants, and several species are now known. The spine-like emergences on the lower stem have no vascular tissue, but the stem has a small central stele. Where did Psilophyton go when the gang broke up?

Priscilla (the Phylogenist) did not spare my feelings for old friends. She said: "Nobody seems to have come up with a good term for whisk ferns + ophioglossid ferns, but the phylum Psilophyta is defunct now. Ditto several other plant phyla. Although whisk ferns certainly don’t look like they have macrophylls, their close cousins do, so their ancestors probably did. Both extant groups have reduced root systems, whisk ferns more drastically than ophioglossid ferns."

The whisk fern she's talking about is Psilotum, a living representative of the entire group these days. (Or, classification being what it is, perhaps not even related anymore to this ancestral group.) I'm still trying to figure out why whisk ferns (or Psilophyton) are linked with grapeferns (ophioglossid), or ferns for that matter. Is it the horizontal rhizome? The circinnately coiled tips?

Continued help from Priscilla: "Monilophytes refers to the fern clade – whisk ferns, horsetails, lepidosporangiate ferns. I’m feeling lazy, so here’s a link to a reasonable listing on Wikipedia." The horsetails and their relatives were once known as the Sphenopsida, so now we know what happened to them—they got absorbed by the Monilophytes. Elsewhere, they're known these days as Equisetopsida. [Or were, on December 7th when I first downloaded this tree; now the Equisetopsida are gone too—having been converted into the Polypodiopsida. Compare the cladogram in the previous post. Plus c'a change, eh?]

A couple of the boys turned up elsewhere; you'll be glad to know they've done well for themselves. One source tells me “The Asteroxylales were originally included under the Protolepidodendrales, but they are a much more primitive lineage and have been given their own taxon (of ordinal rank). They stand midway between the Zosterophyllopsida and the more developed lycophytes.”

Judging by name recognition, I conclude that my old friends Asteroxylon and Zosterophyllum have now found good homes hanging out in and around the Lycopsida. At least by the classification scheme below:

Class Lycopsida ("club-mosses") [this list from Palaeos]

  • Order Drepanophycales (ancestral types)
  • Order Protolepidodendrales
  • Order Asteroxylales
  • Order Lycopodiales (Club "mosses")
  • Order Selaginellales (Spike "mosses")
  • Order Lepidodendrales = Lepidocarpales (Scale trees)
  • Order Miadesmiales
  • Order Pleuromeiales (intermediate forms)
  • Order Isoetales ("quillworts")

I can easily believe a lycopsid affiliation for Aster; you could almost lose him in a patch of Lycopodium lucidulum; except for those frilly tops, he'd blend in quite well. He's a Scot, though, from the Rhynie Flora of Early or Middle Devonian age (400 million years ago, give or take). He and two other genera (Rhynia and Horneophyton) were described by Kidston and Lang in 1920.

As you can see, Zosterophyllum looks nothing like his pals, especially Asteroxylon, above—not surprising, as he comes from the other side of the planet. Zoster is an Aussie, a member of the Baragwanathian Flora of Late Silurian age. I think he and his cohorts, Yarravia, Hedeia, and Baragwanathia, were the earliest land plants known back when I drew these mug shots. Zoster was, as you see here, then interpreted as having an aquatic habitat. The sporangia were 3.5 cm across, distributed radially on a terminal spike. Baragwanathia, however, could pass for a long-lost brother (or grandfather, given the age difference?) of Asteroxylon. [Details here and here. Speaking of Dr. Cookson, it was this color picture of Cooksonia that got me started on this nostalgic trip in the first place. Thanks, Phytophactor!]

Just to complete the story, here's the "plant evolution" diagram I included in my paper (Fuller & Tippo, 1954, apparently my college botany textbook—how scary is that?). There at the bottom are all those other algal and fungal divisions that made up the "Thallophytes" in the classification post; and there, in a central spot, are my wonderful Psilophytales, giving rise to the ferns and gymnosperms, and anchoring plant life on land for the first time. So, though a lot has changed in the phylogenetic world since then, here's the end of my paper:
The Psilopsids are the group that best demonstrates the attainment of the necessary characteristics that enabled plants to conquer the terrestrial environment, and make possible a whole new sequence of evolution.

Defunct or not, they deserve to be remembered.

1 comment:

Christopher Taylor said...

The whisk fern she's talking about is Psilotum, sole living representative of the entire group these days. (Or, classification being what it is, perhaps not even related anymore to this ancestral group.)

You forget Tmesipteris, the other living genus of the Psilotaceae. Tmesipteris has a more 'leafy' appearance than Psilotum, but like Psilotum it lacks true leaves. But while Psilotaceae are derived ferns, Psilophyton is still holding its old basal position as a trimerophyte.

I don't what all the details uniting Psilotaceae and Ophioglossaceae are, but I know that one character they both share is subterranean gametophytes.