|The nature of pre-Devonian tracheid-like tubes
|In the discussion on early vascular land plants an important part is played by different organic microfossils. One category comprises tube-like objects of pre-Devonian age, called tracheid-like tubes. Recognition of the taxonomic affinity of these microfossils is very important because tracheids are one of the fundamental elements of the xylem and are treated as a hallmark of the vascular plants. Banks (1975) was of the opinion that the tubes previously described as tracheid-like were of animal rather than plant origin. He suggested also that tracheid-like tubes presumably originated from graptolites or chitinozoans. Gray & Boucot (1977:166), following personal communications of A. Urbanek, W.B.N. Berry and K.M. Towe, stated that there was little likelihood that the tubes of graptolites and tracheid-like tubes "could beconfused or that our tracheid-like tubes could have originated from graptolites". Gray & Boucot (1980) claimed that tracheid-like tubes had a plant source.|
Graptolite Net | Graptolites & Graptoliters | Organic Microfossils | Scolecodonts | Amber
|In the course of my studies of Ordovician and Silurian organic microfossils I have found, however, an aberrant benthic graptolite of an uncertain taxonomic position. This form, described as Maenniligraptus ursulae Mierzejewski, 1985, derives from a calcareous erratic boulder of Ordovician age found in Poland on the Baltic beach (Mierzejewski 1985). An unusual stolonal system is characteristic of this graptolite. There is a great resemblance between fragments of these stolons and the tracheid-like tubes. In fact, the stolons as well as the tracheid-like tubes have helical or annular thickenings showing evidence of branching, and do not appear to be septate. Gray & Boucot (1977) noted that the tracheid-like tubes were consistently less than 100 micrometers in diametr, sometimes near 50 micrometers or less. The graptolite stolons mentioned above are mostly 30-50 micrometers or even 60 in diameter. In this case, there is a little doubt that some tracheid-like tubes from pre-Devonian deposits are fragments of encrusting graptolite rhabdosomes.|
|Stolons of Maennilligraptus ursulae with characteristic inner rings and spiral thickenings.|
Piotr Mierzejewski, the Count of Calmont and Goethe Lab - Centrum Paleobiologii
|Moreover, I suggest that some of the tracheid-like tubes should also be assigned to rhabdopleurids. It has been a common belief among palaeontologists up till very recently that fossil remnants of the Rhabdopleuroidea are extremely rare. The first Silurian rhabdopleurid was described in 1977 (Mierzejewski 1977). Later, I found that the Early Palaeozoic genera Cylindrotheca Eisenack, 1934 and Kystodendron Kozłowski, 1959, described as 'chitinous' hydroids, represent in fact stolons connected with sterile cyst buds of rhabdopleurid coenecia (Mierzejewski 1986). The zooidal tubes of these forms, consisting of encircling fusellar rings, closely resemble some tracheid-like tubes. The diameter of the zooidal tubes in Kystodendron ex gr. longicarpus (Eisenack) sensu Kozłowski from the Caradocian of Poland fluctuates within the range 50-170 micrometers.
These previously unknown structures throw a new light on the problem of the Early Palaeozoic tracheid-like tubes believed to be of a plant nature. I do not suggest, however, that all up to now described tracheid-like tubes are of graptolite or rhabdopleurid origin. But I believe that in future discussion on early vascular land plants, isolated tracheid-like tubes should not play any part at all.
|Based on: Mierzejewski, P. 1982. The nature of pre-Devonian tracheid-like tubes. - Lethaia 15, 148.|