In the Late Carboniferous there were herbaceous as well as tree horsetails. The trees could reach a height of up to 20 m. They provided also an important contribution to the forming of coal.
Horsetails are characterized by articulated trunks and branches with diaphragms (nodes). The side branches, the leaves and the spore cones are always situated on the nodes and the leaves are attached in whorls.

Calamites cistiiCalamites
Calamites is the name of the genus to which most of the horsetail trees belong by far. The genus is often divided in subgenera. Most of the fossils are characterized by ribs in the longitudinal direction and transverse nodes. In these cases the fossils are the petrified fillings of the central cylinder of a trunk or a rhizome (an underground axis with rootlets in whorls). The ribs are moulds of the inside of the wood cylinder.
In case of good preservation oval scars of canals running through the marrow can be seen right under the node (click on the photo on the right). When de conservation is even better, very small leaf scars are visible above the node.
Sometimes smooth or weathered trunks are found. These are stems of which the outside has been (partially) preserved.

Subgenera of Calamites
The genus Calamites is (sometimes) divided into the following four subgenera.
In this diagram the horizontal lines represent the nodes and the little circles the branch scars.

Stylocalamites has hardly any branch scar.
Crucicalamites has branch scars at regular distances from each other at every node. They stagger per node.
Diplocalamites has two opposite branch scars at every node. Per node they turn by 90 degrees.
Calamitina has nodes with densely packed scars followed by several nodes without scars. On top of the node with scars is a short internode (part of the stem between two nodes). This one is followed by in length increasing internodes, until the next node with scars occurs.

This classification and the reconstructions of trees of Calamites, often occurring in restorations of vegetations, originate from the twenties. Meanwhile it has become clear that this classification is rather artificial, however, useful for identification. Newer reconstructions show these trees with a growing pattern comparable with that of a fern tree or a palm, with a branchless trunk and on top a crown which in these giant horsetails existed of several whorls of branches. These were periodically thrown off when a new one was formed.

Calamites cistiiCalamites (Stylocalamites) cistii can be identified by the fact that the internodes are longer than wide. There are no branch scars. Calamites suckowiiCalamites (Stylocalamites) suckowii, however, has internodes which are wider than long. Also no branch scars.
Calamites undulatusCalamites (Stylocalamites) undulatus has zigzag-shaped ribs. C. rugosus or carinatusCalamites (Diplocalamites) rugosus or C. carinatus
Both species have two or three large branch scars with a free inner space. They can be separated only when the outside of the trunk has been preserved: C. rugosus is then showing a rough surface and C. carinatus a smooth one.
Calamites goeppertiiCalamites (Calamitina) goeppertii has a ring of branch scars at some of the nodes. In this species the outside of the stem has often been fossilized. The scars at a node are mostly standing close together.

Cone of a horsetail tree
Palaeostachya or Calamostachys

Spore cones of horsetail trees
Cones of a horsetail tree
Palaeostachya and Calamostachys are spore cones of horsetail trees. They can be distinguished from the cones of clubmoss trees because the sterile and fertile scales are attached in whorls. It is often impossible to separate the two genera in compressed fossils, like those of Ibbenbüren. The difference can only be seen in case of very good preservation. In Palaeostachya the stalked sporangia are found in the axils of the scales, in Calamostachys they are attached in the middle of the internode. After Stewart & Rothwell (1993).

Cone of a horsetail tree
Palaeostachya or Calamostachys

Annularia and Asterophyllites
Annularia and Asterophyllites

The leaves of the horsetail trees were relatively small. They are divided in two genera.
- Annularia: with whorls of leaflets which are fused at the base and as a consequence
lying in one plane; the whorls are circular or oval (photo left)

- Asterophyllites: with whorls in which the leaflets are bent upwards (photo right).

 Annularia radiataAnnularia radiata has narrow, linear leaflets which are acuminate. The average length is 10 - 15 mm. Mostly isolated whorls are found. Annularia galioidesAnnularia galioides has very small leaflets (usually 2 - 4 mm) with a somewhat rounded apex. Per whorl 8 leaflets on average.
Asterophyllites equisetiformisAsterophyllis equisetiformis has whorls of rather stiff, soemwhat bent, upward pointing leaves. The species A. longifolius has whorls with more leaflets (about 30) and they are longer and less stiff. Asterophyllites grandisAsterophyllites grandis has whorls of about ten very small (2 - 4 mm) upward curved leaflets.
Asterophyllites charaeformisAsterophyllites charaeformis has still smaller leaflets than the previous species. They are often somewhat hook-shaped. In a whorl rhere are only 4 or 5 leaflets.

Calamites ondergronds
Roots of Horsetail trees
Horsetail trees had rhizomes. These are underground stems with nodes from which rootlets sprang. These rootlets were not articulated. From time to time a new offshoot grew from such a node, from which a new tree originated. In this way forests of Calamites trees came into existence in which only few other plants and trees could survive. After Taylor et al. (2009).


Two species of root fossils are distinguished:
Pinnularia capillacea
(photo above) has roots of which the lateral rootlets are arranged in two rows.  

In Myriophyllites gracilis the lateral rootlets are irregularly attached.

This is a genus of herbaceous horsetails with at the nodes whorls of wedge-shaped leaflets which can be laciniate or not. Usually the whorls lie as circles or ovals on the splitting surface of the rock. The nodes are somewhat thickenened and the stems are threelateral or sixlateral. The number of leaflets in a whorl is mostly 6, 9 or 12. Some species have several different leaf shapes at the same plant (heterophylous plants).

Sphenophyllum has often hairy stems and leaflets ending in hooky structures. Though individual stems can reach a great length, their diameter is always relatively small and almost constant. Therefore it is assumed that these stems could not stand upright independenly and that Sphenophyllum was a climbing plant or a plant forming a dense bushy vegetation such as nowadays our blackberry.

Sphenophyllum cuneifoliumSphenophyllum cuneifol ium has whorls with triangular leaflets with sharp teeth at the outside. The plant is heterophylous and has deeply incised leaflets at the thicker axes. The species is very common. Sphenophyllum myriophyllumSphenophyllum myriophyllum is a rather rare species with very narrow leaflets which are forked just above the attachment point.