Archaeopterids †

Earliest true trees on Earth

The archaeopterids (Order Archaeopteridales) were a group of progymnosperms, meaning that they were spore-bearing plants that possessed a cambium with robust woody growth. This combination of traits is not found in any living plants, since all extant spore-bearing plants lack robust woody growth. The group is named after the taxon Archaeopteris, which is the earliest known woody tree found in the fossil record (Middle Devonian). This plant had fern-like foliage that bore sporangia on specialized fertile leaves, which is the form-genus for Archaeopteris.

For many years, it was assumed that Archaeopteris was a fern, until the 1960s, when Charles Beck was able to demonstrate that these fern-like leaves were actually connected to fossil wood known as Callixylon. From a distance, these plants probably appeared quite conifer-like, but upon closer inspection, the leaves would appear fern-like. These trees were a major component of the Late Devonian and early Mississippian ecosystems and may have played a role in the Late Devonian extinction event.

Ecology & Form

Major component of Late Devonian ecosystems
Originally thought to be ferns due to their foliage
First true woody trees with a vascular cambium
Probably appeared quite conifer-like, possibly like a Taxodium or Metasequoia, which possess fern-like needles on shoot systems

Stem

Protostele-eustele with large amounts of pycnoxylic wood (secondary xylem)
- Trunk diameters in excess of 1 meter with estimated heights of up to 30 m have been reported
- Callixylon † is a form-genus for these woody stems
Pseudomonopodial and monopodial branching with axillary buds

Leaves

Laminate leaves that were roughly fan-shaped, but highly variable (see Archaeopteris below)
Archaeopteris † is a form-genus for branching systems with sporangia

Reproduction

Sporangia adaxial on special, helically arranged leaves (see Archaeopteris below)
Sporangia are elongated and pointed with lateral dehiscence
Heterosporous with spores that are radial, trilete, pseudosaccate
- Microspores are 70 micrometers, and align with the form taxon Aneurospora
- 16-32 megaspores (110-400 micrometers) are in a megasporangium; assigned to Contagisporites

Geological Age

Middle Devonian (Eifelian) -Mississippian

Classification

└Lignophytes

└Progymnosperms †

└Archaeopteridales †

Above: Archaeopteris † root system at the Cairo fossil forest site (Credit: Charles Ver Straeten)

Above: Archaeopteris † root system at the Cairo (NY) fossil forest site (Image credit: Bill Stein)

Diversity

Actinopodium nathorstii †

Hoeg, 1942; Schweitzer, 1999
Middle Devonian of Spitsbergen
Permineralized axes with medullated actinostele with wedges of mesarch xylem, and some secondary xylem
Maybe the main axis of Archeopteris (Gensel & Andrews, 1984)

Actinoxylon banksii †

Matten, 1968
Givetian of New York
Several orders of branching, with ultimate axes bearing sub-opposite leaves in a decussate arrangement
Stele contains actinostele with 6 ribs; each rib contains one protoxylem point at the peripheral tip, and another along the radius of the rib
Secondary xylem is present

Archaeopteris †

Dawson, 1871; Lesquereux, 1880; Fairon-Demaret et al., 2001
Devonian - Carboniferous taxon for vegetative and fertile branch systems from North America, Russia, China, Morocco, Europe and Australia
Lateral branch systems were pseudomonopodial in nature, developing from primordia and not axillary buds
Leaves are helically arranged, but appear opposite and decussate in penultimate axes
- Lamina was high-variable (i.e. dissected, entire, obovate, spatulate) and anisophylly occurs in some species
Fertile ultimate axes usually occur as the basal most units of branching system
- Fertile leaves are helically arranged on fertile axes, with two rows of fusiform sporangia
- Heterospory is present

A. acuta

A. archetypus

A. browniana

A. changyangensis

A. elschanica

A. fimbriata

A. fissilis

A. gaspiensis

A. grandifoliolata

A. guangdongensis

A. halliana

A. harttii

A. hibernica

A. hitchcockii

A. howittii

A. hubeiensis

A. intermedia

A. jakuschkiana

A. kuzbassica

A. latifolia

A. laxa

A. magnacensis

A. minor

A. osmanica

A. pseudofimbriata

A. roemeriana

A. rogersii

A. rotundifolia

A. sibirica

A. tatarica

A. vologdinii

Callixylon †

Zalessky, 1911
Twenty species of Callixylon are described from North America, Morocco, Europe, China, and several parts of Russia
Late Givetian to the Mississippian
Pycnoxylic woody stems up to 150 cm in diameter, and 10 m in length
- Eustele with pith and mesarch xylem of thick-walled tracheids and narrow rays
- Growth rings present
- Tyloses have been discovered in Callixylon, indicating that plants already had a mechanism to seal off infection or rot early in wood-making evolution (Decombeix et al., 2022)

C. arnoldii

C. beckii

C. brownii

C. clevelandense

C. henkei

C. huronense

C. kazakhstanum

C. newberryi

C. owenii

C. petryi

C. schmidtii

C. seamrogia

C. trifilievii

C. velinense

C. wendtii

C. whiteanum

Eddya sullivanensis †

Beck, 1967
Late Devonian (Frasnian) from New York
Possible immature plant, or sapling, of Archaeopteris
30cm long, with shoots, leaves, and well-developed roots
Leaves are fan-shaped, with dichotomous veins, and helically-arranged
Stem is an eustele with mesarch development and small amount of wood

? Fuellingia gilkinetii †

Leclercq, 1928; Kräusel & Weyland, 1960; Schweitzer & Geisen, 2002
Middle Devonian of Germany
Axes up to 4 cm in diameter and 1 m in length
Fan-shaped, helically arranged leaves are attached
Sporangiophores are attached to midrib of wing-shaped leaves, with numerous helically arranged lateral branches
Originally called Psygmophyllum gilkineti (Leclercq, 1928) and Platyphyllum fuellingi (Kräusel & Weyland, 1960)

? Langoxylon asterochlaenoideum †

Stockmans, 1968; Scheckler et al., 2006
Middle Devonian of Belgium
Fern-like, permineralized stem with possible archaeopterid affinities
"Langoxylon compares with Middle Devonian archaeopterid progymnosperms like Actinopodium and Actinoxylon, but has a mixture of anatomical characters that were previously used to subdivide Euphyllophytina into Moniliformopses and Radiatopses types. Langoxylon is thus a ‘fern-like’ Middle Devonian euphyllophyte of uncertain phylogenetic relationship" (Scheckler et al., 2006)

Siderella scottii †

Read, 1936
Tournaisian from the New Albany Shale of Kentucky
Permineralized axes with 7-10 primary xylem (px) lobes, with patches of px that is reminiscent of peripheral loops at the tips of the lobes
Anatomically, this taxon is larger, but otherwise identical to the penultimate axes of Archaeopteris
Siderella leaves may have been distichous, unlike the decussate leaves in Archaeopteris (Beck & Wight, 1988)

Svalbardia †

Hoeg, 1942; Schweitzer, 1999
Middle Devonian
Planated axes that may represent a portion of Archaeopteris trees

S. avelinesiana

S. banksii

S. boyi

S. fissilis

S. osmanica

S. polymorpha [type]

S. scotica

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