Medullosids †

Seed fern with large seeds

The Medullosales are a strange group of Late Paleozoic seed plants with fern-like foliage. These plants had pulpy seeds the size of avocado pits. In addition they had large, wing-less pollen. These features seems to indicate that these plants employed animals for pollination and/or dispersal, which would make this group the first on Earth to have such a symbiosis. In many phylogenetic studies, the medullosids are a sister group or closely-related to the cycads.

Ecology & form

Tree fern-like growth
Possibly grew in arid and wet env'ts
Late Pennsylvanian–early Permian medullosids were thriving under seasonally-dry climate on wet, rocky soils showing proximity to the groundwater level (Luthardt et al. 2021)
In forested landscapes, they occurred as part of the forest understory.
Permian medullosids probably had a (semi-)self-supporting growth habit with a high water-conducting potential of these plants. Anatomical differences with tropical relatives of Carboniferous age might reflect an evolutionary process that was driven by environmental changes during the late Paleozoic (Luthardt et al. 2021).

Classification

└Lignophytes

└Medullosales †

Geologic Age

Mississippian - Middle Permian

Plant-Animal Interactions

Several damage types (DTs) on leaves include margin feeding, hole feeding, and surface feeding, but also galling, piercing and sucking, as well as oviposition scars (Molina-Solís et al. 2023)
Carboniferous taxa had adaptations such as curved and glandular trichomes that probably helped to dissuade invertebrates from predating them

Stems

Persistant leaf bases (Medullosa)
Strange eustele w/ cambium, which appears polystelic
Manoxylic wood

Leaves

Large fronds with bifurcate petiole (Myeloxylon)
Some pinnae, in form genus Neuropteris, are broad, with a thin epidermis, uniform mesophyll, numerous stomata, and lack papillae and hairs
Other pinnae, in form genus Alethopteris, are narrow, with a thick epidermis and hypodermis and sunken stomata, as well as adaxial palisade layer, abaxial spongy mesophyll, and epidermal hairs

Roots

Adventitious and prop-like

Classification

└Lignophytes

└Medullosales †

Geologic Age

Mississippian - Middle Permian

Reproduction

Ovule-producing structures

Avocado pit-sized seeds (Pachytesta or Trigonocarpus)
7 mm to 6 cm in length
Many layered seeds: fleshy outer layer, stony middle layer, and fleshy inner layer

Pachytesta †

Brongniart, 1874; Hoskins and Cross 1946

P. crenulata †

Raymond & McCarty 2009
Late Pennsylvanian
Large, ellipsoidal ovules, with three-layered integument (sarcotesta, sclerotesta, endotesta) with three commissured primary ribs most strongly expressed at the apex
- Sarcotesta of three layers: the exterior cuticle suggests an epidermis of uniform parenchyma cells; a layer of irregular, randomly oriented parenchyma cells, which is lacunar near the exterior of the ovule; and an inner solid layer of tabular, radially-oriented parenchyma cells.
- Sclerotesta of two layers: outer layer of interwoven sclerenchyma fibers that form longitudinal ridges; and an inner layer composed primarily of longitudinal fibers, with some fibers wrapping around the ovule perpendicular to its long axis, in a tangential direction.
- Endotesta a thin cuticle except at ovule apex.

P. illinoensis †

P. duquesnei †

Vallois and Nel 2023
Middle Pennsylvanian, Bruay Formation, France
Carboniferous medullosalean ‘seed’ attributed to a female reproductive structure of Alethopteris
The peculiar micropylar extension of the new ‘seed’, as long as half of the total ‘seed’ length
A possible function of the extremely elongated micropylar extension suggests it is unlikely adapted to a pollination-drop mechanism, but rather...
1. ...linked to a protection mechanism against arthropod predators
2. ...and/or to a highly specialized pollination mechanism either by very tiny insects entering these ‘seeds’ through this tube (e.g. figs and fig wasps as modern analogs)
3. ...or an insect with elongated mouthparts able to go inside the ‘seed’ (e.g. tube flower and moths with extremely long proboscis), to feed and transport pollen at the same time
If #2 or #3 provie accurate, this would be the earliest evidence of pollination in the fossil record
The attribution of Whittleseya elegans as male organs of an Alethopteris sp. is possible

Above: Reconstruction of Pachytesta illinoensis (from Fig 4, Raymond & McCarty 2009)

Pollen-producing structures

Clusters of elongate sacs formed into a variety of cup-, bell- and cigar-shaped configurations (Aulacotheca, Dolerotheca, Potoniea, and Whittleseya)
- Extremely large wing-less pollen in most
- May indicate required interaction with pollinators to disperse pollen
Anatomical evidence indicates they were borne on the fronds, attached the rachis
Pollen is considered pre-pollen by some researcher since is displays a proximal trilete mark (as in Potoniea), and assumed to germinate proximally similar to microspores of spore-bearing plants
- This is in contrast to the distal, equatorial or other typical apertures of seed plant pollen grains.
- Most other medullosids produced large ovoid pre-pollen with a monolete mark, and assigned to the genus Schopfipollenites.

Alethopteridaceae

Alethopteris †

Sternberg 1825
Pennsylvanian - Early Cretaceous
Pinna are narrow, with a thick epidermis and hypodermis and sunken stomata, as well as adaxial palisade layer, abaxial spongy mesophyll, and epidermal hairs
A. ambigua; A. crassa; A. evansi; A. halli; A. hartungi; A. lacoei; A. marginata; A. roesserti; A. serli; A. valida; A. whitney

Neuralethopteris †

Cremer 1893
Late Devonian - Pennsylvanian
N. biformis; N. pentias; N. pocahontas; N. schlehanii; N. sergiorum; N. smithsii

Lonchopteris †

Brongniart 1828
Bi- or tripartite leaves similar to Alethopteris except with veins that display anastomoses or pseudo-anastomoses
Stomata are hypostomatic and anomocytic
L. bricii; L. virginiensis

Lonchopteridium †

Leaves similar to Alethopteris and Lonchopteris and occupying an intermediate morphology between these form genera

Medullosids †

Seed fern with large seeds

Ecology & form

Classification

Geologic Age

Plant-Animal Interactions

Stems

Leaves

Roots

Classification

Geologic Age

Reproduction

Pachytesta †

Alethopteridaceae

Alethopteris †

Neuralethopteris †

Lonchopteris †

Lonchopteridium †

Cyclopteridaceae

Laveineopteris

Callipteridium

Margaritopteris

Douropteris

Neurodontopteridaceae

Neuropteris

Reticulopteris

Odontopteris

Macroneuropteris

Neurodontopteris

Neurocallipteris

Barthelopteris

Lescuropteris

Palaeoweichselia

Parispermaceae

Paripteris

Linopteris