This is Mazon Monday post #67. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Trigonocarpus sp. was described in 1825, by Alexandre Brongniart, who was a French chemist, mineralogist, geologist, paleontologist, and zoologist back in the early 1800's. It is the seed part of an extinct order of plants called Medullosales. In the Mazon Creek literature, they are commonly referred to as "Seed Ferns" and include some of the more recognizable species from the deposit. This group includes Alethopteris, Odontopteris, Laveiniopteris, Neuropteris, and Macroneuropteris. Wikipedia has a passage describing the seeds of this group.
Ovules in different medullosalean species could vary from maybe 1 cm to over 10 cm long - the latter being the largest known ovules produced by any non-angiosperm seed-plant.[6] It was traditionally believed that the ovules were borne directly on the fronds, replacing one of the pinnules on the ultimate pinnae. However, there is a strong possibility that this reconstruction was based on the chance finds of ovules having been preserved just lying on a piece of pinna rather than in organic attachment to it. A number of cases are now coming to light that suggest that the seeds were borne in clusters on relatively slender, branching axes,[7][8] and that these trusses of ovules would have been produced from the top of the trunk among the crown of fronds.
The seed megaspore was surrounded by two layers of tissue: a vascularised nucellus and a usually three-layered integument; the nucellus and integument were completely free except at the base of the ovule.[9] There has been some debate as to the exact homologies of these tissues, and it has been argued that the vascularised nucellus was in fact the nucellus and integument that have become fused together, and that the 'integument' was homologous to a cupule that contained only one ovule.[10]
Most medullosalean ovules preserved as casts or adpressions show three longitudinal ribs and are assigned to the fossil genus Trigonocarpus. When such ovules are preserved as petrifactions, they are assigned to the fossil genera Pachytesta or Stephanospermum, depending mainly on differences in the apical form of the ovule. Another group of medullosalean seeds, usually associated with parispermacean fronds (see later), have six longitudinal ribs and are referred to as Hexagonocarpus when found as adpressions or casts, and Hexapterospermum when found as petrifactions.
Jack Wittry has a description on pages 233 - 235 in "A Comprehensive Guide to the Fossil Flora of Mazon Creek".
Trigonocarpus sp. Brongniart, 1825
epidermis and sarcotesta
DESCRIPTION: These seeds are large, radiospermic, and range from 7 mm to 6 cm in length. They are rounded at the base and taper gently from the middle toward the distal end where a small aperture may be found. In life, the seed was divided into layers. Starting from the outside toward the center are the epidermis, sarcotesta, sclerotesta, and endotesta (which surround the innermost nutshell-like nucellus layer), and are collectively known as the testa. The details of layer stuctures are known from permineralized examples, but they are difficult to deter mine in the authigenic cementation fossils found in the Mazon Creek area. The epidermis (see Fig. 1) is rarely found; it is generally seen free from the seed and appears as a distorted, tough, flat skin. Next is the sarcotesta (see Fig. 3), which quickly rots away and is seldom preserved. It was fleshy and sometimes grooved. Below this is the tough, fibrous, and, in some species, deeply grooved sclerotesta (see Figs. 4, 5). Below this layer is the most commonly found and recognizable part: the hard, shiny shell of the centermost female gametophyte called the nucellus (see Figs. 6 through 8). It often displays the three-part organization from which the seed gets its name.
REMARKS: Trigonocarpus is rare and the third most common seed taxon in the Mazon Creek flora. In North America, 43 species of Trigonocarpus have been described. When found as authigenic cementation fossils like those in concretions, they can display many different aspects, caused either by compression or desiccation, and depending on which layer of the seed is exposed. It is now believed that the species count is inflated and contains many synonymous names. From permineralized examples in coalball peels where complete seeds with better details can be observed, only 14 species have been described. Due to this fact, no attempt is made to describe species here.
George Langford had photos of each part of the seed in his 3rd book "Fossil Flora and Fauna of the Pennsylvanian Period, Will County, Illinois", which was unfortunately never published.
Specimens
A couple seeds from the Mazon River trip. These were both collected in 2020.
Photo from ESCONI member George Witaszek, from Fossil Friday #62.
