In the world of fossils it goes without saying that Paleobotany is often the most overlooked, either because, in general, plants are seen as «boring» or because plant fossils are more difficult to interpret, as it is common for leaves, trunks/wood/stems and reproductive organs to be identified with their own genera and species because they are found separately, rarely together. …and even when all are found together, the problem can arise of not knowing to which family (or even group) they should be attributed (I’m looking at you, Umaltolepis).
If we look at Mesozoic Paleobotany, we will find many rarities, especially in extinct families within current groups (Hirmeriellaceae in Conifers)…but also groups or grouped plants that have no known modern relatives. Here enter the Pteridospermae, commonly referred to as «seed ferns», which are plants whose grouping is considered to be, in part, artificial, since it is based on joining non-Angiosperm botanical remains that cannot be related to any of the current groups: coniferoids (conifers or cordaites), ginkgophytes or cycadophytes (cycads or bennettites). Here we will not discuss the affinities of these plants (For this Anderson, Anderson & Cleal, 2007; Novíkov & Barabaš-Krasni, 2015), but one genus linked to this grouping: the Dichopteris leaves, from the Lower Jurassic (Pliensbachian-Toarcian) of the Mediterranean region (Italy, Montenegro and Albania, as well likely Morocco).
Dichopteris: a basic approach
In 1856-1868 De Zigno studied the diverse Lower Jurassic flora found in the Grey Limestones of Veneto (Lessinia Province, Northern Italy), which he published in his «Flora fossilis formationis Oolithicae», recognizing more than 60 species of plants, among which he described the genus Dichopteris from leaves +70 cm long, bifurcate rachis and odontopteridian-type venation of the pinnules. De Zigno recognized seven species: D. visianica, D. microphylla, D. paroliniana, D. angustifolia, D. rhomboidalis, D. lanceolata and D. laevigata, with D. visianica being the type species. De Zigno noted similarities with other leaf genera of Pteridospermae, such as Pachypteris and Thinnfeldia, which led authors such as Schimper (1869) to make this genus synonymous with Pachypteris, to later racapacitate and re-validate it (Schimper, 1874). Later, Grandori (1913a,b) took the specimens and found a few epidermal features, especially the epidermal cells from the upper cuticle of the pinnule apex and rachis, and assigned all specimens to D. visianica. Harris (1964) suggested again the combination of this genus with Pachypteris, with both D. laevigata and D. lanceolata being synonymized with P. lanceolata. This was in part corrected by Wesley (1965), that agreed with the use of Dichopteris, while he limited the genus to two species. It wasn´t until Thévenard et al (2005) when this genus was truly well revisited, focused on the type species D. visianica, that the authors suggest to be the only valid one. This work is going to be the main reference for this Blog entry.
So, what is Dichopteris visianica? Following Thévenard et al (2005) definition this genus represents very large Pteridospermae fronds, among the largest ones ever described in literature, measuring up to 70 cm. This fronds show usually oneforked (bifurcated) rachis and a clear bilateral symmetry (Fig. 1; Fig. 2). At the base of the frond, the main rachis is massive, up to 2.5 cm wide, then becames smaller due to branching, around 1.5 cm (Fig.2).
Fig. 1: A complete Dichopteris visianica frond, part of De Zigno´s original description of the Grey Limestones of Veneto (Rotzo Formation, Italy)
Fig.2: Selected Dichopteris visianica from the Rotzo Formation (Pliensbachian, Italy) of Thévenard et al. (2005)
This genus possesses a number of characters that clearly differentiate it from Pachypteris, be it an odontopteridian venation (that of Pachypteris is sphenopteridian), shallowly constricted bases of the Pinnule (variable in Pachypteris), etc. In fact, by its characters it is more similar to Ctenozamites and even to the well-known Dicroidium (Thévenard et al., 2005).
Regarding the appearance and growth of this plant, the high proportion of fronds does not favour a significant branching system of the stem. In fact, suggest a more simplified, perhaps more monocaulous, in a similar way to Angiosperms like Carica papaya (Caricaceae, Check Bruy 2018 for examples of grown trend). However, one cannot deny a simplified dichotomous branching, observed in the genus Nypa fruticans (The Mangrove Palm, Fig. 3), or even a mixed monocaulous and low branching model (Nypa model can be seen in Chomicki et al, 2017). Nevertheless, the size of the entire plant is unknown nor whether it was growing in the understorey or canopy. The leaf morphology is consistent with both possibilities, yet the ultrastructural pattern of the cuticles appear to support an understorey habit, especially the shade leaf pattern (Thévenard et al., 2005).
Fig. 3: Extant Angiosperm Nypa Nypa fruticans, a dichotomous branching plant. Image taken from Printerest
Fig. 4: The Reconstruction of Dichopteris visianica A, frond; B, pinna; C, pinnule featured in Thévenard et al. (2005)
The Importance of the Environment of Dichopteris visianica
Dichopteris visianica comes mostly from the so-called Grey Limestones of Veneto, historically assigned to the «Noriglio Grey Limestone Formation», part of the Calcari Grigi Group, which was redescribed, reassigning these facies with Plants to the Rotzo Member (Bosellini & Broglio Lorica, 1971), that was upgraded to a proper formation recently (Masetti et al, 2012). These sediments have been attributed an Upper Sinemurian-Lower Pliensbachian age (Fugagnoli & Loriga Broglio, 1998), however works such as Franceschi et al. (2014) have managed to prove that in reality the formation is limited to the Pliensbachian stage only. The Formation stands out for having very variable sediments, although most of it is made up of ooidal, peloidal, bioclastic and intraclastic limestone, with marly and clayey horizons, representing shallow tropical lagoons (Bahamian-type lagoons according to Wesley, 1965), only a few metres deep (Fig. 5). These lagoons were closed seawards by oolitic shoals and bars, and bordered landwards by marshes (Bosellini & Broglio Lorica, 1971). The linkage of these sediments with the Trento Carbonate Platform, and especially with the Lithiotis reefs, should be emphasized.
Fig. 5: Location and Paleogeography of the Rotzo Formation after Posenato et al. (2013)
The flora of these facies developed in a Bahamic biome, where they grow abundantly, with a transition from the lagoon to solid ground, probably through a complex coastal marsh and/or swamp, with Mangroves and Taxodium-like swamps developed on shallow marshy and/or submerged settings, as proven by the presence of a network of carbonated rootlets with associated Marine fauna (Neri et al., 2015; Neri et al, 2017). This means that in the lagoon some areas were permanently submerged, but others were periodically emergent or less frequently flooded depending on the tides in the lagoon, with finely crystalline aragonite is being precipitated from ocean waters in areas of shallow water bordered by mangroves (As seen in Weasley, 1965; Fig. 6).
Fig. 6: Paleoenvironment of the Rotzo Formation lagoon from Franceschi & Bernardi (2021)
In this context, the leaves of Dichopteris visianica appear with exceptional preservation, indicating that they were deposited close to the mother plant, in calm waters with little or no current, an aspect that may have been reinforced by the Mangroves present (Weasley, 1965; Thévenard et al., 2005). In fact, this genus is usually deposited with the strata with abundant carbonate roots (Mangrove Strata, Thévenard et al., 2005), an aspect that may suggest that the leaves of Dichopteris visianica could be those of the Mangrove plant, reinforcing its resemblance to the Nypa palm. Unfortunately this cannot be verified unless a complete plant is found. Even so, the finding of the largest Pteridospermae leaves known in the fossil record in a Mangrove environment is curious, and beyond the size of its leaves, other aspects may help to find parallels with the genus Nypa. If a similar environment can be demonstrated for both after cuticle data, although in the case of Dichopteris visianica, had it resided in Mangroves, it would have surely done so in the shade of larger Plants (The Hirmeriellaceae in Taxodium-swamp or Mangrove arrangement suggested by Neri et al, 2017, may be an example).
Finally, it is interesting to mention the case of other localities where this genus has been identified. And there is a curious fact about the presence of Dichopteris sp. in other Pliensbachian Mediterranean localities, these being the Budoš Toarcian limestones in Montenegro (Pantić, 1981) and the Mangrove Pliensbachian sediments in Shkodra, Albania (Barbacka et al., 2019). And is it that the collected Pteridospermae leaves are assigned to Pachypteris? o Cf. Pachypteris, taking as reference the work of Harris (1964), where Dichopteris is a synonym of said genus or using Pachypteris as a wastebasket taxon. Therefore, it is possible that these leaves are misidentified Dichopteris. Finally, this genus may have been collected from strata with the same carbonate root pattern (seen in the Rotzo Fm), from the Col de Ghnim Toarcian in the Moroccan Atlas (Azilal Formation, Unpublished). Although all these possible records of the genre outside the Rotzo Formation need to be reviewed.
References
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Dactylioceras 