Conifers of the Crato Formation
|Fig. 1 A selection of conifers from the Crato Formation.|
Greetings fellow connoisseurs of palaeobotany,
I thought I’d write just a quick blog post, since I have a little free time and I just finished this series of illustrations for another project. Hopefully I can wrap it up reasonably quickly without rambling on for too long, but we’ll see how it goes (edit: well, as you’ve probably already guessed, this post grew a little larger than expected, so I hope you still persevere to the end… please?).
The Crato Formation (as some of you will undoubtedly already know) is a Lower Cretaceous formation from the northeast of Brazil, composed of a series of very finely laminated limestones and shales which have yielded hundreds of fossil taxa which inhabited a warm dry desert-like environment surrounding either a lake or a lagoon (the exact depositional environment is still hotly debated; Ribeiro et al., 2021). It is approximately 115-120 million years old, although this age is also uncertain (Martill et al., 2021; Melo et al., 2020) and is famous for the exceptional preservation of many of its fossils, which include hundreds of insects, tens of vertebrates and, most importantly, about 80 different species of plant!!! I would love to cover all of them at some point, but that will have to wait. Reconstructing an entire fossil flora is proving to be a lot more time-consuming than I expected, so it’s gonna be a while before I finish, but for now I finally have enough to do a blog post on some of the most abundant plant fossils of the Crato, the Conifers!!!
A quick note on conifer taxonomy
Okay, so what are conifers? This is a seemingly simple question, but has a slightly confusing answer, which I’ll cover briefly here. Extant conifers belong to three different orders within the class Pinopsida: the Pinales, the Araucariales and the Cupressales. During prehistory several other orders of conifers existed (such as the Voltziales), however these will not be covered in this blog post. The class Pinopsida itself belongs to the clade Acrogymnospermae, which includes all extant gymnosperms (the conifers, gnetophytes, Ginkgoales and Cycadales). Within Pinopsida the Pinales (Pinaceae) are usually found to be 'basal', with the Araucariales (Araucariaceae and Podocarpaceae) and Cupressales (Sciadopityaceae, Cupressaceae and Taxaceae) being sister to each other (Fig. 2). In some phylogenies, the gnetophytes (a group of weird gymnospermous plants represented today by Welwitschia, Gnetum and Ephedra) are found to be deeply nestled within the conifers (although they are more often found to be outside of Pinopsida; Christenhusz et al., 2017; Li et al., 2019), and when this is the case the term ‘conifer’ becomes a paraphyletic name (Fig. 2). Despite this complication throughout the remainder of the blog I will use ‘conifer’ in its vernacular form to mean plants belonging to either the Pinales, Araucariales or Cupressales.
|Fig. 2 Simplified phylogenetic trees showing the interrelationships within the 'conifers', and the possible positions of Gnetophyta.|
The Conifers of the Crato
Within the Crato Formation two main families of conifers are present: Araucariaceae, represented today by the monkey puzzles and kauri trees of South America and Australia (Christenhusz et al., 2017), and Cheirolepidiaceae, an extinct family of conifers which were widespread during the Mesozoic (Taylor et al., 2009). I will deal with these two families separately below. Remains of the conifer morphotaxon Brachyphyllum are among the most abundant of plant remains in the Crato Formation (along with the fern Ruffordia goeppertii; Mohr et al., 2007) and are also the most well studied, so I will discuss this morphogenus separately too. In addition to the Araucariales and Cheirolepidiaceae the leaf morphogenus Podozamites is also present, and fossils of these isolated leaves are very common. While they are often assigned simply to Podozamites sp., specimens of P. lanceolatus have been reported from the Crato Formation (Lima et al., 2012). Some of the conifers described below possess Podozamites leaves, and the diversity of forms present could indicate multiple source species (Fig. 3). However, not all leaves from the morphogenus Podozamites necessarily belong to conifers: the gnetophyte Cerania heterophylla, from the Crato Formation, also possesses Podozamites leaves when young (Kunzmann et al., 2009).
|Fig. 3 A selection of Crato Formation Podozamites morphotypes. The two specimens on the right likely belong to Agathis, based on their similarity to modern Agathis leaves and the known Crato Agathis species. Scale bar 20mm.|
Crato Brachyphyllum species
The morphogenus Brachyphyllum is a globally distributed leaf taxon found throughout the Mesozoic, which is characterised by having rhomboidal (diamond-shaped) leaves which are wider than they are long, and arranged spirally around the branch. Within the Crato Formation three species are well documented; a fourth species has been identified, B. insigne, but this identification was based on poorly preserved cone remains, and thus its presence is uncertain (Lima et al., 2012; Batista et al., 2021). Brachyphyllum obesum (Fig. 4.a-d) is the most abundant species of Crato conifer; like other Brachyphyllum species it has small (less than 1cm) rhomboidal leaves which are arranged helically around the central stem of a branch which they are adpressed to. The leaves were thick, almost succulent, and their margins are simple and adhere slightly to overlapped leaves. Branches are arranged oppositely or suboppositely and there can be up to 4 orders of branching (Fig. 4.d). While fossils of B. obesum typically show a striated surface texture this is an artifact of taphonomy (how the leaf was preserved after death) and in life the leaf surface would have been smooth or coriaceous (leathery) (Kunzmann et al., 2004). Terminal branches have been found with attached spherical Araucaria cones, confirming the araucarian affinities of B. obesum.
A second, very similar Brachyphyllum species from the Crato Formation is B. sattlerae (Fig. 4.e-f), named after Dr. Ellie Sattler, a fictional palaeobotanist from the novel Jurassic Park (Batista et al., 2020). While it is almost identical in overall appearance to B. obesum (hence it only recently being identified as a different species) it differs in having 3-4 ridges on each leaf, with these leaves being arranged decussately (or in a very shallow helix or spiral) around the branch. In life these two species likely would have appeared near identical from a distance unless their leaves were coloured differently, or they possessed a different growth form, both of which can only be speculated about. These two species likely grew as medium to tall trees and likely possessed a growth form similar to modern Araucaria species (Fig. 7.d). The leaves of these species indicate adaptations to xeric (dry) environments, and they likely lived in a transitional brushland or coastal maquis (dense semi-arid bushland adapted to regular burning; Lima et al., 2019) as individual trees with an evergreen understory of shrub-like gymnosperms. Batista et al. (2021) further suggested that their canopies may have had an irregular outline, similar to Cupressus dupreziana (a conifer native to the Sahara Desert). Some extant species of Araucaria also possess irregular canopies, and many species display different growth forms depending on the growing environment. Permineralized Araucariaceae wood from the Crato Formation possibly belonged to these trees (Santos et al., 2020).
The third species of Crato Brachyphyllum is rather different from those previously described (to be honest it’s just plain weird); B. castilhoi (Fig. 4.g) is of uncertain taxonomic placement, but may belong to Cheirolepidiaceae, however this assignment is very tentative. It is characterised by having incredibly thick branches covered in small helically arranged rhomboidal leaves, with unusual club-shaped side shoots, which were arranged oppositely along the branch. Based on its thick stems and tiny leaves this plant was likely adapted to dryer environments that the other two Crato Brachyphyllum species, and its rarity in the Crato Formation further supports the idea that it lived further away from the Crato depositional environment, perhaps being restricted to xeric shrublands surrounding an inland desert. Its growth form is also very uncertain, but I have reconstructed it as a small tree or shrub with an open canopy (Fig. 7.g), similar to several xerophilic Araucaria species (A. muelleri and A rulei).
In addition to the previously described araucarian Brachyphyllum species (B. obesum and B. sattlerae) several other Araucariales existed in the area surrounding the Crato depositional site, however these were less common so perhaps hailed from more distant environments. The first of these conifers is Lindleycladus sp., a plant known from isolated elongate leaves and terminal branches with leaves arranged in a simple helix around reasonably thick and smooth caducous (short lived and shed) stems. Although this is uncertain, it appears to have affinities with the Araucariales and its leaves (which are thick, elongate, petiolate and have parallel veins) are similar to Agathis and some podocarps (in particular the aquatic Retrophyllum; Kunzmann et al., 2004), although the helical leaf arrangement is different from Agathis (which has decussately arranged leaves). In life it likely would have grown as a tree (Fig. 7.a) in less water stressed environments than the aforementioned Brachyphyllum species. While it is tempting to speculatively reconstruct is as aquatic or semi-aquatic tree (like the unusual Retrophyllum, which it resembles), this life mode is unlikely given its uncertain taxonomic placement, the highly unusual life mode of Retrophyllum (which is almost unique among modern conifers), and the relative scarcity of Lindleycladus remains in the Crato Formation (an aquatic conifer would be expected to have an increased preservation potential, relative to terrestrial conifers).
Two specimens, the first composed of a terminal shoot with elongate strap-like parallel veined leaves, and the second an isolated male pollen cone, have been assigned to cf. Agathis sp. (Fig. 5; Martill et al., 2005). The leaves of the first specimen were described as being arranged spirally, however, I have reconstructed them as being decussately arranged in accordance with modern Agathis (the specimen in question is flattened and poorly preserved at its apex, with leaf bases indistinct, so my deviation from the original description does not necessarily contradict the fossil evidence; a re-examination of the specimen is needed to confirm its phyllotaxy). As the male pollen cone was found in isolation, we cannot confirm whether it belonged to the same taxon as the terminal shoot specimen, however I have reconstructed them as the same plant here to show how Agathis male cones are arranged on the stem, in opposite or subopposite pairs from axillary buds. As previously mentioned, the variety of Podozamites leaf shapes present in the Crato Formation could indicate the presence of multiple Agathis species (the far-right leaf in Fig. 3 is comparable to the leaves of cf. Agathis sp. in Fig. 5, and the middle-right leaf of Fig. 3 is almost identical to several modern Agathis species, so likely also belongs to this genus). Aside from the abundance of Podozamites leaves in the Crato (which represent a multitude of plants both coniferous and not) there are very few specimens of Agathis known from the Crato Formation, indicating that trees (Fig. 7.b) of this genus were either locally uncommon or more likely restricted to habitats away from the Crato depositional site, perhaps in well-watered woodlands and gallery forests surrounding rivers to the south, east and west.
|Fig. 5 Crato Formation Araucariales. Scale bars: cf. Agathis sp., 50mm and 10mm; cf. Wollemia sp., 20mm; Araucariostrobus, 10mm; Lindleycladus sp. 20mm; Brachyphyllum obesum and B. sattlerae, 20mm; Araucaria/Araucarites spp. 20mm.|
A second Araucariaceae which was also probably restricted to more humid habitats is cf. Wollemia sp. (Fig. 5; Martill et al., 2005). It is known from a single specimen composed of a terminal cone attached to a stem with two in-situ leaves at a node; few details of its life appearance are preserved, but I have reconstructed it like a modern Wollemi pine (Wollemia nobilis) with leaves arranged spirally (but with twisted bases producing two layers of leaves forming a shallow X around the stem), however it differs in having a smaller cone than most W. nobilis individuals (although smaller cones are sometimes present) and (based on the two preserved leaves) slightly more rounded, less elongate leaves. In growth form it is most conservative to reconstruct it as a medium to tall tree which has a tendency of coppicing at its base, like the modern Wollemi pine (Fig. 7.c). Like Agathis, the rarity of this conifer in the Crato Formation and the xerophobic nature of its modern representatives suggest that cf. Wollemia sp. was restricted to more humid environments away from the Crato depositional environment.
Several different types of isolated araucarian cones are known from the Crato Formation. Araucariostrobus sp. is known from specimens of immature ovulate (female) cone which are attached to a stems with Brachyphyllum-type leaves (Fig. 5). Most other araucarian cones from the Crato Formation are assigned to Araucaria sp. (Mohr et al., 2007), although Araucarites vulcanoi ovuliferous scales have been reported (Lima et al., 2012), as have the leaves of Araucaria imponens (otherwise known from the Paleocene of Antarctica; Bernardes-de-Oliveira, 2013; Cantrill and Poole, 2012). Duarte (1993) described the leaves of Araucaria cartellei from the Crato Formation, although they have not been figured since and thus I have been unable to produce a reconstruction of them (I’ve been unable to attain a copy of Duarte, 1993, despite my best efforts). As previously mentioned, the presence of Araucaria cones connected to Brachyphyllum-bearing branches indicates that some of these cones belong to the Brachyphyllum-plant, however the diversity of cones present in the Crato (which vary in both size and shape: contrast the spherical cone of Fig. 4.b, with the larger elongate one of Fig. 5 bottom right), and the addition of several Araucaria leaf taxa, suggest a diverse assemblage of araucarian conifers were present in the Crato.
The second family of conifers with representatives in the Crato formation is Cheirolepidiaceae. This extinct family of conifers persisted throughout the Mesozoic and were very taxonomically and morphologically diverse, inhabiting a range of habitats. Within the Crato Formation they are represented by several taxa. The best studied of these is Pseudofrenelopsis (Fig. 6) which is known from two separate species: P. capillata and P. sp. (Batista et al., 2017). Specimens were initially described as Frenelopsis by Kunzmann et al. (2006) but were placed into Pseudofrenelopsis by Sucerquia et al. (2015) on the basis of their leaf arrangement and anatomy. Both Crato species are near identical and only distinguishable by the absence of hairs on the leaf surface and margin of P. sp.. The leaves enclose the stem completely in a closed sheath and have a triangular pointed tip; they are arranged singly at nodes in a spiral and the internode length increases along the branch to the tip. These conifers were xerophilic (adapted to dry environments) or halophytic (salt tolerant) so likely formed either small to medium trees (Fig. 7.e), similar to the growth form suggested for the British P. parceramosa (Alvin, 1983), or small bushes, as has been suggested for some Frenelopsis (Poyato-Ariza and Buscalioni, 2016) and cheirolepidiaceous Brachyphyllum species (Karakitsios et al., 2015). Male cheirolepidiaceous cones have thus far not been described from the Crato Formation, but I have included them in the reconstruction to show their form and attachment. Female cheirolepidiaceous cones are known from the Crato, however; these were almost heart-shaped and varied in size up to ~10 cm across (Fig. 6). An isolated cone figured in Ribeiro et al. (2021) bares some similarities to other cheirolepidiaceous cones; it is more elongate than the cones reconstructed here, and is about 5 cm long, however its taxonomic placement is uncertain.
The only other definite Cheirolepidiaceae from the Crato Formation is Tomaxellia biforme (Fig. 6; Kunzmann et al., 2006) This species is known from unusual terminal shoots which are heterophyllous (possessing two forms of leaves); proximally (the basal end of the stem) the stems are covered in short helically arranged Brachyphyllum-type leaves which are adpressed to the stem, however distally (towards the apex of the stem) the leaves become longer (Pagiophyllum-type leaves) and spread out at an angle of 50-70% away from the stem. This produces stems which in life would have appears to be almost fanned or brush-like at their tips. All the leaves were thick and fleshy, indicating some adaptations to a dry environment. Unfortunately, little material is known of this taxon from the Crato Formation, so it was likely an uncommon element of the palaeoflora surrounding the Crato depositional environment. I have conservatively reconstructed it as a small to medium sized tree, but there is no direct evidence for its true growth form.
A further two species from the Crato Formation may also be assignable to Cheirolepidiaceae. The first of these is Brachyphyllum castilhoi, which I described earlier. And the second is perhaps the most unusual (and awesome looking) Crato conifer: Duartenia araripensis (Mohr et al., 2012)!!! This crazy looking conifer is known from branches possessing an unusual anisotomous (asymmetrical dichotomous; aka, the branch splits in two and one side grows larger than the other, and the larger side alternates each time it branches) branching pattern, which produces a ‘zig-zag’ shaped ‘main axis’ with several subsequent orders of branching producing hexagon-like shapes when the branches overlap (it superficially resembles the spiny black olive or ‘Chemistree’, Bucida spinosa). These branches were covered in spirally arranged Brachyphyllum-type leaves which were coriaceous (leathery) and hairy. The terminal branches were thorn-like and while they were still likely covered in leaves (as I have reconstructed) they may have been exposed as woody thorns. So basically, this is a conifer with badass thorny geometric branches covered in dragon-like scaly armour!!! Okay, fanboy moment over, back to business… Unfortunately, not much can be said about its growth form, so I have reconstructed it here as a small tree with a layered canopy (although I must emphasise that this is very speculative, it could just as easily be a multi-stemmed shrub; Fig. 7.h). D. araripensis shows many xerophytic adaptations, so it likely inhabited the inland xeric shrubland and desert boarders (so whatever its growth form, it likely wasn’t a massive plant).
|Fig. 6 Crato Formation Cheirolepidiaceae. Scale bars: Pseudofrenelopsis capillata and P. sp., 50mm; Cheirolepidiaceous cones, 100mm; Tomaxellia biforme, 10mm; Brachyphyllum castilhoi, 50mm; Duartenia araripensis, 100mm.|
A VERY quick note on the Ginkgoales
While they’re technically not, ginkgoes are often informally called conifers, especially within horticulture (they’re grouped in with the conifers in my own copy of ‘The Tree and Shrub Expert’ by Dr. D.G. Hessayon, for example), so I’d like to say something briefly on the Crato Ginkgoales. Okay, are you ready? Great, let’s go….
So the most important thing to know about the Crato Ginkgoales is that THEY DON’T EXIST!!! There are no known remains even tentatively assignable to the Ginkgoales at all from the Crato Formation. The nearest we get are possible Czekanowskiales which were mentioned by Mohr et al. (2007), but these have never been figured, described or even discussed since (unless in reference to Mohr et al,. 2007). The only reason that I bring this up is that including ginkgoes in Mesozoic palaeoart has become almost a standard in situations were the artist wants to include some plant life without wanting to do mountains of extra research, and while in many cases the inclusion of a maidenhair tree is appropriate (the species often depicted usually isn’t though, but that’s a story for another time), it is not in the Crato Formation, based strictly on the available data. Ginkgoales were present in Aptian South America, several species are known from the Baqueró Group of Pategonia (I reconstructed a few for Mario Coiro’s blog post on the Anfiteatrode Ticó Formation), but they appear to have been absent from equatorial regions of the continent at this time. This is of course subject to change, but for the time being it is safest to avoid including any Ginkgoales in your Crato Formation reconstructions.
So, it looks like we’ve come to the end of our tour through the Crato Formation (at least for now). As you have just seen the Crato landscape was populated by a diverse array of conifers which grew in a variety of habitats. Most taxa show at least some adaptations to a dry (or seasonally dry) environment; however, several uncommon conifers were xerophobic and would have been restricted to more humid environments. A lot of taxonomic uncertainty still surrounds many of the Crato conifers, and only a few species have been well studied.
I hope that you’ve enjoyed reading about them – as expected I wrote a lot more than I intended to, but if you’ve made it to the end, you can give yourself a well-deserved pat on the back, and I’d also like to give you a very big ‘Thank-you’ for caring about fossil plants.
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