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This Triassic Cycadophyte is part of a collection I am making this week with three others for the new Smithsonian deep time exhibit!

The University of Maryland article

http://cmns.umd.edu/news-events/features/1582

Potomacapnos apeleutheron, an early flowering plant from North America

Well this has been fun!

My latest paper with Leo Hickey has been getting a bit of press, and was featured on iO9 and the New York Times Science Page!

A while back I posted an entry about the known flowering plants from the Aptian of North America, but I’ve learned a lot since then and a new post is due. However, at the moment I am working hard to finish my thesis, so it may be a bit longer. In the mean time I thought I’d take moment to provide some additional information to readers who already saw the Smithsonian article or the University of Maryland press release. The Aptian collections from the Potomac Group (lower Zone I) contain the oldest flowering plants from North America. There are several different species known, many of which have not been described. One of the most important is Acaciaephyllum, which appears to be the oldest monocot. Now, Potomacapnos demonstrates the presence of eudicot-like plants as well, which is why I picked that one to describe as a new species. The two (Acaciaephyllum and Potomacapnos) are not from exactly the same collections, but they are both from the oldest unit in the Potomac Group. This means that by the time we start picking up plant megafossils in North America, much of the basic structure of angiosperm phylogeny must have been laid out. For me, that inspires a wanderlust for field work places like Portugal, South America, Africa, and Asia where there is much more field work to be done and layers slightly older than the Potomac Group might reveal exciting new species that fill today’s apparent morphological gaps between the monocots and eudicots, or between the aquatic water lilies and the tropical shrubs and lianas of the ANA-grade like Amborella and Austrobaileya.

Potomacapnos apeleutheron Jud et Hickey

Potomacapnos apeleutheron Jud et Hickey

A paleobotanical puzzle

The plant fossil record is composed of fragments that represent different parts of the plant body and different stages of the plant life cycle. Part of the challenge in paleobotany is putting the pieces back together and getting a concept of the whole organism. Recently I was looking through the Smithsonian’s collections from the Patuxent Formation in Virginia because I was searching for fossils of early flowering plants. One of the most common plant fossils in the Patuxent Fm. are the leaves known as Dioonites buchianus. Dioonites leaves look like the leaves of a cycad, but there are several extinct groups of gymnosperms that had cycad-like foliage. In Cretaceous collections, cycad-type foliage typically comes from plants either in the order Cycadales, or in the extinct order Bennettitales.

Dioonites buchianus leaf

Dioonites buchianus leaf with fern

The standard way to tell whether compressed cycad-type leaf fossils are cycads or bennes is to examine the stomata. Stomata are the pores in the leaves that plants use to transpire water and take up carbon dioxide. Bennettitalean stomata have a unique development and morphology that makes them recognizable. They have thickened cuticle on the outer and dorsal walls of the guard cells, they are arranged more or less in rows, and the stomatal pores are oriented perpendicular to the veins (Taylor et al. 2009). Lets have a look using an epifluorescence microscope…

dioonites buchianus epidermis

Epidermis of Dioonites buchianus

dioonites buchianus stomata

Stomatal pore of Dioonites buchianus

BINGO! Dioonites buchianus is a benne. In both photos, the veins run from the lower right to the upper left, but they are not visible.

I also note that it seems to be a general rule that the leaflets or blade of bennettitalean leaves attaches along the upper surface of the petiole (rachis), whereas in cycads it generally inserts along the middle of the rachis. In D. buchianus, the leaflets attach along the top.

Dioonites buchianus

Dioonites buchianus, scale=1cm

I also noticed that in some of the collections there are a few fossils that belong to the species Williamsonia virginiensis. Williamsonia fossils are cones or parts of cones produced by some bennettitalean plants. This intrigued me and I decided to test the hypothesis that the same plants produced the leaves called Dioonites buchianus, and the cones Williamsonia virginiensis.

USNM 3404 cpt

Williamsonia virginiensis

Williamsonia virginiensis

Williamsonia virginiensis

Williamsonia virginiensis cones consist of bracts (modified leaves) arranged around a central scar. The central scar is where the ovulate receptacle attached. The ovulate receptacle is a distinctively bennettitalean structure that bears the megasporophylls and seeds. See an example here, on the right. Sometimes these structures are found isolated with exceptional preservation (Stockey and Rothwell 2003). Unfortunately, I haven’t seen any of these ovulate receptacles in the collections.

USNM 3404

Williamsonia virginiensis

In the lieu of finding the Williamsonia cones and Dioonites leaves actually connected in a single fossil via a stem, I had to employ alternative approaches for demonstrating affinity. One way is to analyze association data, and another is to demonstrate morphological and structural similarities.

First, I looked at a table of all the individual sites where plant fossils have been collected from the Patuxent Formation and what species were found. I noticed that only some of the collections included both D. buchianus and W. virginiensis, but that fossils of Williamsonia virginiensis were never found without abundant fossils of D. buchianus from the same site. If I had found that each was often found without the other, I’d be more likely to conclude that they came from different species.

Next, I decided to compare the epidermal structure of the bracts of the Williamsonia cones with the Dioonites leaves.  Above we saw that the cells on the surface of the leaves have wavy (or crenulate) margins and stomata that are sunken, arranged in rows with the pores oriented perpendicular to the veins, and surrounded by two thickened cells that fluoresce brightly under the scope. Only one Williamsonia had the original carbon of the bracts preserved and thus the potential to see the epidermis under the scope. 

LJH 71 117 williamsonia

Williamsonia virginiensis

Cuticle of W virginiensis

Cuticle of Williamsonia virginiensis

The margins of the epidermal cells are less crenulate, but the stomata have similar structure! Although I think the similarity supports the hypothesis that these two go together, what I saw still surprised me. I expected to see the epidermal cells with the crenulated margins, and no stomata. I suppose the various illustrations that I have seen over the years of bennettitalean flower-like cones with petal-like white or otherwise colored bracts is what was behind this expectation. But the stomata are there, and they are abundant! This means that in life the bracts were probably green, and based on the density of stomata, I’ll bet they were important in supplying photosynthate to the developing ovules/seeds!

Stockey and Rothwell 2003

Taylor et al. 2009

Potomac Group exhibit

I want to share this interactive website from the Smithsonian with my followers! There is a significant plant component, but also some dinosaurs or whatever…

Its all about Cretaceous fossils from the Potomac Group, some of which I wrote about in my second post.

http://www.mnh.si.edu/exhibits/backyard-dinosaurs/index.cfm

Sphenopsids

I like horsetails. Living Equisetum stands alone representing an entire class of land plants, and one could argue that it is the most successful genus of vascular plant in the world. Species are found on every continent except Antarctica. [Note: Equisetum is introduced in Australia today, but extinct sphenopsids including Equisetum grew there during the Mesozoic, as is also the case for Antarctica.] Studies of well-preserved fossils from the Early Cretaceous and Jurassic have shown that they have been doing basically the same thing for ~150 million years (Stanich et al. 2009; Channing et al. 2011); and Triassic (~225Ma) compression fossils suggest the genus may be older. Fossil evidence demonstrates that Equisetum is probably the oldest living genus of vascular plant.

Living and extinct horsetails are easily recognized by their distinct jointed stems with whorls of leaves and sometimes branches borne at the joints (nodes). The spore-producing structures are aggregated into a strobilus or cone.

Deep/old branches of the tree of life that have few genera or species today, like horsetails, often turn out to have a rich fossil record of extinct diversity that encompasses a much broader range of morphology, life history, and ecology than can be found among modern representatives.

A recent paper in Review of Palaeobotany and Palynology re-emphasized this point for me.  Rößler et al. (2012) described a fossil horsetail from the Permian of Germany (~250-300 million years ago) that they called Arthropitys bistriata. This plant was a large tree (15 meters or more) with wood and growth rings, and a branched crown! Hardly the stream-side herb in the photo above.

There have been other descriptions of fossil horsetails showcasing their extinct diversity in the last few years. Neocalamites horridus (Shuquin et al. 2012) from the Triassic of China looked like a giant Equisetum covered in sharp prickles, and Sphenophyllum costae (Bashforth and Zodrow 2007) from the Pennsylvanian of Nova Scotia was an elaborate bramble with distinct orders of branching that produced a range of leaf types, from fan-shaped leaves for capturing light, and hook shaped leaves for climbing and support.

Last fall my wife and I found our own horsetail fossils while collecting plant fossils in the Cloverly Formation in Wyoming. This specimen is broken at the node, so what you are seeing from the bottom up is a stem, with a whorl of flimsy looking branches in the middle of the photo. The branches subtend (are directly below) a whorl of leaves that are mostly fuzed into a collar with free tips, kinda like Bart Simpson’s hair. The node is under that collar, and that’s where the top of the plant broke off.

Rößler et al. 2012 The largest calamite and its growth architecture – Arthropitys bistriata from the Early Permian Petrified forest of Chemnitz. Review of Palaeobotany and Palynology 185 p.64-78

Petrified Forest National Park

The record of petrified wood, across all continents, and back to the Devonian period tells about the history of earth’s forests. Petrified wood preserves information about things like taxonomic identity, canopy structure, seasonality, and productivity. Coupled with an understanding of the sedimentogical context of the rocks in which the wood is preserved we can learn even more, like the soil preferences and spatial structure of the trees (if they are preserved in place) or about the events that resulted in the trees’ burial and preservation.

I was recently on a trip with my wife that took us through Petrified Forest National Park during National Park week (not by accident) where the Triassic Chinle Formation is exposed and the remains of a ancient forests are preserved. I’d never been before, but I’d seen the wood in museums, and am familiar with some of the vertebrate paleontology that is going on involving some Smithsonian scientists; so I was excited to finally experience the place.

Obviously, I expected to see a lot of petrified wood, but even so I was impressed! Vast fields of deep red silicified wood dot the landscape not far from historic route 66 which runs through the park. Walking trails wind through some of the fields, taking the visitor past impressive specimens, while in other areas visitors can look down into valleys littered with wood. As erosion exposes the grey-blue and red sediment of the ancient floodplain deposits (which make for beautiful backdrops), the wood-bearing horizons are occasionally exposed. When this happens the mud and sand washes away, but the heavy petrified wood rests on the surface.

The wood comes from several levels in the Chinle Formation, but most of it comes from the Petrified Forest Member and the Sonsela Member. These units are a sequence of channel fills and floodplain deposits which no doubt supported vegetation, but the trees that produced the wood known as Araucarioxylon arazonicum, (Triassic conifer wood) were not preserved in growth position. None of the logs are upright and none of them have roots, though the largest specimens show the basal flare. The wood is generally found in the cross-bedded conglomerates and cross-bedded conglomeratic sandstones. These are the channel fills, which rest on scour surfaces with 1-7 meters of relief. This all suggests that ~210 Million years ago there was high energy flow capable of transporting and then burying the tremendous logs, sometimes forming log jams.

The longest petrified logs measured at the park are ~43 meters (~140 feet) and up to 3 meters in diameter. The canopy may have been 60 meters (~200 feet) high. More than ten other types of petrified wood have been identified, but most of them are rare. In addition to conifers, there are also tree-ferns, a Ginkgo relative, and Calamites. The wood doesn’t have annual growth rings, which tells us that growing conditions were generally good year-round; a conclusion that is consistent with the presence of tree ferns which generally don’t tolerate cold temperatures.

If you get a chance to go through Petrified Forest, I highly recommend it. We had a great time and they do a good job of painting the picture of the ancient ecosystem, including the early dinosaurs and other animals.

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