Paleobotanical mystery – what is this unidentified plant?

One of the things I love about paleobotany is that any project inevitably leads to new discoveries and new questions. Palebotanists spend a lot of time looking at things that nobody has ever looked at before, so if you like discovery, paleobotany could be for you. As an example, my assistant and I recently found a fossil of a small herb inside of piece of fossil wood from a tree, and now we are trying to figure out what it is and how it got there…

We made this discovery when were examining petrified wood preserved in marine deposits of Panama that are ~18 Million years old. The fact that the wood is preserved in marine deposits means that the pieces were washed out to sea and then fossilized. Sometimes, wood fossils that are preserved in marine deposits have holes made by small wood-boring clams after the wood was washed out, but before it was fossilized. These clams can still be found breaking down wood in the ocean today. Here is a photo of one of our fossils showing the holes that were probably made by wood-boring clams.

Miocene fossil wood

We cut the fossil wood with a rock saw and then examine the cut faces. Normally, we examine the anatomy of the wood, but this specimen turned out to have a tiny plant fossil in one of the holes, seen below cut in cross section. This is strange because if the holes were made by clams, then they were made after the wood was washed out to sea. Unless it was somehow washed in after the clam, the hole must have been made before that to allow this other small plant to grow inside of it.

plant axis in a hole. the words "vascular bundle" are written on the fossil wood tissue, and the the 6-lobed axis is in a hole.

Plant axis in cross section. The words “vascular bundle” are written on the fossil wood tissue that surrounds the 6-lobed herbaceous axis.

We think that this is a stem and not a root because the roots of dicots have vascular tissue in the center, whereas this thing has vascular bundles around the outside, one per lobe. The tissue in the center of the stem was probably thin-walled parenchyma cells that are not preserved. Below is a close up of one lobe with a vascular bundle. The group of cells in the very center of this photo are the water-conducting xylem cells.

close up of a vascular bundle

close up of a vascular bundle

Does anybody recognize what this is? I found an image of a six-lobed stem of Clematis (Ranunculaceae) in cross section HERE that has me intrigued. Whatever it is, I’m sure the fossil record of Neotropical herbs is sparse, so it would be nice to have an identification. Here are two more pictures.

plant axis in a hole in wood.

plant axis in a hole in wood.

plant axis in a hole in wood

plant axis in a hole in wood

Tasmania – where the red fern grew

A few years ago someone gave me a brick-red plant fossil from Tasmania labeled “Osmundacaulis.” Recently, I’ve been making thin sections of petrified wood, but I began by practicing on a few of my own fossils. I started with the Osmundacaulis specimen and in honor of fossil day here are some photos of the fossil in cross section.

Osmundacaulis roots and stipes

What you are seeing is a small part of the outer portion of tree fern trunk, in cross section . The marks across the top are millimeters. You can see a complete trunk in cross section here. All of the preserved tissue is either roots or frond bases (stipes). There are seven stipes in cross-section, but only 5 are mostly complete. An arrow is pointing to one, and there is a close-up of that stipe in the next photo. All of the rest of the reds and yellows are minerals that have either replaced or preserved the tiny roots.

Here is the close up of the stipe at the arrow in the last photo. Notice the oval-shaped band and the U-shaped bundle inside. The outer oval provides support and mechanical strength to the frond, and the U-shaped bundle includes the vascular tissue, the xylem and phloem.

Osmundacaulis stipe

I’ve never been anywhere near Tasmania, but it has a magical place in my mind and I really hope that someday I’ll get to visit.

Also here is my dog.

happy and alive

A fossil fruit from Panama

Follow this LINK to my post on the Panama Canal Project blog

Dracontomelon macdonaldii

Triassic cycadophytes from Arizona

Check out @paleonate‘s Tweet: https://twitter.com/paleonate/status/453896477119496193

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

Triassic

Here is a complete blog post

http://nmnh.typepad.com/smithsonian_fossils/2014/04/triassic-plants.html

 

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

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