Hunting For Fossil Cycads
in Petrified Forest National Park, Arizona
As part of a coöperative venture between the Western Interior Paleontological Society
(WIPS) and the
Petrified Forest National Park, I had the opportunity to do some fieldwork in the park in October 1992. The rocks exposed there are mostly
the upper Triassic Chinle Formation, a series of interbedded sandstones, claystones, and shales representing lakebed and
floodplain deposits. All of our work was to be in the lower Petrified Forest member of the Chinle, in rocks approximately 220 million
years old.
Five of us drove down from Denver to meet with park paleontologist Vince Santucci and assist him with various research
and reconnaissance projects. We were joined by a sixth WIPS member from Arizona, and all shared a condominium
generously provided by the park service for our use.
Although the park is most famous for its amazing deposits of petrified wood, especially the Arizona state fossil,
Araucarioxylon arizonicum, it also contains fossils of a remarkably varied vertebrate fauna, including remains of large amphibians, many types
of thecodont reptiles, a variety of fish, and some of the earliest dinosaurs. Leaf fossils are found in certain claystone layers
and represent cycadeoids, ginkgos, ferns, horsetails, and conifers; the stems of two types of true cycads
(Lyssoxylon and Charmorgia) have also been found in the park.
Although my specialty is fossil reptiles, I was excited about the possibility of discovering some cycad fossils since I grow many
of the modern genera as a hobby. (I have been a member of the Cycad Society since its founding in 1977.) So while part of our
field crew went to survey the Blue Mesa area for evidence of large vertebrates, Doug and I chose to work in another area and look in
one of the grey claystone layers for plant fossils. (Unfortunately, I am not at liberty to reveal specific locality information due
to problems with vandalism in the park. A recent survey found that an average of 3-1/2 lbs of petrified wood per vehicle leaves
the park illegally. Ever heard of Fossil Cycad National Monument in South Dakota? It isn't even a National Monument any
more because vandalism has denuded it of all the cycads.)
I traced the grey claystone layer around the base of a small hill, looking for a place where it would be relatively easy to remove
the overburden and get to some fresh rock. A deep, narrow arroyo on the west side of the hill provided me with the access I needed.
It was late in the afternoon and the arroyo was in shadow by the time I began quarrying, but in the hour or so that I worked before
my rendezvous with Doug, I uncovered enough plant hash (mashed up leaves and stems) to determine that I had found a
promising locality.
Doug, Vince, and I returned the next day with a large pick for removing the overburden in an expedient manner. Once this had
been done, Doug and I remained to work the quarry. It's hard to explain, but I could just "smell" cycad fossils (then again, maybe it
was merely optimism). But before lunch I began finding detached leaflets that looked very much like those of a modern
Zamia in their shape and venation. Doug was working on the opposite side of the wash and stratigraphically a little higher than I was, where
he was finding mostly plant hash, some of which looked like the giant horsetail
Neocalamites. It was after lunch that, getting tired,
I was swinging my rock hammer rather nonchalantly at some broken, blocky stones when out popped a beautiful pinnate leaf!
A spontaneous "Hallelujah! There it is!" sprang from my lips. The frond was not complete, but the rachis and about a dozen
pinnae
on each side were preserved as a black carbonaceous film in the rock. Doug was suitably impressed and immediately stopped
what he was doing to help me search for the counterpart, which he found on the floor of the arroyo. It lacked the carbonized film
but showed the perfect mirror image as a shallow impression. It was spooky to see how closely the frond resembled one of a
modern Zamia, given the incredible timespan that separates them. The fossil lacked the conspicuous constriction of the pinna
bases common to the modern Zamias, and the pinnae were squared off at their ends rather than tapering as in the modern forms in
my collection. But it was obviously a cycad-like leaf.
The adrenalin began flowing and I didn't feel tired any more. I just knew there were more cycads waiting to see their first
glimpse of daylight in 220 million years! We immediately christened the locality "Cycad Wash" and worked the quarry for the remainder
of the afternoon. By the time we had finished, we had a backpack full of carefully-wrapped specimens, almost all of which
appeared to be leaves of the same cycad. When the sun was getting low, we shouldered our hammers and packs, and set out to hike back
to civilization, knowing that some of the fossils we were carrying would be good enough to be curated into the Petrified
Forest museum collection. (Private collecting is not allowed in national parks.)
Back at the condo that evening, everyone exchanged stories and showed each other what we had found. The other crew had
some nice teeth and scutes of phytosaurs and reports of awe-inspiring deposits of petrified wood in all the colors of the rainbow. We
all felt privileged to see the backcountry areas of the park where vandalism has not yet removed all but the large logs. We showed
our finds to Vince and he selected the best specimens to keep. I was allowed to retain a couple of lesser-quality cycad leaf fossils
to take back to the Denver Museum of Natural History for their paleobotany collections. Vince also provided me with some
literature from the park's library so that I could read up on the fossils I'd found.
The leaves were those of Zamites ("rock Zamia")
powelli, a common member of the Triassic flora of the United States. It is
thought to have been a deciduous plant of moist habitats. Though in macroscopic form it is impossible to tell the fossils we found from
true cycad leaves, the microscopic structure of the epidermis places Zamites in the extinct group called bennettitaleans or
cycadeoids. Both groups are gymnosperms, or "naked seed" plants, meaning that they bear their seeds in cones rather than in flowers, and
are more closely related to modern ginkgos and conifers than to any other living groups. In fact, flowering plants were yet to
evolve, and would not appear in the landscape until the Cretaceous. The cycadeoids resembled true cycads in growth habit and
gross morphology, except that the structure of their cones was different (bisexual in some species, as opposed to male and female
cones always being on separate plants in true cycads), and they bore them in the leaf axils, rather than at the apex of the trunk as
true cycads do; this left conspicuous scars or pits on cycadeoid trunks. The stems from the former Fossil Cycad National Monument
are actually those of cycadeoids, and you will frequently see them all called "fossil cycads" in the literature. Cycadeoids appear in
the fossil record in the Triassic, whereas true cycads go back another 40 million years or so, being known from the lower Permian.
They are thought to have evolved from seed fern ancestors, probably some time in the Carboniferous. The closeness of
relationship between the cycadeoids and the true cycads is still a matter of debate.
The remainder of our time was spent doing fieldwork in other areas, and on projects not pertaining to fossil plants. But for me,
the time I spent looking for cycads was the highlight of the trip. Vince has now left the park, and we haven't been back, although
WIPS has coöperative agreements with the Forest Service and the Bureau of Land Management and runs some very interesting trips
to other areas each year. Perhaps some time in the future we will return to Cycad Wash, and unearth that cone that is still
lurking somewhere just below the surface.
Want to see more fossil cycads? Or read a little about their history?
Watch online videos of Petrified Forest National Park at TravelRapidly.com.
