those birds unknown, that left only footprints

Big toes! Huge bird!!

Along the rim of Bull Canyon, on the north slope of the La Sal Mountains in southeast Utah, my field assistant and I followed footprints in sand. Three-toed two-footed creatures had passed this way when the sand was wet, before it turned to rock. They were common then, traveling in packs.

Similar 3-pronged impressions occur 2000 miles to the east, in the Connecticut River Valley in Massachusetts. Some are famous—among the earliest to be studied and published. They were found in 1835 by Dexter Marsh, who was laying a flagstone sidewalk. He showed the slabs to the owner of the property who gave them to a physician who then gave them to state geologist Edward Hitchcock.

"They consist of two slabs, about forty inches square, originally united face to face; but on separation, presenting four most distinct depressions on one of them, with four correspondent projections on the other; precisely resembling the impressions of the feet of a large bird in mud." (Hitchcock 1836, italics mine)

Sandstone slabs, each 36.5" x 34"; depressions (molds) on left, projections (casts) on right (source, click on "Fossil Slabs Found by Dexter Marsh").

Hitchcock was understandably excited. Very few bird fossils had been found anywhere, and geologists had decided that because most birds were lightweight creatures of the air, they were unlikely to be submerged and preserved on the bottom of lakes, oceans and such. "Even when they chance to perish in the water, they float so long upon the surface, as to be most certainly discovered, and devoured by rapacious animals."

As it turned out, such marks were fairly common in the area. Hitchcock studied slabs from five quarries, concluding that the impressions must have been made by birds:

1. These impressions are evidently the tracks of a biped animal. For I have not been able to find an instance, where more than a single row of impressions exists.

2. They could not have been made by any other known biped, except birds. On this point, I am happy to have the opinion of more than one distinguished zoologist.

3. They correspond very well with the tracks of birds.

Some of Hitchcock's drawings of modern-day bird tracks, from his 1836 publication (source).

However, many of the tracks Hitchcock studied were too large to have been made by the birds we know—to 18 inches long and 13 inches wide, and separated by 6-foot strides. Therefore these tracks must have been made by large birds now extinct. Eminent geologists of the day agreed with Hitchcock (2). But by the end of the century they had been "proven" wrong. These tracks were not avian, they were reptilian (Dean 1969).

Science marches on of course, and we now know that in a sense, Hitchcock and his colleagues were correct. The creatures that left footprints in the Connecticut River Valley and southeast Utah were indeed birds. But they also were dinosaurs, specifically theropods (study the images below before discussing this at cocktail parties).

Dinosaur classification (3). While all birds are dinosaurs, not all dinosaurs are birds; similarly, birds are a subset of theropods (from Zureks).

Evolution of birds from a dinosaur ancestor; Manti-La Sal National Forest, Bull Canyon Tracksite.

You might be wondering why dinosaurs traveled the rim of Bull Canyon. Well ... actually they didn't. There was no Bull Canyon 157 million years ago. Instead this was a broad coastal plain, where large bipeds could cruise along at 2.5–3.5 mph (Hunt-Foster 2016).

The Bull Canyon Tracksite includes at least 50 well-preserved large theropod tracks, to 18 x 14 inches in size. But aside from footprints little is known about these creatures, for no bones have been found. So rather than naming a species, paleontologists named their tracks: Megalosauripus. These are ichnofossils—"a fossil record of biological activity by lifeforms but not the preserved remains of the organism itself." They're also called trace fossils, the term I learned.

 Theropods passed this way, in a pack perhaps.

They were big! (40-pound dog for scale).

Megalosauripus is a theropod track, not the theropod itself.

The wet sand where theropods once walked is now sandstone, part of the Moab Member of the Curtis Formation, dating from 157 million years ago (Late Jurassic). The setting was dynamic—changing sea level, oscillating shoreline, occasional sand dunes—making classification and dating of rock units difficult (Mathis 2021). But no matter. Whatever geologists decide to call the rock, its theropod tracks go on and on and on. They occur across Arches National Park, east to the Bull Canyon area and the Colorado–Utah state line, and perhaps as far south as Blanding. This is the Moab Megatracksite, also known as the Dinosaur Freeway. A conservative estimate of its size is 700 square miles; as of 2016, c. 3000 tracks had been reported from 30 sites (Hunt-Foster 2016).

Dinosaur Stomping Grounds, aka Jurassic Dancefloor, with at least 2000 theropod tracks (Sierra Club).

Our visit to the Bull Canyon Tracksite last fall was but a brief introduction. Many more opportunities to commune with large extinct birds await. Fortunately many of the sites are on public land, and there's a handy guide available (Hunt-Foster 2016). We shall return!

Dreaming of giant birds after a day in the field.

Notes

(1) The title of this post comes from a poem by Henry Wadsworth Longfellow—To the Driving Cloud. He referred to fossil bird tracks in several poems (Dean 1969).

(2) Edward Hitchcock was a cleric and amateur geologist. In his time, so-called amateurs made major contributions, his study of fossil birds being a good example. Charles Lyell confirmed Hitchcock's findings, and included them in lectures and later editions of Principles of Geology. Louis Agassiz and others also spread the word (more in Dean 1969).

(3) Are you wondering, as I did, why birds are NOT included in the seemingly eponymous Ornithischia (lower right in diagram)? That group includes dinosaurs with hips that superficially resemble those of birds. Maybe the name predates the realization that birds evolved from a theropod.

Sources (in addition to links in post)

Dean, DR. 1969. Hitchcock's Dinosaur Tracks. American Quarterly 21:639–644. https://www.jstor.org/stable/2711940

Hitchcock, E. 1836. Ornithichnology—description of the footmarks of birds, (Ornithichnites) on New Red Sandstone in Massachusetts. American Journal of Science and Arts, XXIX:307-40. Internet Archive.

Hunt-Foster, RK, et al. 2016. Tracking Dinosaurs in BLM Canyon Country, Utah. Utah Geological Association, Geology of the Intermountain West, Vol. 3. PDF

Mathis, A. 2021. Moab, Goblin Valley, and the Curtis Formation. Moab Happenings Archive.

The Monthly Fern: Water Clover & its odd spores

"It is worth clarifying that these plants are not clovers." (Photo by Bill Dodd).

For July, South Dakota's fern-of-the-month is the Hairy Water Clover, Marsilea vestita. Among our ferns it's quite the oddball—in habitat, behavior, leaves, and especially the spores. They suggest that one of its ancestors was the evolutionary start of seed plants!

Water Clovers grow nearly worldwide. On the order of 45 to 65 species are recognized (experts disagree on number), of which 5 are native to North America. M. vestita, the Hairy Water Clover, is the only species in South Dakota (so far). It's known from many sites across the state, in shallow water and on mud. Plants are rooted (not free-floating) and tolerate seasonably dry conditions, for example persisting after a pond has dried up. In fact, wet followed by dry can aid in reproduction and dispersal (Montana Field Guide).

Marsilea vestita is rhizomatous and colonial; in wet habitat, leaflets usually are horizontal (MWI).

M. vestita in a field; in drier habitat, leaflets often are ascending (Mary Ellen (Mel) Harte photo).

Hairy Water Clover can grow as tall as 20 cm on moist soil, or to 40 cm in water (in order to reach the surface). Leaves are dimorphic—sterile and fertile—but neither is fern-like. Sterile leaves have blades with four rounded triangular leaflets, and look a lot like four-leaf clovers! No other plant can be confused with Marsilea (1). 

Sterile leaves of Hairy Water Clover have divided blades on long slender stalks (MWI).

The sterile leaves also are unusual in behavior. Water Clovers are the only ferns known to be nyctinastic—moving with the onset of darkness. During the day leaf segments are nearly horizontal. Then as the sun sets they bend upward, forming a packet of sorts (Montana Field Guide has an account of this and other interesting features of Water Clover).

Fertile "leaves" are located near the base of sterile leaf stalks. In shape and size they resemble beans or peppercorns (source of another common name, Pepperwort). Being unusual they of course have a special name—sporocarp (= spore body); oldtimers like Linnaeus called them capsules.

Marsilea vestita. The hairy sporocarps contain 2 kinds of spores (lower right). Britton & Brown 1913.

Young sporocarps are greenish, hairy and slightly soft. With maturity they dry out, darken and become very hard. In this state they can survive for many years; the record is said to be 100. The Marsilea sporocarp is an effective unit of dispersal, often by way of waterfowl digestive tracts.

Like almost all ferns, Water Clovers have sori—clusters of sporangia which contain the spores. But the arrangement is quite different. In a typical fern, sori are located on the underside of leaves. In Marsilea, they're neatly arranged inside the sporocarp.

A typical fern; sori are clusters of sporangia, which contain dust-sized spores (2). USDA Forest Service.

Marsilea sporocarp with sori; below it, a sorus with sporangia, which release spores (no source given).

A cross-section through a Marsilea sporocarp (above) reveals an orderly but complicated interior. Inside the container-like sori are 2 kinds of sporangia. This is where things get exciting. They produce 2 kinds of spores—male and female!

Most ferns, 99% in fact, release a single type of spore—tiny, 1-celled, asexual. But not Water Clovers. They're among the 60 fern species (out of c. 10,500 total) with male microspores and female megaspores. These are heterosporous ferns; interestingly, all are aquatic (more here).

When a Marsilea microspore bursts open, many sperm (aka spermatozoids) swim off in search of an egg to fertilize—not unlike sperm of typical ferns. It's the megaspores that are so unusual. Not only are they 10 times the size of a typical fern spore, they're complex, with specialized parts.

Marsilea megaspore, c. 0.8 mm long, with 2 cells (no source given).

Shortly after leaving its sporangium, the megaspore divides to become two joined cells. The upper cell will produce an egg, which gives off chemicals to attract sperm. If a sperm successfully wriggles through the opening and reaches the egg, fertilization takes place, leading to development of an embryo and then a baby fern.

In comparison, the megaspore's basal cell is huge, and rich in carbohydrates and fats. These will sustain the developing fernling in its first days, before it can photosynthesize. In this way, a megaspore is like a seed, which supplies nutrients for its young seedling. Perhaps an ancestor of Marsilea was the evolutionary beginning of seed plants (more here).

Some readers may be wondering where the gametophytes are—those tiny independent plantlets that are the sexual stage of ferns. Good question! Water Clovers do have gametophytes, but they are minute and NOT independent (another similarity to seed plants). For more about the life cycle of heterosporous ferns, see (3) in Notes.

Simplified?

Now we finally arrive at the long-promised answer to the burning question, "How many spores would fit in a typical [empty] can of soda?"

Fern spores are truly tiny. A handful looks like a pile of dust. To show just how small they are, Robbin Moran (2021) calculated the number of average-sized spores that would fit in a typical can of soda, which has a capacity of 355 milliliters. For spore volume, he used 125,000 µm3, assuming for simplicity that a spore is a cube. What do you think? How about a ballpark estimate?

Hmmm ... 777,000?

Maybe 10 million?

According to Robbin's calculations the answer is 4,440,000,000 (4.4 billion). Yikes, that's a lot! Yes, spores are tiny indeed (4).

Notes

(1) While Water Clovers are easily recognized, distinguishing the Hairy Water Clover from others in the genus is not easy, requiring sporocarps. If you intend to document an occurrence, be sure to collect both types of leaves. See Marsilea in Flora of North America for a species key and descriptions.

(2) It seems sporangia are readily mistaken for spores, as a search for images of "fern spores" suggests. Many of the images actually are sporangia clustered in sori.

(3) In the previous Monthly Fern, I made a big deal out of the 2-stage life cycle of ferns, which involves separate tiny green sexual plantlets—gametophytes—that give birth to baby ferns. Heterosporous ferns have gametophytes, but they are minute and not independent. They develop inside the persistent spore wall, where they give rise to either sperm-producing antheridia or egg-producing archegonia (gametophytes of typical ferns usually have both). For the female gametophyte, developing inside the spore wall provides additional protection for the embryo, but there's no access to sunlight to photosynthesize food for the fernling. That's why the nutrient-rich basal cell is so important.

Life cycle of Marsilea, a heterosporous fern (labels added). See Milne Publishing for details.

(4) I couldn't find Robbin Moran's article online. If you'd like to read more about fern spores, and all of Robbin's calculations and conclusions (e.g. 4.44 billion spores taken together has a surface area nearly equal to 8.5 ping-pong tables), send me an email address and I'll send you a PDF file.

Sources (in addition to links in post)

Hooker, WJ, and Greville, RK. 1831. Figures and descriptions of ferns, principally of such as have been altogether unnoticed by botanists, or as have not yet been correctly figured. Vol. 2. BHL

Milne Publishing. Marsilea. Accessed June 2025.

Montana Field Guide. Montana Natural Heritage Program. Hairy Water Fern—Marsilea vestita.

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Moran, R. 2021. Fern Spores, Soda Cans, and Ping-Pong Tables. Fiddlehead Forum (May–Dec).

Pinson, J. About Ferns, American Fern Society.

PremaBotany (Prema Iswary). December 2018. Marsilea.

A Restless Region on the Colorado Plateau

Castle Valley, 20 miles east of Moab, UT; Round Mountain rises from the valley floor on the right.

The Colorado Plateau is a thick block of crust in the Four Corners area of the American Southwest, an immense stack of sedimentary rock. It's remarkably stable, remaining a region of geological calm even when severe deformation was underway right next to it—uplift and faulting of the Rocky Mountains to the east, and stretching and breaking of the Basin and Range Province to the west. This is why the Plateau's sedimentary rocks are largely horizontal, like the deposits they once were (source).

This is not to say the landscapes are boring. In fact they're spectacular—sweeping vistas with colorful roughhewn features. For the last 10 million years the Plateau has been rising, invigorating streams and accelerating erosion. The result is a seemingly endless collection of buttes, arches, rimrock, fantastical spires, deep winding canyons, and more.

The Colorado Plateau covers c. 130,000 sq mi; note complex topography in adjacent areas (source unknown).

Valley of the Gods near Bluff, UT; vertical and horizontal erosional features are common on the Colorado Plateau.

Entrenched meander of the San Juan River, cut through horizontal strata; Goosenecks State Park.

Looking down Castle Valley. Is this a standard Colorado Plateau landscape? Bryant Olsen photo.

Castle Valley appears to be dominated by vertical and horizontal features, as is typical on the Colorado Plateau. Is it another example of recent uplift and erosion? Only partly. Its story is much more complicated—repeated flooding, prolonged deposition, unusual deformation, weird intrusions, and finally ... collapse.

About three hundred million years ago, not far east of today's Castle Valley, the great Uncompahgre Range was rising. At the same time the Paradox Basin was subsiding along the range's base, and filling with sediments eroded off the mountains. Critical to the Castle Valley story, seas repeatedly flooded the Basin (1). Then whenever sea level dropped, the saltwater left behind evaporated and deposited evaporites, including lots of salt (halite). At least 29 such cycles took place over a period of at least 8 million years. The result was extensive deposits of evaporites at least 4000 ft thick—the Paradox Formation (2).

Ancestral Rocky Mountains c. 300 million years ago; darker blobs are major ranges; Castle Valley location approximate (modified from Soreghan et al. 2009).

Being evanescent creatures, it's difficult for us to think of geologic structures as ephemeral. But the rock record clearly shows that they are. Even mountains have lifetimes. The great Uncompahgre Range is now gone, razed by erosion. The Paradox Basin also disappeared, filled to overflowing with sediments and then deeply buried. But in a sense both are still with us. Uncompahgre sediments are widely displayed in colorful rocks across the Colorado Plateau. And the Paradox Basin maintains a ghostly presence—dramatic, but difficult to explain.

After deposition of the last Paradox evaporites, the region was inundated by tropical seas—source of the impressive layers of limestone, sandstone, siltstone and shale that line Plateau drainages. Then about 200 million years ago, when the supercontinent Pangaea started to come apart, there was a shift to terrestrial deposits—dune sand, volcanic ash, and sediments from rivers, lakes, and inland seas. Under this immense "lithic layer cake" lay the Paradox salt, deeply interred but not dead (source).

Salt is a strange kind of sedimentary rock. Sediments such as sand and mud can be compressed to form dense rocks, but salt remains nearly unchanged under pressure. It's weaker and less dense than the rock around it, but also plastic—with properties of both solids and liquids. It can flow to escape from its "stressful surroundings", deforming any rocks in its way (source). Deformation can occur at the scale of landscapes, and the old Paradox Basin has many fine examples: fins and arches of the Fiery Furnace, closely-spaced meanders on the Colorado River, and at least seven parallel northwest-trending valleys, including Castle Valley.

Dotted elongate blobs are parallel northwest-trending valleys. Blue circle marks intriguing overlap of Castle and Spanish Valleys with La Sal Mountains (3). Modified from Doelling 1985.

Castle Valley begins at the base of the northern La Sal Mountains and extends northwest about 12 miles. It's a broad valley, to 2 miles wide. The southwest wall is capped by a nearly continuous outcrop of the erosion-resistant Wingate Sandstone, known as Porcupine Rim. On the other side of the valley, the wall is less continuous but equally dramatic—carved into mesas, buttes, rimrock and spires.

Porcupine Rim—Jurassic Wingate Sandstone caps southwest wall of Castle Valley.

Northeast side of Castle Valley; strata tilting away from valley center visible at arrow.

Geologists find Castle Valley intriguing. The floor is much broader than would be expected for its little streams, and quite flat. Rock layers on both sides of the valley tilt down away from the valley center (more easily seen on the northeast side, photo above). Most exciting is what lies beneath the surface. Wells drilled in the center of the valley revealed a long steep-sided bed of salt to 1000 feet thick! Castle Valley must be a salt anticline, an elongate convex uplifted fold cored by salt. On this geologists agree. But as to how it formed and what happened to it ... that's another matter.

Castle Valley (Google Earth). But where's the anticline?! Valley walls hint at what happened.

The diagram below shows a common explanation for salt anticlines. In the top panel, flowing salt accumulates to form a convex fold, pushing up overlying rock layers. Castle Valley salt is thought to have flowed and formed an anticline 300 to 200 million years ago (Ornduff 2006, Trudgill 2011).

Salt anticline in cross section; at the time of the top panel, Castle "Valley" would have been a long ridge.

Now the Castle Valley anticline is mostly gone. The second and third panels in the diagram show a possible demise, but first, a major disturbance very close by needs to be considered—uplift of the La Sal Mountains about 28–25 million years ago (3).

The La Sals are not a mountain range but rather clustered peaks. They're similar to volcanoes except that magma never reached the surface. Pioneering geologist AC Peale called them "eruptive mountains of a peculiar type ... igneous and yet non-volcanic". Recent studies indicate that magma stopped just 1–3 miles below the surface, making them shallow intrusions, specifically laccoliths.

La Sal Mountains rise 8000+ feet above the Colorado Plateau—a major disturbance! (source)

La Sal high country: La Sal Peak (right) is intruded trachyte; Castle Mountain (left) is still capped with sedimentary rock (Ross 1998).

Now we're faced with another question. If magma never reached the surface, why are the La Sal "intrusions" visible? Instead of being 1–3 miles below the surface, their tops stand over a mile above the Plateau. The likely answer is the recent uplift and erosion of the Colorado Plateau mentioned at the beginning of this post.

Starting about ten million years ago, both the Colorado Plateau and the Basin and Range Province (to the west) have been rising. But while the latter was stretched and faulted, forming its eponymous basins and ranges, the Plateau remained a single block. Eventually it rose about kilometer higher than the Basin and Range. Why? That's a puzzle not yet solved (source). In any case, streams were steepened and invigorated, and erosion sped up enough to reveal the La Sal laccoliths.

Recent uplift and erosion probably explain the demise of the Castle Valley anticline as well. Erosion and/or faulting of overlying rock would have exposed the salt to water. Being salt, it of course dissolved. When enough was removed, rock layers at the crest fractured and collapsed, creating a breached anticline. But others think differently. Regional extension may have been the cause, perhaps related to ongoing extension in the Basin and Range Province. Or as Naqi et al. (2016) safely concluded, "formation of the salt valleys might be attributed to multiple factors (i.e., extensional forces, salt dissolution, and internal salt flow) rather than a single mechanism."

A salt anticline's demise may start with salt dissolution, followed by collapse of rock layers at the crest.

Breached anticline; dashed line shows former continuity across crest (Grabau 1920, A Textbook of Geology).

Let's visit!

Castle Valley is a great destination for geotrippers. Enough remains of the anticline to see and appreciate what happened. The tilted rock layers of the flanks are now the valley walls. Imagine them reaching higher and arching across the broad floor. Consider the depth of the valley below the now-imaginary crest and think about how much salt and rock must have been removed! Then look toward the head of the valley, at the dark hill rising from the floor. That's Round Mountain—a little relative of the La Sal intrusions. It was exposed when the anticline was breached and deeply eroded.

Castle Creek Road (paved) runs the length of the valley, providing easy access. Spires, buttes and mesas on the northeast side can be reached from several parking areas. Round Mountain is a short distance south of Castle Valley Road via a rough eroded 2-track; I parked just off the paved road and walked. Tour the southwest side via the Porcupine Rim trail—highly recommended, though maybe not on weekends and holidays.

Castleton Tower is a short hike from Castle Valley Road.

La Sals on left, Round Mountain on right, rabbitbrush in foreground.

En route to Round Mountain, Porcupine Rim beyond.

Looking up Castle Valley from Porcupine Rim, La Sal Mountains on horizon; redrockrubi.

Refreshing shade, courtesy uplift and erosion of the Colorado Plateau.

Notes in addition to links in post

(1) Cyclicity of Paradox deposition is well documented, but the cause is debated. Glaciation seems to be most popular, specifically sea level change with alternating glacial and interglacial periods. Other possibilities include rise of the Uncompahgre Range and climate change. Trudgill (2011) concluded that glaciation-driven changes in sea level was the main cause; tectonics and/or climate change may have made lesser contributions.

(2) In my reading, I found a range of estimates for Paradox deposition: 29 or 33 cycles over 8 to 15 million years, producing evaporites 4000, 6000 or 8000 ft thick (Ornduff et al. 2006, Trudgill 2011, USGS).

(3) Geologists have long wondered whether the La Sal Mountain intrusions and salt anticlines such as Castle Valley are related. Thomas Harrison, who surveyed the Paradox Basin area in 1926, discussed the possibility in his report (1927):

"It is interesting to know that igneous intrusive rocks of very considerable importance are closely associated with the saline anticlines. The laccolithic La Salle Mountains occupy an area on and between two parallel anticlines ... A small isolated igneous stock [Round Mountain] surrounded by gypsum occurs in the Castle Valley salt [anticline]."

Harrison noted that salt anticlines were thought to be associated with lines of weakness dating from Precambrian time. Their uplift was followed by subsidence, and massive accumulation of sediments. Perhaps this "heavy load" generated heat that "liquefied rocks within the zone of fracture, resulting in the [magma] which formed the laccolith, and the Castle Valley stock" [Round Mountain].

Today's geologists may chuckle at the idea of a "heavy load" of sediments melting igneous rock below. In contrast, "lines of weakness dating from Precambrian time" are taken seriously. Many northwest-trending faults cut basement rocks in the Paradox Basin area. Ross (1998) concluded that "the locations of the La Sal Mountains intrusive centers along the trend of subsurface faults ... suggest that the faults were avenues of weakness for the ascent of magma in the upper crust. This is especially true for the northern and southern clusters of peaks [which coincide with salt anticlines]".

Sources

Doelling, HH. 1985. Geology of Arches National Park. Utah Geological Survey, to accompany Map 74. PDF

Harrison, TS. 1927. Colorado–Utah Salt Domes. Am. Assoc. Petroleum Geologists 11:111–133.

Ornduff, RL, Wieder, RW, Futey, DG. 2006. Geology Underfoot in Southern Utah. Mountain Press Publishing. For salt anticlines see Vignette 29, A Sea of Fins; for La Sal Mountains see Vignette 32, Intruders in a Sedimentary Domain.

Ross, ML. 1998. Geology of the Tertiary intrusive centers of the La Sal Mountains, Utah; influence of preexisting structural features on emplacement and morphology, in Laccolith complexes of southeastern Utah; time of emplacement and tectonic setting. USGS Bull. 2158: 61-83. PDF

Snyder, NP. 1996. Recharge area and water quality of the valley-fill aquifer; Castle Valley, Grand County, Utah. Report of Investigation 229. Utah Geological Survey. PDF

Soreghan, GS, et al. 2009. Hot fan or cold outwash? Hypothesized proglacial deposition in the upper Paleozoic Cutler Formation, western tropical Pangea. J. Sed. Res. 79:495-522.

Trudgill, BD. 2011. Evolution of salt structures in the northern Paradox Basin: controls on evaporite deposition, salt wall growth and supra-salt stratigraphic architecture. Basin Research 23:208–238. https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2117.2010.00478.x

US Geologic Survey. Mid 2000s. Geologic Provinces of the United States: Colorado Plateau Province. Internet Archive WayBackMachine.

Monthly Fern: polypodies & fern sex (or do they?)

Left, Polypodium saximontanum, leaves to 25 cm long (Matt Berger); right, P. virginianum, leaves to 40 cm long.

For May, the South Dakota "Monthly Fern" series features polypodies—Polypodium saximontanum and P. virginianum. The genus name comes from the Greek "poly" meaning many and "podion" meaning little foot, referring to the bumps (old leaf bases) on the creeping stems. Polypodies occur worldwide, but are more common in the Northern Hemisphere. About 100 species are recognized, with 11 in North America. They grow mostly on rock (source).

Creeping stem (rhizome) of Common Polypody; bumps on right are old leaf bases. © 2007 Robbin Moran.

South Dakota's two polypodies have a notable distribution—one on each side of the state. P. saximontanum, Rocky Mountain Polypody, grows on granite outcrops in the Black Hills, in the far west. P. virginianum, Common Polypody, is restricted to a small area of Sioux quartzite, in the far east very close to Minnesota (where it's common). It's a good thing they live 350 miles far apart. They're very similar and would be difficult to distinguish if their ranges overlapped.

South Dakota polypodies: Rocky Mountain Polypody in Black Hills (green dot); Common Polypody in Minnehaha County (pink dot); SEINet search May 2025.

Rocky Mountain Polypody on granite, Black Hills, SD (JD McCoy).

Common Polypody on Sioux quartzite, Palisades State Park, SD.

Both species have evergreen leathery deeply-lobed leaves with straw-colored stems. Common Polypody leaves tend to be longer and wider than those of Rocky Mountain Polypody (see first photo). No other clear differences in vegetative characters were found in published descriptions.

As for reproductive structures—which are critical for fern id as we've been told repeatedly—the South Dakota polypodies again are very similar. Both have round sori (spore clusters) arranged in two rows on the underside of leaf lobes; indusia (covers) are absent. The spores are yellow, so much so that even though they're housed in brownish sporangia, they give the sori a yellowish cast.

Common Polypody, P. virginianum (MWI).

Rocky Mountain Polypody, P. saximontanum (Kelly Fuerstenberg).

It is possible to distinguish Common and Rocky Mountain polypodies based on their sori, but it isn't easy. Both have sporangiasters—tiny transparent jelly-like blobs separating the sporangia (1). In P. virginianum, most sporangiasters have gland-tipped hairs, while in P. saximontana, gland-tipped hairs are few or absent. Be forewarned—sporangiasters are said to be so small that one needs macro photos or a good hand lens to examine them.

Polypodium virginianum sori, with sporangiasters with gland-tipped hairs. © 2007 Robbin Moran (arrows added).

Sporangiasters with and without glandular hairs, Polypodium amorphum; very helpful photo by James Thomas.

While we're on the subject of "reproductive" structures, let's address a common misconception (fern buffs excepted). Strictly speaking sori, sporangia and spores are not reproductive structures, for ferns cannot reproduce themselves directly. Instead, their spores give rise to plants quite unlike the parent fern.

This leads us to the fern life cycle, the so-called bugbear of beginning botany students. But we can dispense with complicated details and off-putting terms and still understand and appreciate the curious life of ferns. They and their relatives the lycophytes (formerly "fern allies") are the only land plants that exist as two different free-living beings—kinda like a butterfly and its caterpillar (2).

I find it helpful to first think about flowering plants (angiosperms) with their familiar sex organs. Flowers have eggs in ovaries and sperm in pollen. When a cell of each joins in fertilization, the result is a seed. If conditions are right, the seed germinates and grows into a plant like its parent.

Angiosperm life cycle; note the single free-living being—the plant (modified from source).

But ferns are different. Instead of seeds, they produce millions of tiny asexual spores. If conditions are right, a spore germinates and grows into a minute green plant very different from its parent, even though they have the same DNA. I find this so cool to think about! Unfortunately there seems to be no user-friendly term for these little beings, only "gametophyte" or "prothallus".

Gametophyte of Polypodium vulgare (light microscope at x4 magnification); Viséan.

John Lindsay, a British surgeon working in Jamaica, was the first to describe fern gametophytes (1794), though he didn't call them that and didn't fully understand what they were. Hoping to figure out how ferns reproduce, he had sprinkled "dust" from a fern leaf (today's spores) on dirt in a flowerpot. "I placed the pot in a window of my room, watered it daily, and every day or two examined a small portion of the [dirt] by the microscope ... but observed no alteration till about about the 12th day after sowing." At that point the soil began to turn green "as if it were covered with some small moss".

Lindsay made a nice drawing showing the stages of fern development he observed (full illustration here).

Excerpt from Lindsay's drawing: top, germination; lower right, tiny scales; lower left, first fern leaves.

With the microscope Lindsay could see particles of dust germinating—"pushing out their little germ, like a small protuberance, the rudiment of the new fern" (8–11 above). After a few more weeks the "moss" had grown enough to be visible to the naked eye, looking like small scales (13). These grew to be roundish and bilobate, similar to liverworts (14). Finally a tiny leaf emerged from the scale (15), followed by larger ones (16) until there was a fern like the one that produced the dust. Understandably, Lindsay concluded the dust was fern seed.

It wasn't until the 1840s that botanists finally got rid of fern seed. It had become obvious that despite their alluring beauty, ferns are not sexual creatures. That honor belongs to their gametophytes.

A typical gametophyte, with antheridia and archegonia (source).

On the underside of a gametophyte are little bumps that come alive in the presence of water. Some release wriggling spiral filaments that swim away. Others open to receive a spiral filament if one happens by. These are sex organs: male antheridia release wriggling sperm, and female archegonia each contain an egg. If a sperm wriggles down the neck of an archegonium, it arrives at a large cell—an egg. Fertilization produces a zygote, which develops into a baby fern growing out of the gametophyte (15 in Lindsay's drawing above). If conditions are right, it will become a full-sized fern, thereby completing a life cycle.

Here's the life cycle of a fern, emphasizing the two independent free-living beings that make it so cool! (3)

Fern life cycle—green fern (aka sporophyte) and brown gametophyte (Sigel et al. 2018, much modified).

Once again I'm ending a post without addressing a promised topic. With ferns, it's too easy to go down a rabbit hole! So I will do another Monthly Fern for June—about "the burning question" of how many fern spores fit in a Coke can. What's your guess? Here's a hint: a typical soda can holds 0.355 liters (1.5 cups). And here's an average-sized spore:

Polypodium virginianum spore. Copyright © 2007 by Robbin Moran.

Notes

(1) Sporangiasters may help keep sporangia from drying out prematurely (source). Moran (2017) notes that "In immature sori they form a continuous, protective covering over the young sporangia, thus acting like an indusium."

(2) In thinking about ferns and gametophytes, butterflies and their caterpillars came to mind. In both cases, two forms are produced from the same DNA by using different genes. This is dramatic in butterflies and caterpillars, but not so much in ferns and their gametophytes. In fact, Sigel et al. (2018) found a nearly 90% overlap in genes expressed in Polypodium amorphum ferns and gametophytes. And there's an even bigger difference. Butterflies and caterpillars are both diploid (two sets of chromosomes); there is no independent haploid form that produces gametes—no equivalent of the fern gametophyte. So my comparison of butterflies and ferns was a stretch.

(3) Strictly speaking all land plants alternate between sporophyte and gametophyte life stages (spore- and gamete-producing). But only in ferns and lycophytes are both stages free‐living beings. In seed plants only the sporophyte is free-living; in mosses and liverworts, only the gametophyte is free-living. More here.

Sources in addition to links in post

Lindsay,  John. 1794. Account of the Germination and Raising of Ferns from the Seed. Trans. Linn. Soc, London 2:93–100. BHL.

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Moran, RC. 2017. Division Polypodiopsida, Ferns in New Manual of Vascular Plants of Northeastern United States and Adjacent Canada. NYBG Press Digital Content (not available as of June 2025, pers. comm.)

Rothfels, C. 2022. Fiddleheads: Fern life cycles and identification. Online workshop for Jepson Herbarium (videos).

Sigel, EM, et al. 2018. Overlapping patterns of gene expression between gametophyte and sporophyte phases in the fern Polypodium amorphum. Front. Plant Sci. 9:1450. FREE

USDA Forest Service. Fern Reproduction.