Monday, April 26, 2010

Predators and Prey – Seeing Out of the Corner of Your Eye

This is a posting about predators and prey, and the ingenuity of scientists in picturing the unobservable.

One prompt for this posting came the other day when, as I sat at my desk, dogs outside began an insistent barking. This was one of those moments when I am reminded that, even in this paved over, abused urban landscape, we are part of the ebb and flow of nature. My little schnauzer, German-bred to hunt rats and other vermin, was wonderfully excited, throwing sharp barks up past the back gate. My neighbor’s basset hound, bred by the French (French-bred?) to track down small game such as rabbits, stood in her yard baying loudly.

Cause of the commotion? Nestled in the grass in front of a stack of firewood on a slope overlooking the backyards sat a red fox, a member of the Canidae family which includes those domesticated, yammering canines.

The appearance of this demure creature explains those sudden bursts of barking that had punctuated the quiet of the neighborhood in the evenings and early mornings for several weeks. The interloper, probably pushed out of a more secure environment, was leading a life uncomfortably near to humans, houses, and those dogs. The fox retained her cool. Bemused by the racket she was causing, she yawned a couple of times, exposing her long white canine teeth, sat back and stared for awhile. Then, tiring of this and having the somewhat more important job of survival to attend to, she briefly scratched under her chin and faded away. The dogs continued their noise.

Three relatively closely related predators in close proximity for a moment – reminder that it’s a dog eat dog world, or, as William Buckland (grand subject of my previous posting) put it, “The stomach, sir, rules the world. The great ones eat the less, and the less the lesser still.”


Several weeks ago, I commented about the demise of a fossil collection as an intact collection and the spread of its specimens to other collections, including mine. A couple of microfossils that came my way puzzled me. Embedded in a piece of shale were conodont fossils, something unknown to me. That’s embarrassing to admit because conodonts are incredibly common. They are nearly ubiquitous in marine sedimentary beds from the Cambrian to the Triassic, and have a “fossil record . . . generally held to be among the best of any group of organisms.” (Mark A. Purnell and Philip C.J. Donoghue, Between Death and Data: Biases in Interpretation of the Fossil Record of Conodonts, Special Papers in Palaeontology, Volume 73, 2005, p. 7. Available among the publications posted at Purnell’s website) Yes, conodont fossils are nearly everywhere. Here’s one of those new to my collection (their small size makes them a photographic challenge particularly with a modest digital camera).

They come in many different shapes as shown in the picture below taken by the late Charles Drewes, biologist at Iowa State University and included on his website.

Following their discovery in 1856 by the Russian biologist and embryologist Heinz Christian Pander (or, as it is sometimes recorded, Christian Heinrich Pander), conodonts were the focus of a prolonged debate – what were they? animal or plant? the whole organism or part of it?

With the later discovery of a few fossils including the soft parts of the conodont organism, it became clear that the hard parts we almost always see in isolation are fossilized mouthparts. The conodont is now known to have been a primitive vertebrate, one of the Agnatha (jawless fish) whose only extant representatives are hagfish and lampreys.

But, how did those mouthparts work? Of course, for these long gone creatures, as Mark Purnell of the University of Leicester observed, “direct observation of feeding is not possible.” As a result, saying something conclusive about the behavior of conodonts and how they used their body parts is difficult, as it is for other extinct organisms.

The advice in science for these situations appears to be: “If you cannot see it directly, try some indirection.”

In one of those nifty bits of cleverness that attracts me to the study of science and scientists at work, Purnell theorized that, if they were teeth used for crushing and shearing, there would be evidence of that, in the form of microscopic wear patterns on the fossils, similar to those well known from the teeth of mammals and certain reptiles. Absent those signs of wear, these mouthparts would likely have been a filtering device for bits of food. He looked closely and found wear patterns, ergo, teeth. And, from this, he concluded that the first vertebrates were predators . . . ah, Conodont the Predator, though what they preyed on remains an open question. (Mark A. Purnell, Microwear on conodont elements and macrophagy in the first vertebrates, Nature, Volume 374, April 27, 1995; and M.A. Purnell, Feeding in Conodonts and other Early Vertebrates, from Palaeobiology II, 2001, available at Purnell’s website.)

Crinoids and Echinoids

Another example of scientific resourcefulness in visualizing behavior in the distant past came my way recently in the form of a new article about crinoids. This also involved predators and their prey.

Crinoids can make beautiful fossils, sometimes appearing to be flowers fixed for an instant in the middle of a breeze. They are marine invertebrates commonly known as sea lilies (those with stems) or feather stars (stemless). Crinoids were nearly done in during the massive End-Permian extinction (about 251 million years ago, touched on in a previous posting), but recovered rapidly beginning in the Triassic, diversifying greatly, expanding into new territory, and developing new behaviors and physical attributes. Perhaps most dramatically, following the End-Permian extinction, many types of crinoids became mobile, “a trait not found among Paelozoic crinoids.” (Tomasz K. Baumiller, et al., Post-Paleozoic crinoid radiation in response to benthic predation preceded the Mesozoic marine revolution, Proceedings of the National Academy of Sciences, early edition, March 30, 2010, volume 107 (13).)

These changes in crinoids have been attributed by some scientists to the impact of predation, particularly by fish, which, it is argued, prompted crinoids to seek out safer environments, developing direct methods of escaping predators. Geologist and paleontologist Tomasz Baumiller and his colleagues theorized that, in addition to attacks by fish, predation by dwellers of the sea bottom might have also contributed significantly to these evolutionary developments in crinoids. They focused specifically on echinoids (sea urchins), animals that also were almost wiped out in the End-Permian extinction. Echinoids, like crinoids, bounced back, undergoing diversification in habitat and physical changes, including the strengthening of mouthparts.

To answer the question of whether predation by echinoids played a role in the evolution of the crinoid, particularly, the development of motility, Baumiller et al. were faced with that ever present problem of determining what happened in the very distant past. Once again, the ingenuity that scientists being to bear on addressing this challenge impressed me.

Baumiller et al. had living types of crinoids and echinoids to work with. Using live specimens in aquaria, they established that, indeed, echinoids do prey on crinoids, but, of course, that didn’t prove a predator-prey interaction between them in the Triassic. It did suggest it was possible, and, beyond that, offered a clever way to test whether that interaction had occurred. Baumiller et al. discovered that crinoid pieces that had been ingested by echinoids were scratched and pitted in very specific ways. They then looked for those characteristic marks in crinoid fossils from the Triassic, and they found them, in spades – 20% of 2,500 crinoid fossil fragments studied had those same pits and scratches. The research offered support of the proposition that bottom dwelling predators, such as echinoids, preyed on crinoids during the Triassic, spurring the development of mobility.

Concluding Thought

Scientists are powerfully creative in dealing with problems, or visualizing events, that cannot be approached directly. Evolutionary biologist Geerat Vermeij identified three ways to determine predation from the fossil record, the last two decidedly indirect – (1) luck out and find fossils showing the predation occurring, if not freezing the capture in time, at least showing the remains of the prey in the predator’s gut; (2) find evidence on prey fossils of predator action; or (3) infer the behavior from predator morphology. (As described by E.M. Harper in Dissecting post-Paleozoic arms races, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 232, 2006, p. 324.)

The two examples described in this posting reflect Vermeij’s indirect methods, exhibiting a degree of artistry I find impressive, and, in my mind, adding an extra dimension to the effort. I may particularly appreciate them because I prefer the indirect or oblique approach to almost everything. Approaching your prey from an unanticipated direction means sometimes succeeding unexpectedly or capturing something unexpected.

I am fond of the poem Bach, Winter by Jane Mead, which suggests something hidden (in this case, music) may be lost if you come at it head on. Here are the poem’s opening verses.

Bach must have known – how
something flutters away when you turn
to face the face you caught sideways
in a mirror, in a hall, at dusk –

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