Why the Graboids from "Tremors" Totally Would Not Have Worked, and the One Thing They Got Right, Part II

We resume my review of the “graboids,” giant carnivorous worms from the 1990 cult classic “Tremors.” My previous post ended midway through a lengthy diatribe criticizing the flaws in their supposed fossorial lifestyle. I’m not quite finished with the criticism, but I do have a few good words to say about the monsters, and the movie as well. So, without further ado…

"Miss me?"

Problem #3: Internal rumblings

It is established fairly early in the movie that graboids are blind*, and detect and follow their prey via seismic waves (= vibrations) – entirely logical adaptations for a burrowing organism. While this detection is shown to be quite sensitive and spatially precise, it is not very discriminatory. Through the course of the movie, graboids are attracted to not only to prey, but to a hand shovel, pogo stick, tumbler, chest freezer, unmanned riding mower, running water, and possibly a jackhammer. 

*Strangely, later in the movie, the graboids supposedly learn that motor vehicles keep their prey safe, and manage to locate and disable a pickup truck and SUV, even though neither is running at the time….

 

But if this style of prey detection is true, graboids have either extremely bad luck or a terrible sense of direction. Nevada, where the movie takes place, is the third most seismically active state in the nation, just behind Alaska and California (you may have heard about earthquakes in those two…). If you look at the geologic map of Nevada to the top left (from here), you may notice that mountain ranges are arranged in parallel bands with an eerie resemblance to the stretch marks on a pregnant woman’s belly (bottom left; from here). This is not a coincidence – in CliffsNotes®-style plate tectonics, the plate being subducted under the west coast (the one ultimately responsible for the San Andreas Fault) is dragging and stretching the western United States along with it, expanding Nevada to several times its original width. Inherent to this is a lot of seismic activity. Graboids trying to pinpoint the footsteps of a puny human amidst all the seismic background noise is akin to trying to find a buzzing housefly in the middle of a dubstep concert.

Incidentally, the co-option of seismic waves by animals is not limited to science fiction. Several modern organisms are suspected of using seismic waves for communication, probably the most notable being the elephant. Elephants, both Asian and African, appear to generate and detect low-frequency waves for long-distance communication. Some paleontologists have also hypothesized that hadrosaurids (duckbilled dinosaurs) may have participated in similar communications, using their elaborate headgear as resonance chambers.

So what’s the good news?

Fortunately, the producers of “Tremors” did manage to include one impressively accurate feature on the graboids (possible unintentionally, but I’m willing to overlook that). In several scenes, graboids display prominent finger-like or fringe-like lateral projections. I assume these to be the gigantic equivalent of setae: miniature hair-like structures found on many organisms, including our invasive friend the earthworm. In earthworms, these structures are very small – even at the microscopic scale in the image below, you may still believe I’m pulling your leg when I say the structures really are there.

asknature.org/strategy/29204f007bb1d29b7c2f63a580fd7d4d

But they are present, and damned effective. Earthworms use them to “grip” the soil and assist with locomotion – an important trait for a cylindrical, slimy invertebrate. They also help the earthworm resist attempts to remove it from the ground - anyone who has ever tried to pull up earthworm probably learned that even if you have the grip of a professional free-climber, the best you will end up with is half an earthworm. Scale this trait up to a whale-sized worm, and very well-anchored organism would result. In fact, in a scene early in the movie, one of a graboid’s “tongues” manages to clamp onto the axle of a pickup. The pickup is able to get away after flooring the gas, but it only succeeds in pulling the “tongue” out by its base, leaving the rest of the graboid likely in exactly the same position it started, albeit in much pain. 

Unfortunately, the graboids in the movie never fully utilize their extreme stubbornness. Towards the end of the movie, the town’s survivors hatch a plan to escape on a bulldozer – reflecting on the graboids’ previous motor vehicle destruction, one of the characters says something along the lines of “[The bulldozer] weighs more than 30 tons. There's no way they could lift that!” Well, it turns out the graboids wouldn’t need to bother. If setae can allow an earthworm to resist something 100,000 times its size, they sure as hell can allow a graboid to immobilize a bulldozer. One graboid would simply need to bite down on the dozer and hold it place, then patiently wait for the trapped prey to “jump ship” (and the graboids are shown early in the film to do just that).

For the record, I don’t mean to sound too critical of the movie – as I said, it is one of my favorites. It effectively blends horror and comedy, portrayed Reba McEntire and the father from “Family Ties” as survivalist gun nuts, and accurately depicted “bromance” decades before Judd Apatow wasted film on nothing but two hours of hairy, pasty-white men waving their dongs at the camera to bad 80’s pop music. But if the producers or writers had consulted with a biologist (or, apparently, a bored paleontologist), they could have created a rare good science/cult classic combination, and saved the bandwidth needed for these last two blog posts.

Although, considering what most of the internet is used for,
perhaps that's not such a great loss after all...

The Awesomeness of Beecher's Trilobite Bed

Trilobites are remarkable animals, deserving not only multiple posts, but probably their own blog (if one exists, I haven’t found it yet…). They are one of the most abundant, well-known, and best-studied organisms from the fossil record. If anyone goes through a “rockhound” period in his or her life, it’s almost a guarantee they picked up a fossil trilobite. For those unfamiliar, trilobites kind of looked like a cross between a pill bug and cockroach (see below), but lived in the ocean. However, 99% of the time, it’s just the exoskeleton of trilobites that is preserved. For better preservation, there are two prime trilobite fossil beds in the world, the Hunsrück Shale of Germany, and Beecher’s Trilobite Bed in New York.

Beecher’s Trilobite Bed is the result of a turbidity flow (think an underwater “mudslide” of very muddy water, image at left from here) which picked up trilobites, carried them a short distance, and essentially buried them alive. For taphonomic reasons, this rapid burial exquisitely fossilized the trilobites as pyrite, fossilizing not only exoskeletons but legs, antennae, muscles, and even internal organs (image below from PDF here). For a more lengthy explanation of how Beecher’s Trilobite Bed was formed, there’s an excellent chapter by Etter (2002), and Cisne’s original taphonomic study from 1973 is now publicly available. But the Beecher bed is a definite taphonomic outlier for a couple reasons, both of which probably need a brief introduction:     

Trilobites were rebels before it was cool

First, in the ocean, only three things are needed to produce pyrite (iron sulfide: FeS₂): sulfur, iron, and organic carbon. Chemical studies of the Beecher bed (Briggs et al., 1991) suggest the seawater was already rich in sulfur and iron – all that was needed was organic carbon, readily provided by the washed-in trilobites. Due to the abundance of sulfur and iron, it is plausible that every trilobite (as in, 100%) washed in by the turbidity flow acted as a nucleation site for pyrite and ended up getting fossilized. This is highly unusual – preserving 10% of a paleo-environment would make a paleontologist drool (personal observation).

Second, pyrite can form two ways in the ocean: through “bacterial sulfate reduction” (BSR) or through diagenesis (after burial). BSR occurs in open seawater, while diagenesis occurs in the seabed sediment, with no contact with the seawater above. How the pyrite is derived affects the sulfur isotopes in the mineral: it is commonly thought that BSR-derived pyrite is enriched in “lighter” ³²S, while diagenetic pyrite is enriched in “heavier” ³⁴S. Understandably, soft parts (like legs and antennae) decompose faster than hard parts (like exoskeletons), so soft parts would need to fossilize first, right? Well, as it turns out, the legs and antennae of the Beecher trilobites tend to be enriched in the “heavier” ³⁴S, while the exoskeletons are enriched in the “lighter” ³²S (Briggs et al., 1991). This suggests the exoskeletons were fossilized first, before burial, and the softer parts stuck around and were fossilized after burial. This is kind of like a root beer float where the root beer evaporates before the ice cream even melts. Briggs et al hypothesized this could have occurred due to changes in water chemistry as the exoskeleton decomposed, but there could be a greater disturbance in the force...

Wait, what do you mean “it is commonly thought”?   

The paradigm on sulfur isotopes, as it turns out, was recently challenged by a publication in Geology led by Justin Ries, a marine geologist at The University of North Carolina (pictured in the field at left, from here). Ries and colleagues examined the sulfur isotopes in a 10 million-year stretch of carbonates from Namibia (Ries, 2009: PDF available at link above). They found that the pyrite derived by BSR was actually enriched in the heavier ³⁴S isotope, enough to be labeled “superheavy pyrite.” Needless to say, this throws a monkey wrench into the current notion that the bacterially-derived pyrite should be “lighter.”

The translation of this article towards the Beecher bed is uncertain – Ries et al attribute the anomalous pyrite isotopes to large-scale (possibly global) low atmospheric oxygen, but it is unlikely such conditions existed when the Beecher bed was deposited. They also consider the anomalous pyrite could have resulted from stratification of the water column, low water levels, or aerobic reoxidation (mixing oxygen back into the water). Personally, I could easily be convinced that turbidity flows, like the kind that produced the Beecher bed, could carry oxygenated water. But, to my knowledge, it’s uncertain how the isotopes of subsequent  diagenetic pyrite would be effected – i.e., could it end up being “lighter” than the bacterially-derived pyrite, or would it be “super-duper heavy”? Like any good scientific study, it creates more questions than it answers… 

 

REFS

Briggs, D.E.G., Bottrell, S.H., and Raiswell, R., 1991, Pyritization of soft-bodied fossils: Beecher’s Trilobite Bed, Upper Ordovician, New York State, Geology, v. 19, p. 1221–1224. 

Cisne, J.L., 1973, Beecher’s Trilobite Bed revisited: ecology of an Ordovician deepwater fauna, Postilla, v. 160, p. 1–25.

Etter, W., 2002, Beecher’s Trilobite Bed: Ordovician pyritization for the other half of the trilobite, in Bottjer, D.J., Etter, W., Hagadorn, J.W., and Tang, C.M., eds, Exceptional Fossil Preservation: a Unique View on the Evolution of Marine Life, Columbia University Press, New York, p. 131–142.

Ries, J.B., Fike, D.A., Pratt, L.M., Lyons, T.W., and Grotzinger, J.P., 2009, Superheavy pyrite (δ³⁴S_pyr > δ³⁴S_CAS) in the terminal Proterozoic Nama Group, southern Namibia: a consequence of low seawater sulfate at the dawn of animal life, Geology, v. 37, p. 743–746.