Crusher Teeth, Spines, and the Fall of Trilobites

If you have food in your jaws you have solved all questions for the time being. —Franz Kafka, Investigations of a Dog

Paladin transilis, Upper Carboniferous, Zhirnovsk, Volgograd region, Russia
Paladin transilis, Upper Carboniferous, Zhirnovsk, Volgograd region, Russia. This beautiful trilobite had extremely well developed genal spines for its time. Many late Paleozoic trilobites generally resembled Paladin overall, but had small genal spines or no spines at all. Specimen is about 1.5 cm across genals.

The decline of trilobites accompanies the expansion of gnathostomes (jawed vertebrates). Family-level trilobite diversity nearly held steady with only minor decline throughout the Silurian Period just as gnathostomes began their evolutionary radiation. Trilobite diversity then underwent a series of significant step-wise declines throughout the Devonian Period. This was a time of major expansion for vertebrates, including those with “crusher-type” dentitions, the most likely trilobite hunters. Only four trilobite families survived into the Carboniferous Period.

These crusher-teeth occurred in many common groups of fishes of middle and late Paleozoic age, including bony fishes (Osteichthyes), placoderms, and shark-like fishes (Chrondrichthyes, especially holocephalians). Many fishes with  crusher-teeth likely preyed largely on hard-shelled invertebrates as they do today. It seems plausible, then, that these predators exerted selective pressure on trilobites. It is also therefore reasonable to believe, as some do, that vertebrates played a role in the decline and ultimate extinction of trilobites.

Syntheotodus trisulcatus, Maple Hill Formation, Upper Devonian, Kalona, Iowa
Syntheotodus trisulcatus crusher tooth in occlusal view, Maple Hill Formation, Upper Devonian, Kalona, Iowa. This is an early holocephalian tooth. Holocephalians were likely a much more diverse and widespread group in the middle and late Paleozoic than they are today. Extant holocephalians, the 39 species of chimaeras and ratfishes, are mostly deep water forms that feed on hard-shelled benthic invertebrates. Specimen is 8 mm across.

But correlation, of course, is no proof of causation, especially given the multitude of other changes that occurred during this interval of earth history. Some would even argue that:

A belief in the causal nexus is superstition.—Ludwig Wittgenstein

Lagarodus tooth plate, Harrodsburg Limestone Formation, Washington County, Indiana
Lagarodus sp. (Psammodontidae, Holocephali) tooth plate, Harrodsburg Limestone Formation, Mississippian Period, Washington County, Indiana. Such flat, plate-like crushing teeth would have made short work of a trilobite. Specimen is 1.6 cm across.

Philosophy aside, spines in trilobites are often interpreted to have had a defensive function as they do in many extant marine and aquatic forms. Some predatory fishes of insufficient size, for example, may have difficulty swallowing other fishes because the prey fish can splay out fin spines making passage down the gullet impossible. But spines are no guarantee of safety. Diving birds and waders, fish-eating specialists, can easily manipulate prey into a head-first orientation and eat the spiniest of fishes, even those of large relative size. For every measure, there is a countermeasure. This was likely as true in the Paleozoic as it is today.

The proliferation of dorsal spininess in Devonian trilobites may have been a response to threats from jawed fishes and other predators, ammonites, for example. In the case of soft-bodied predators this makes sense, but I’ve always been skeptical that spines could have been of much protection from vertebrate predators, especially specialized ones. Specialized vertebrate predators are often just too formidable for any invertebrate prey, not matter how thorny. Triggerfish, for example, bite off echinoid spines until the animal’s body is exposed and then eat the soft-tissues. Holocephalians graze on shellfish the way cows graze on grass, groupers grab crabs, and so on.

Further, an evolutionary arms race between spines and teeth would have clearly and immediately favored teeth. Vertebrate teeth are, after all, made of hard, phosphatic tissues and are inherently more than a match for the calcite of the trilobite exoskeleton, no matter how spiny. Trilobite spines, if they were defensive structures at all, were likely only effective against a specific, most likely unknown, soft-bodied threat.

A final observation indicating that spines may have had little to do with defense is that it is the dorsally spiny trilobites (like Comura) that disappear at the end of the Devonian Period. The trilobites that survived into the late Paleozoic, a time when waters teemed with the most menacing piscine predators of the era, were the most conservative forms. Many late Paleozoic trilobites even lacked genal spines.

The reason trilobites retreated into the shadows at the end of the middle Paleozoic and ultimately disappeared near the end of the Permian Period will likely never be completely understood. An analysis of spines versus predators or trilobite predation in general, although an attractive place to look for easy answers, seems unlikely to yield convincing answers about extinction.

Double-crested Cormorant with "Plecostomus, Fiorenza Park, Houston, Texas
Double-crested Cormorant with “Plecostomus” (Pterygoplichthys multiradiatus), Fiorenza Park, Houston, Texas. Loricariid catfish are covered in dermal armor and have formidable fin spines as wicked as any trilobite’s associated with their fins. Cormorants dispatch and eat them with ease.

©2017 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.

Evidence of Great Biocrises in the Field and the Drawer

Life on our planet has been a constant series of cataclysmic events, and we are more suitable for extinction than a trilobite or a reptile. So we will vanish. There’s no doubt in my heart. –Werner Herzog

Griffithidella doris, Lake Valley Formation, New Mexico
Griffithidella doris, Lake Valley Formation, Mississippian Period, New Mexico. A series of biocrises in the Late Devonian Epoch whittled trilobite diversity down to only four families by the opening of the Carboniferous (Mississippian) Period. Smaller trilobite is 1.0 cm long.

Extinction must frequently be on the mind of many trilobite collectors. Every species of once-living thing in their cabinets has been extinct for hundreds of millions of years. What’s more, the diversity of specimens in those cabinets reflects the steady background rate of extinction as well as the mass extinction events sprinkled throughout the Paleozoic Era.

Most noticeable, perhaps, is how the Late Devonian events shape a fossil collection. The Lower Devonian drawers are chock full, the Upper Devonian drawers are sparse, and (unless you’ve made a concerted effort), the late Paleozoic drawers have ample space for additional specimens!

Other transitions are just about as apparent. Ordovician drawers stuffed with asaphids, for example, bear little resemblance to the Silurian drawers—no doubt reflecting the big trilobite die-off at the end of the Ordovician Period.

Megistaspidella triangularis, Kunda level, Ordovician Period, St. Petersburg region, Russia
Megistaspidella triangularis, Kunda level, Ordovician Period, St. Petersburg region, Russia. The Megistaspidae is a group containing large, spectacular shovel-nosed forms. Megistaspids are confined to Lower and Middle Ordovician rocks. Trilobite is about 13 cm long.

Each of the five major mass extinction events of the Phanerozoic, no doubt, looked different to the organisms experiencing them, from cataclysmic bolide impacts to seas draining away or becoming choked with organic detritus . . . .

As a natural history enthusiast who spends a great deal of time in the field (mostly photographing birds), I accept the concept of the Anthropocene, the Age of Man. I also accept that we are experiencing the the sixth great (anthropogenic) mass extinction event of the Phanerozoic Eon. I have no doubt that the fossil record of the future will show evidence of a geologically instantaneous extinction event dating to . . . now.

Older Holocene terrestrial strata will record a diverse vertebrate fauna, and nearshore marine strata will preserve reef facies bristling with invertebrates. Younger Anthropocene strata will show a much decreased biodiversity and a much greater abundance of cow, pig, chicken, human, and dog bones interlaced with scrap metal, broken concrete, and plastic debris!

One more subtle aspect of the unfolding anthropogenic extinction event, I think, is the human importation of often destructive exotic species into many parts of the world.

And now for a bit of shameless self-promotion . . . .

Save the Date (January 18, 2017): A New Two Shutterbirds Presentation at the Houston Audubon Nature Photography Association (HANPA)

Prothonotary Warbler on bottlebrush, Catholic Cemetery, Dauphin Island, Alabama
Looking for a Sweet Treat: Prothonotary Warbler on Bottlebrush, Catholic Cemetery, Dauphin Island, Alabama. Bottlebrushes are Australian plants, but birds everywhere love them because of the copious nectar and pollen they produce. Sweet calorie-rich nectar must be a wonderful treat after a grueling trans-Gulf of Mexico flight! Canon EOS 7DII/600mm f/4L IS (+1.4x TC). High-speed synchronized fill-flash.

Exotics Gone Native!

Synopsis: Human-introduced exotic plants and animals are all around us, and many of them are doing nicely, thank you very much. It’s sometimes hard not to notice them while out photo-birding. The proliferation of these organisms can be troubling to nature lovers, particularly eco-purists. Are these foreign organisms adversely affecting our native plants and wildlife? And if so, how badly? Are some helpful to our native species? Certainly some, like bottlebrush, are helpful to the bird photographer! Whatever your stance on exotics, perhaps the healthiest thing to do is treat them as just another opportunity to experience new species in the wild—even if they are out of place. In this talk, Chris Cunningham will share images of some frequently encountered exotic species and discuss their place in our native landscape. (Note: If this topic is too upsetting, Chris and Elisa will share and some images of native wild birds from their most recent outings to West Texas, the Coastal Bend, and central New Mexico, too!)

Time and Place: 7:00 PM, January 18, 2017 at the Edith L. Moore Nature Sanctuary, 440 Wilchester Blvd., Houston TX 77079. For additional details, please see the Houston Audubon HANPA website.

©2016 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.

Appreciating Late Paleozoic Trilobites

Sic transit gloria mundi . . . .

Paladin morrowensis, Run-away Bay Formation, Pennsylvanian Period, Wise County, Texas
Paladin morrowensis, Graford Formation(?), Pennsylvanian Period (Missourian), Runaway Bay, Wise County, Texas. Trilobite is 4.0 cm long.

The story of trilobite evolution from the late Cambrian on, is one of a series of drops in diversity to eventual extinction near the end of the Paleozoic Era. By the opening of the Carboniferous Period, for example, only four families of trilobites remained. By the beginning of the Permian Period, only three remained, down from a maximum of sixty-three at the end of the Cambrian Period (see Gon, 2003), the heyday of the trilobites.

For about one-half their evolutionary history, then, trilobites were a low-diversity group. Luckily for the collector, late Paleozoic (Carboniferous and Permian Periods) trilobites, like Ameura, are among the most aesthetic of the entire group (I think!). Despite being interested in the entire sweep of trilobite diversity, my own collecting activities have focused on the late Paleozoic Era more than any other interval of time.

Cummingella belisima, Lower Carboniferous Epoch, Belgium
Cummingella belisima, Lower Carboniferous Epoch, Lemay, Tournei, Belgium. Trilobite is 2.0 cm long.

To my knowledge, all Late Paleozoic trilobites belong to the Order Proetida. I would characertize this group, especially the late Paleozoic forms, as a conservative bunch. They are generally small in size (5 cm long would be exceptionally large for a late Paleozoic specimen). They may be largely smooth with only minor ornament (Like Ameura or Ditomopyge) or covered in a pustulose or tubercular surface texture (like Breviphillipsia). Genal spines may be present or absent. In some forms, like the Paladin (above) or Pseudophillipsia, the genal spines may be impressive.

Breviphillipsia sampsoni, Chouteau Formation, Mississippian Period, Sedalia, Missouri
Breviphillipsia sampsoni, Chouteau Formation, Mississippian Period, Sedalia, Missouri. Trilobite is 1.9 cm long.

But what contributed to the once diverse trilobite clan being whittled down to a relative handful of similar forms in the middle to late Paleozoic Era? From the beginning of the Silurian Period to about the mid-Middle Devonian, trilobite diversity more or less held its own, with only minor losses.

In the Middle and Late Devonian, however, the world was hit by a number of biocrises that took a heavy toll on trilobites. Odontopleurids, harpetids, and dalmanitids, for example, became extinct at the end of the Frasnian Age (the so-called Kellwasser event). Phacopids became extinct near the end of the Famennian Age (the so-called Hangeberg event). The end of the Devonian Period mass extinction, considered one of the major five such “events” of the Phanerozoic Eon, then, was not likely a single event but rather the result of a series of widespread changes.

Ditomopyge decurtata, Florena Shale Member, Beattie Limestone, Cowley County, Kansas
Enrolled Ditomopyge decurtata, Florena Shale Member, Beattie Limestone, Lower Permian Epoch, Cowley County, Kansas. The Florena Shale is fun to collect in: it has a feel similar to many early Paleozoic shale units in that some weathered outcrops are literally covered in invertebrate skeletal debris. Echinoid sclerites and chonetid brachiopod valves are especially prominent—and occasionally one finds a trilobite! Specimen is 1.0 cm across the genals.

One intriguing interpretation of the cause of the biocrisis affecting epicontinental marine ecosystems that trilobites called home in the late Middle and Late Devonian Epochs, is the expansion of terrestrial land plant communities. Algeo et al. (1995) argued that more land plants lead to enhanced soil formation and weathering. This may have lead to the epeiric seas of the world being inundated with more organic detritus than they could accommodate, thus leading to anoxia (and hostile marine paleoenvironments) and deposition of widespread black marine shales in Eurasia and North America. These geochemical changes may have affected the atmosphere and climate (global cooling), and even caused glaciation.

Mass of Phillipsid Trilobite Molts, Pennsylvanian Period, Wise County, Texas
Septarian Nodule Containing Mass of Phillipsid Trilobite Molts, Pennsylvanian Period, Wise County, Texas. Specimen is 9 cm across.

Oddly, I can see parallels between the world of the Late Devonian Epoch and today. For many years I considered the rise of Homo sapiens to be akin to the bolide impact at the end of the Cretaceous Period. After all, humanity’s rise and extermination of numerous organisms, such as Tropical and Neotropical migrant songbirds, mammalian megafauna, and reef communities (to name a few!), will appear as a geologically instantaneous event in the stratigraphic record—just like the deposition of a thin iridium-enriched clay layer.

Upon reflection, though, the monumental expansion of human population and the concomitant impacts on ecosystems (producing the current sixth or anthropogenic mass extinction event) does seem more like the expansion of Devonian terrestrial plant communities than a cataclysmic event of astrophysical origin.

I certainly lament the demise of the trilobites, but the expansion of early terrestrial ecosystems certainly provided opportunities for diversification of other arthropod groups, especially insects, relative to the bare rock and sediments (or perhaps with the minor algae, bryophytes, and psilophytes) of the early Paleozoic landscape. In one sense, then, our current landscape, replete with insects, is a consequence of the changes that brought the era of trilobitic seas to an end.

Dragonfly on agave, Houston, Texas
Needham’s Skimmer Dragonfly (Libellula needhami) on Agave Thorn, Houston, Texas. The earliest undisputed insects are Early Devonian in age. The earliest true dragonflies are Late Carboniferous in age, although fossils of similar forms predate that time.

References

Algeo, T. J., R. A. Berner, J. P. Maynard, and S. E. Scheckler. 1995. “Late Devonian oceanic anoxic events and biotic crises: “Rooted” in the evolution of vascular land plants?” GSA Today 5 (3): 64-66.

Gon, Samuel M., III. 2003. A Pictorial Guide to the Orders of Trilobites. 88p.

©2016 Christopher R. Cunningham. All rights reserved. No text or images may be duplicated or distributed without permission.