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Trilobites Don't Bug Me At All!
"But ask now the beasts, and they shall teach thee; and the fowls of the air, and they shall teach thee: Or speak to the earth, and it shall teach thee: and the fishes of the sea shall declare unto thee." Job 12:8
From age 6 on besides school, my spare time was spent being involved at the church (Lutheran) as an Acolyte, Cub, then Boy Scouts, or out on my own in a local woods or stream since I found nature fascinating.
On my 13th birthday, being a serious and precocious boy, I randomly opened my Bible and stuck my finger randomly in it and read the above verse which was the beginning of my studies in biology and geology. Because the rocks in the area of my birth and youth were of ancient marine coral reefs, that became the primary field of my interest and study (although my Irish & Scot grandmothers has also taught me much herbalism which I've always muddled with).
Of all the fossil creatures I began to find on my hikes, the most elusive of all were trilobites, that extinct arthropod relative to the horseshoe crabs. For four years I would only find a few fragments of them, usually of their moulted exoskeleton tail, occassionally a head part, sometimes the middle. Finally, in the summer of my 18th year, I found a perfect enrolled specimen on a family camping trip near Cincinnati. That fall in my first field trip in a geology class in north west Ohio at Bowling Green University, I would find a baggy full of fragments and a perfect enrolled and, rarest of all, a perfect outstretched specimen of Phacops rana milleri (which was recently renamed Eldredgeops milleri). Although I would still maintain a very active interest in the entire marine ecosystem, along with a bit of paleobotany, fresh water and terrestial animals, my primary focus over the years has been collecting trilobites, although my mid through late 20's is when my academic focus was on art and philosophy.
Hopefully, beginning 2014 I can devote more time in collecting both general marine and specifically trilobites once I start my river adventure.
Posting here a discussion of the little beasties and a sampling of the complete ones in my collection, which number about 350 species of complete specimens, and another 700 species of partial specimens.
Introduction is a bit technical although as simplified as I could make it, in matters of my critters I am most surely a boffin.
For Rachel Victoria H. and her Extatosoma tiaratum: may they never have to go through parthenogenesis again!
(Diorama scene of ancient ocean floor)
model of a living Dipleura sp.
model of a living Neoasaphus sp.)
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INTRODUCTION TO THE TRILOBITA
Trilobites are the most diverse group of extinct animals preserved in the fossil record. Ten orders of trilobites are recognized, into which there are over 180 families of trilobites, and about 5000 genera, which contain the 15,000+ described species of trilobites.
The major reference for Trilobites is The Treatise of Invertebrate Paleontology (Part O, Arthropoda 1: Trilobita, 1959, revised: 1997", an international on-going editorial project with worldwide contributors in their field of expertise. The information and images (other than my own photos of specimens in my collection) used in this discussion are derived from both Treatises. -
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Morphology
The major trilobite body features that are typically preserved are summarized and illustrated below. These hard pieces of exoskeleton are preserved readily because of their mineral content, high in calcite (CaCO3). Most of these terms for major body features figure prominently in trilobite descriptions in taxonomy.
The trilobite body is divided into three tagmata (major sections), a cephalon with eyes, mouthparts and antennae, a thorax of multiple articulated segments (that in some species allowed enrollment, and a pygidium, or tail section of fused segments.The name "trilobite" (meaning "three-lobed") is not based on the tagmata (cephalon, thorax, and pygidium), but on the three longitudinal lobes: a central axial lobe, and two pleural lobes that flank the axis.
When describing differences between different taxa of trilobites, the presence, size, and shape of the cephalic features are often mentioned. To the right are cephalic (cranidial) features described when the librigenae are missing The cheeks (genae) are the pleural lobes on each side of the axial feature, the glabella. When trilobites molt or die, the librigenae (the so-called "free cheeks") often separate along the facial sutures, leaving only the cranidium --that is, the glabella and fixigenae ("fixed cheeks").
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(Exoskeleton: shell)
The chemical composition of the trilobite exoskeleton, first and foremost, a dorsal shield which protected the animal’s vulnerable inner organs and tissues, has been examined and described by various scientists. It was found to show a very high degree of calcification, i.e. primarily consisting of low-magnesium calcite, a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO³) with only a small portion of organic material to it.
It has to be noted that the unmineralized exoskeletons of insects and other arthropods, primarily constructed from purely organic materials, regularly outclass the mineralized shields of trilobites and other marine life forms in terms of resilience against destructive forces (they usually do not crack; a spider falling from a window sill usually survives its rapid descent completely unharmed). This is one reason why the latter have to grow much thicker to even the odds. Fortunately, the waters of the world’s oceans are rich in soluted calcium ions, and it is far more rational for a marine arthropod to afford a thick mineralized shield by absorbing what is available in abundance than to invest a disproportionate amount of energy into building a construction entirely based on organic compounds.
While the prismatic layer forms but the thin surface of the carapace, the major part of the protective shield is formed by the underlying primary layer, a fine-grained substance in which the calcite crystals fail to stick to the extraordinary homogenous grid that is usually found in the prismatic layer. The primary layer at times shows parallel structures which may be attributed to organic material in the living animal. However, as we stated before, the amount of organic material in the trilobite exoskeleton seems to have been very small indeed and if these structures are indeed what we assume them to be, they must have been way below 1 μm in strength.
When judging upon the value of an exoskeleton’s particular design, emphasis should be placed on the specific advantages it gives to its bearer. In trilobites the carapace seems to have served mainly as a protection against natural enemies, not so much as a framework for the attachment of muscular structures, although such a function seems to be evident in many preserved specimens. The thickness of the average trilobite exoskeleton in conjunction with its high grade of mineralization speaks in favour of adabptability's affinity to develop stronger shields for better protection. As a matter of fact, there are examples where trilobites, when living in an oxygen-poor evironment unfavourable to a large predator fauna developed but relatively thin shields
Further indications as to its main purpose as a protective shield can be found in the capability of enrollment with many trilobites and the fact that trilobite exoskeletons seem to show a more or less identical thickness throughout the whole shield (except for those points that appear to have served as attachments for muscular structures as mentioned above – these regions are usually thicker). This circumstance allowed for a very effective defensive position when assaulted by a predator, without any weak points that could have been selectively attacked. -
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Anatomy
The dorsal (top) morphology of trilobites is typically well preserved, and ventral (underneath) feautures such as limbs and antennae are only rarely preserved. Similarly, our knowledge of the internal anatomy of trilobites is very poorly understood. X-ray images of some trilobite specimens indicate a long, central (axial) structure typically considered an alimentary canal (intestines or gut). Sometimes the gut or its contents are also preserved as an axial structure. Only in extremely well-preserved agnostid specimens has a trace of a mouth and anus been detected. The mouth is associated with the hypostome, and the anus opens toward the rear of the pygidium, as might be expected.
The digestive system has been detailed. Their musculature resembles primitive crustaceans. Most, if not all, segments supported at least one pair of appendages. A pair of antenna in front of the mouth was used for sensing. Several pairs of appendages behind the mouth were probably used in feeding. Each thoracic segment had two pairs of appendages. Like crustaceans, one pair was used for walking and the other in respiration, but may also have aided in swimming or gathering of food
Most workers rely on knowledge of living arthropod anatomy and presume that trilobites had similar (if perhaps more primitive) circulatory, nervous, and muscular systems. -
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(appendages)
Much of the information about trilobite appendages is known from examples found in the Cambrian Burgess Shale, especially specimens of Triarthrus eatoni.
(Triarthrus eatoni (dorsal view) with legs & antenna preserved)(Triarthrus eatoni (ventral view) with cephalic and thorax appendages)
Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs (2, 3 or 4 cephalic pairs, followed by one pair per thorax segment and some pygidium pairs). Each exopodite (walking leg) had 6 or 7 segments, homologous to other early arthropods. Exopodites are attached to the coxa, which also bore a feather-like endopodite, or gill branch, which was used for respiration and, in some species, swimming. The inside of the coxa (or gnathobase) carries spins, probably to chew prey items. The last exopodite segment usually had claws or spines. Examples exist of hairs on the legs suggest adaptations for feeding (as for the gnathobases) or sensory organs to help with walking.
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(soft body parts)
Only 21 or so species are described from which soft body parts are preserved, so some features remain difficult to assess in the wider picture.
(dorsal view)(side view)
The toothless mouth of trilobites was situated on the rear edge of the hypostome (facing backwards), in front of the legs attached to the cephalon. The mouth is linked by a small esophagus to the stomach that lay forward of the mouth, below the glabella. The "intestine" led backwards from there to the pygidium. The "feeding limbs" attached to the cephalon are thought to have fed food into the mouth, possibly "slicing" the food on the hypostome and/or gnathobases first. Alternative lifestyles are suggested, with the cephalic legs used to disturb the sediment to make food available. A large glabella, (implying a large stomach), coupled with an impendent hypostome has been used as evidence of more complex food sources, i.e. possibly a carnivorous lifestyle.
While there is direct and implied evidence for the presence and location of the mouth, stomach and digestive tract the presence of heart, brain and liver are only implied (although "present" in many reconstructions) with little direct geological evidence.
Although rarely preserved, long lateral muscles extended from the cephalon to mid way down the pygidium, attaching to the axial rings allowing enrollment while separate muscles on the legs tucked them out of the way.
The pair of antennae suspected in most trilobites (and preserved in a few examples) were highly flexible to allow them to be retracted when the trilobite was enrolled.
Even the earliest trilobites had complex, compound eyes with lenses made of calcite (a characteristic of all trilobite eyes),Some trilobites were blind, probably living too deep in the sea for light to reach them. Other trilobites had large compound eyes that were for use in more well lit, predator-filled waters
Trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied: some trilobites had only one, while some had thousands of lenses in a single eye. In compound eyes, the lenses were typically arranged hexagonally. -
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Growth
Three developmental periods are recognized: a protaspid period, meraspid period, and a holaspid period. In the protaspid period, the larva (called a protaspis) is composed of an unarticulated exoskeletal shield, and often is very simple in form. It is thought that at least some early protaspid larvae were likely to have been planktonic.
The meraspid period is marked by a body with 2 or more articulated segments, and during the meraspid period, each molt meant the potential addition of usually one or two, but rarely greater numbers of articulated thoracic segments to the body of the growing trilobite. Several meraspid molts occurred, until the number of thoracic segments added to the meraspis achieved the number typical of the adult form of the species, and the general pattern of body morphology (shape and ornamentation) grew more similar to that of adults of the species.
When the number of thoracic segments reached that of adult specimens, this marked the holaspid period, after which no further articulated segments were added with each molt. At this point, the growing animal is called a holaspis, and enters into the last period of development, in which the major change is not in form, but in increasing size. It is thought that most of the increase in size in the life cycle of a trilobite occurred during the holaspid period.(protaspid period)
meraspid period
holaspid period
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(moulting)
During their lifetime, trilobites not only went through three different stages of distinctive morphologies (protaspid, meraspid and holaspid stages) but, with age, also grew considerably in size. Just as modern crustaceans with their chitinous carapace need to shed their old armours every once in a while to allow their bodies to develop in size and shape, our Palaeozoic friends were forced to periodically dispose of their rigid calcite exoskeleton and construct an entirely new and bigger one. In order to allow such a "change of suit" the trilobite’s head shield was equipped with a series of predetermined breaking points, the so-called facial sutures
Once the old carapace became too tight for the animal to proceed in its bodily development it was time to moult. The scientific name for this process is "ecdysis". Just like today’s crustaceans, the animal would try to find a protected place in which it could shed its armour without facing too big a risk of its temporary vulnerability being exploited by enemy predators. Specific hormones seem to have played a major part in jump-starting the process. The facial sutures, which have been classified into three different types (proparian, gonatoparian and opisthoparian - defined by where the sutures end, relative to the genal angle of the cephalic shield) started to break open.
The reconditioning of the trilobite exoskeleton after moulting seems to have been a rather accelerated process. For all arthropods shedding their armour is a time of extreme vulnerability. Some scientists assume that the trilobite secreted a thin prismatic layer first, with a very thin primary layer underneath, in order to build a first line of defence. As secretion proceeded, the primary layer became thicker and thicker. This seems to indicate that the prismatic outer layer might have been the more easy part to build. -
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Reproduction
Trilobites are thought to have reproduced sexually, as do nearly all arthropods today. Eggs were presumably laid, but fossilized eggs that may be of Cambrian eodiscid trilobites have been documented only once.
It has recently been suggested that some trilobites may have held eggs and/or developing young within the cephalon (as horseshoe crabs do today), and anterior median swellings of the cephalon (of the preglabellar field) in some specimens are interpreted as brood pouches because they appear only in holaspids (adults) and represent a dimorphism in which the swelling is the only morphological difference.
Ostracods and some other crustaceans show similar brood pouch swellings, although not at the anterior of the body. Because specimens with brood pouches appear only in natant trilobites, it is possible that the eggs or protaspids were released ventrally, anterior of the hypostome. -
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Ecology
The Earth's marine environment in the past was certainly not the same as it is today. It is likely that the ocean's chemistry, including salinity, was different, and the configuration of the ocean basins and continents was entirely unlike our modern globe, because of continental drift.
Biotic environments (the living community of plants and animals) were also different. While there were many species of marine plants and animals, many groups prominent today were missing, or poorly represented. For example, in the Cambrian and Ordovician , there were no jawed fishes, and Crustaceans (crabs, shrimps, etc.) which dominate the arthropod fauna of today's oceans, were present, but not prominent.
Trilobites were among the most prominent of the Paleozoic marine arthropods, and they have only been found in oceanic fossil beds. No freshwater forms have ever been found. They occupied many different ocean environments, from shallow flats and reefs, to deeper ocean bottoms, and even the water column, as floating plankton or free-swimming forms. While a few were wide-ranging pelagic species, most were regional, and their global paleogeography is a fascinating study of how living forms track their changing environments over geological time. Trilobites from different habitats often had specialized forms that were presumably adaptations to their environment
They were able to dig into the bottom sediments in search of food and to conceal themselves from predators. Perhaps some were herbivores on beds of algae (seaweed), or browsers on corals, sponges, or bryozoans. Some may have been filter feeders, orienting with the current and extracting plankton and organic debris.
It is thought that the majority of trilobites were bottom-dwellers, crawling on the sea floor, or within complex reefs, acting as roving predators on smaller invertebrates or as slow scavengers on organic debris.
Nautiloids (a primitive form of shelled squid/octopi) were probably important predators of trilobites. Trilobites certainly were important prey for larger creatures. At first these were large invertebrates, such as predatory worms, nautiloids, sea scorpions (eurypterids), crustaceans, and perhaps Anomalocaridids (sort of a marine scorpian) . When fishes developed and flourished in the Devonan , we can be sure that trilobites were hard pressed by these new predators. A hard exoskeleton and the ability to enroll protected trilobites from predators and sudden unfavorable environmental changes. -
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Taxonomy and Relationships
Although taxonomy of living creatures is undergoing major revisions due to genetic research, trilobites are divided into ten orders, into which there are over 180 families of trilobites, and about 5000 genera, which contain the 15,000 plus formally described species of trilobites.Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Trilobitomorpha
Class: Trilobita (Walch, 1771)
Order Agnostida
Order Asaphida
Order Corynexochida
Order Harpetida
Order Nectaspida
Order Redlichiida
Order Lichida
Order Phacopida
Order Proetida
Order Ptychopariida(Nope, I won't get into Bishop Ussher's "Biblical dating system" versus Geochronology, but I will add a chart of trilobite orders, their duration in the rock layers, and relationships to the other orders.)
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