Ammonoids and the Evolution of Shell Form
Ecdysozoa: The second major bilaterian monophyletic group.
With the exception of Arthropoda, these are minute creatures with no fossil record. Ecdysozoa is diagnosed by one morphological synapomorphy: An external cuticle that is regularly shed.
Arthropoda is united by the following synapmorphies:
- Jointed appendages
- In primitive forms most limbs are similar, but in most various limbs are highly
- Ancestrally limbs seem to be biramous: one branch for locomotion, the other (dorsal) branch
being a gill.
- Various derived forms lose the gills to become uniramous
- Complete body segmentation
- Body of primitive forms is homonomous (all body segments are very similar), but in
vast majority there is some degree of tagmosis (a.k.a. "tagmatization" specialization and fusion of joints)
Issues of the arthropod cuticle:
- Growth strategy: Arthropods are not confined, as are mollusks, by the need to retain their juvenile skeleton. In them, episodes of ecdysis separate distinct instars - discrete growth stages. These can, in some case, be radically dissimilar to one another. Thus, arthropods may optimize their skeletons for different life strategies at different stages.
- Limitation: For an interval after ecdysis, arthropods are vulnerable and unable to support themselves. This places strict limits on absolute size.
Major Arthropod Groups
Pycnogonida (sea spiders) (Devonian - Recent) Greatly reduced trunks with greatly enlarged limbs (which house gut tube diverticula and verisou viscera as well as the usual stuff.) Strictly marine. Specialized feeding limbs and, in males, specialized limbs for carrying egg sacks.
Myriapoda: Millipedes and centipedes. (Silurian - Recent) Homonomous bodies with distinct heads and strictly uniramous limbs. Strictly terrestrial. Among first animals to invade the land.
Hexapoda: Insects and their kin. (Devonian - Recent) Terrestrial or secondarily fresh-water aquatic. Segments consolidated into:
- Cephalon with compound and simple eyes, one pair of antennae, three pairs of feeding appendages.
- Thorax with three pairs of walking limbs and in most, two (but originally three) pairs of wings.
- Abdomen. containing viscera.
Crustaceamorpha: Crustaceans and their kin (possibly including Hexapoda). (Cambrian - Recent) Mostly aquatic but some terrestrial. Segments consolidated into:
Development from nauplius larva.
- Cephalon with compound, two pairs of antennae, three pairs of feeding appendages.
- Trunk with strictly biramous limbs.
Chelicerata. (Cambrian - Recent) Marine and terrestrial. Segments consolidated into:
Includes horseshoe crabs, sea-scrpions, and arachnids.
- Prosoma with compound or simple eyes, no antennae, five pairs of walking limbs.
- Opisthosoma with numerous breathing appendages or their derivatives.
But our principal emphasis is on....
Trilobita: Trilobites. (Cambrian - Permian) Strictly marine.
- Culturally, are to the Paleozoic Era and to Invertebrate Paleonology what dinosaurs are
to the Mesozoic Era and to Vertebrate Paleontology!
- Extremely common fossil makers, particularly in early and mid-Paleozoic
- Range from Early Cambrian, when they were major players in the "Cambrian explosion" to the Permo-Triassic Extinction. They experienced their peak diversity in the Ordovician and were substantially reduced by extinction events at the end of the Ordovician and the Devonian. By the Permian, they were marginal.
- Exoskeletons were highly calcified, and thus very well preserved in comparison to their relatives, who show up only at sites of exceptional preservation (E.G. Chenjiang and Burgess Shale.)
- Body divided into three lobes, two different ways:
- Antero-posteriorly into a cephalon (head, containing most of the viscera),
thorax (generaly with many segments, containing the limbs), and pygidium (fused "tail" segments)
- Ventrally, the only calcified cuticle is in the hypostome, a plate that forms the floor of the mouth.
- Medio-laterally into a central axial lobe (containing the nervous and digestive
systems) and a pair of lateral pleural lobes (overlying the spread of the limbs, including
- The mouth was ventral and opened into a stomach that filled the forehead-like glabella.
- Compound eyes were well developed. Lenses were single crystals of calcite with their crystalline axes aligned with the long axis of the eye. Two major types of eye are known:
- Holochroal: Similar to the compund eyes of insects. Lens elements are contiguous and focus onto a point.
- Schizochroal: Unique among animals to the phacopid trilobites. Lens elements are separated by cuticle. Each element consists of two subunits with slightly different indices of refraction, creating an aplanatic correcting lens such as described by Huygens and Descartes. Each element creates a sharper image and transmits more light.
- Of course, some have no eyes.
- Very diverse feeding habits, including:
This appears to be the ancestral state, present in some outgroups to trilobites such as Naraoia. Present in the largest forms. Presence indicated by spiny gnathobases and elongate, stout hypostomes.
- Particle feeding: Most common forms (numerically and taxonomically), kicking particulate food into their mouths. Indicated by modest gnathobase spines.
- Filter feeders:
At least twice, small, virtually eyeless forms have evolved with broad pitted cephalic brims. They typically inhabited deep marine (i.e. dark) habitats. These are interpreted in two general ways:
- The pits housed sensory organs
- The pits were part of a sieving filtration mechanism. If this were true, it would mark a radical departure in trilobite feeding.
- Chemosymbiotic forms:
Some forms, particularly of the clade Olenimorpha, found in deep water black shales seem poorly equipped to feed at all. Indeed, they resemble a trilobite imitating an ediacaran mat-animal, with a great many thoracic segments and very broad pleural lobes. Some speculation maintains that they were chemosymbiotic forms analogous to the pogonophoran worms of recent deep sea hydrothermal environments.
- Pelagic planktonivores:
During the Ordivician, several groups gave rise to small nektonic plankton eaters. These are characterized by reduced cuticles, and enlarged eyes and limbs.
- Plankton themselves: the tiny eyeless agnostids:
A particularly early and distinctive group, the Cambrian agnostids were, themselves, planktonic, experimenting with the trilobite version of the bivalve morphology found today in planktonic crustaceans. Trilobites being arthropods, we have good growth series for many of them, This enables us to identify agnostids as paedomorphic.
Regrettably, a fifty-minute survey does no justice to the variation in these diverse animals.
Many invite speculation on the function of their conspicuous specializations. Not surprising given that, during the early Paleozoic, their role was to be as abundant and diverse as "the fish in the sea."
Finally: additional information.