Ecdysozoa I: Panarthropoda
Ecdysozoa:Organisms characterized by the regular shedding of an external cuticle and protostomous development.
- Possible sister taxon to Spiralia
Burgess Shale priapulid Ottoia prolifica from Wikipedia
Panarthropoda:This lecture focuses on Panarthropoda, the only ecdysozoans with a good fossil record, Burgess Shale priapulids (right) notwithstanding.
- Distinct limbs with circles of stiffened cuticle.
- Segmented bodies consisting of repeating limb-bearing units
The living panarthropod groups:
- Simple eyes
- Three sets of specialized limbs on the head:
- Sensory antennae at the front
- Mandibles inside the mouth
- Slime secreting oral papillae
- The mouth is on the underside of the head.
- Limbs are moved in part by muscles and in part by coelomic hydrostatic skeleton.
Onychophoran from The British Museum of Natural History
Tardigrade from Reddit.com
Sally-lightfoot crab Grapsus grapsus
- Lobopoda: Creatures broadly resembling marine onychophorans like the Burgess Shale Aysheaia (below).
- Dinocarida: Creatures with lateral swimming flaps and large raptorial (prey-capturing) anterior appendages like Anomalocaris.
- Specialization of limbs
- The position and orientation of the mouth
- Evolution of specialized mouthparts
- Evolution of sense organs
Mureropodia apae from Diario de un Copepodo
Mureropodia apae (early Cambrian) An infaunal creature believably transitional between priapulids and panarthropods, with five pairs of flexible unspecialized limbs, and a proboscis with a terminal mouth.
Aysheaia pedunculata from Biologia - La Vita e la sua Storia
Aysheaia pedunculata (middle Cambrian) The classic Burgess Shale "lobopodan." Sometimes called an ancestral marine onychophoran, but actually lacking man of their synapomorphies. Aysheaia occupies a basal position in recent analyses because of its lack of robust cuticle and specialized mouthparts. I does show the specialization of the first pair of limbs for something (?) like prey capture or as a sense organ.
Chengjiang "lobopodan" Microdyction sinicum from The Virtual Fossil Museum
Stem onychophorans (Cambrian) Closer to Onychophora. Poorly understood prior to 2000. The first representatives to be studied inlucded Hallucigenia of the Burgess Shale, a creature so poorly understood as to be misrepresented to the public as a stilt-walking worm. The discovery of less extreme armored forms like Microdyction (right) facilitated proper interpretation. Considerable Cambrian diversity, then the record falls silent.
Sirius Passet Kerygmachela kierkegaardi Wikipedia
- Jointed appendages paired with
- Soft swimming appendagesin a arrangement reminiscent of proper arthropod biramous appendages. Adding color to this similarity is the fact that in most taxa, these swimming appendages also seem to be house gas-exchange organs.
- Like the previous taxa, they retain and elaborate on the anterior great appendages. On the opposite end, they possess abdominal cerci homologous (?) to those of arthropods.
- Kerygmachela kierkegaardi: (early Cambrian from Sirius Passet) With small eyes and a terminal mouth.
- Pambdelurion whittingtoni: (early Cambrian from Sirius Passet) Up to 29 cm. length.
- A famous example is the five-eyed Opabinia regalis (middle Cambrian Burgess Shale). Unique in that its great appendages are fused into a single mid-line appendage. Derived in:
- The presence of Compound eyes: composed of clusters of ommatidia (singular "ommatidium") - visual elements each with a lens and visual cells.
- The position of the mouth beneath the head.
Chengjiang Anomalocaris saron from Roberto Verzo
- lacking the walking limbs
- Possessing large paired compound eyes
- Their subterminal mouths are encircled by a radial array of calcareous teeth
- Giant appendages invested in full arthropod style jointed cuticle.
Devonian Schinderhannes bartelsi from Palaeos
- The radiodont swimming appendages are replaced by proper jointed biramous appendages
- The dorsal body cuticle is divided into distinct tergites.
- A telson - posterior spine - is present.
Ediacaran Parvancorina minchami from Trilobites.info
- Parvancorina: (Right) Small shield-shaped creatures with body sculpture reminiscent of the cephala and pleural and axial lobes of trilobites. The case for Parvancorina draws strength from its similarity to the Burgess Shale trilobitomorph Skania fragilis.
- Spriggina floundersi: An Ediacaran animal. Rangeomorph frond? Annelid? Panarthropod? Each hypothesis has supporters. The fact that Spriggina has been found in association with trace fossils suggests that it was, at least mobile.
Arthropoda:(Cambrian - Quaternary) Arthropods ("Euarthropoda" of some authors, used herein to refer to the crown group of Arthropoda) with a proper arthrodized (sclerotized and jointed) cuticle covering the body have a copious fossil record, as even unmineralized cuticle is a "tough tissue," facilitating copious preservation in Konzervatlagerstätten. Some groups - Especially trilobites, mineralize their cuticle in addition. During the last twenty years, their phylogenetic pattern has began to come into focus, as well.
Biramous appendage from Copepods from the Bay of Villefranche
- Each body segment is associated with a pair of limbs or their derivatives except for:
- The acron: the first (pre-oral) segment
- The telson: the final abdominal segment (which often takes the form of a spine.)
- Ancestrally limbs seem to be biramous (right): one branch for locomotion, the other (dorsal) branch being specialized as a gas exchange structure (gill).
- Various body segments of various derived forms lose either the dorsal or ventral ramus to become uniramous
- Specialized feeding and sensory appendages are modified from walking limbs.
- Body segmentation is complete.
- Body of primitive forms is homonomous (all body segments are very similar), but in vast majority there is a significant degree of tagmosis (AKA "tagmatization") - specialization and fusion of body segments and their associated limbs. Patterns of tagmosis, the grouping of segments into tagmata - functional regions of the body - are very significant to arthropod systematics, starting with the assessment of the homologies of segments and their limbs.
Schematic internal organs from Earthlife.net
- Nitrogenous waste eliminated by malphigian tubules
- Heart with pericardium and ostia (sing. ostium). Open circulatory system with large haemocoel (coelomic cavity specialized for the transmission of circulatory fluid).
- Highly cephalized: Paired nerve cords run ventrally to the gut tube. For each segment, each cord has a ganglion from which the segment is innervated. Each pair of ganglia is connected by a commissure. The first three meet in a commissure in the head, dorsal to the esophagus, forming a brain.
The living euarthropod groups:
- Xiphosura - horseshoe crabs
- Arachnida - spiders (right), scorpions, ticks, mites, and many more.
- Pycnogonida - "sea-spiders" (but not really)
- Extinct members including Eurypterida - "sea scorpions"
- Diplopoda - millipedes (right)
- Chilopoda - centipedes
Blue crab - Callinectes sapidus
- Hexapoda - Insects and their kin
- A bewildering variety of traditional "crustaceans" (paraphyletic)
Molecular and morphological phylogenies of the last decade (especially the molecular analysis of Regier et al., 2010.) have resolved the major patterns of the relationships of the living taxa, and major fossil ones. Among Euarthropoda, the arthropod crown group, two major groups are evident:
- Arachnomorpha: Trilobites, Chelicerates, and their kin.
- Mandibulata: Centipedes, crustaceans, insects, and their kin.
Things become more interesting when the fossil arthropods of the Cambrian Konzervatlagerstätten are considered. During the last three years, however, a picture has begun to emerge. A survey of "major" groups.
Nereocaris exilis from Sciency Thoughts
- A thoracic shield hinged on the midline
- Two lateral compound eyes plus a third rod-shaped midline eye.
- Numerous undifferentiated thoracic biramous limbs
Fuxianhuia protensa from Evrim Haberleri
Recently, fuxianhuiids have become the poster children of arthropod phylogeny, with spectacular specimens revealing details of the nervous and vascular systems. These reveal that in Fuxianhuiathese systems resemble those of living arthropods. Specifically, the brain is similar to those of most living crustaceans and insects, and its eyes and antennae are innervated similarly by the first and second sets of brain ganglia respectively.
Leanchoillia superlata from Wikipedia
- Kühl et al., 2009: indicate they these are a paraphyletic group closer to Cheliceramorpha than to Trilobitomorpha.
- More recently Legg et al., 2012 and Siveter et al., 2014 recover these as a (mostly) paraphyletic group just outside of crown-group Euarthropoda.
- "Trilobitomorpha": (Cambrian - Permian) Trilobites and their relatives. Whether this group is mono- or paraphyletic is currently unclear. Trilobita is certainly monophyletic.
- Cheliceramorpha: (Cambrian - Quaternary) Chelicerates (horseshoe crabs, sea scorpions, arachnids) and their relatives.
- Naraoida: Similar to trilobites without calcification or segmentation of thorax
- Tegopeltida: Similar to trilobites without distinct cephalon
- Helmetiids: Similar but lacking three body lobes.
Trilobite tagmata from University of Bristol
- Cephalon: (Head) with paired compound eyes, one pair of antennae )in front), and a pair of unspecialized biramous legs for each posterior segment
- Thorax: One pair of biramous legs for each segment.
- Pygidium: Posterior segments fused into a plate. Each segment with a pair of unspecialized biramous legs. In some cases a pair of antenna-like cerci are associated with the last segment.
Facial sutures: The cephala of most trilobites display facial sutures - lines of weakness at which the cuticle of the cephalon breaks apart when the animal molts. In traditional taxonomy, their geometry has great significance. They are classified based on where the suture terminates posterolaterally with respect to the corner of the cephalon:
- Proparian: sutures meet the lateral margin anterior to the corner.
- Opisthoparian: (right) sutures meet the posterior margin of the cephalon.
- Gonatoparian: sutures extend precisely to the corner of the cephalon.
Trilobite body divisions
- 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.
Trilobite thoracic cross-section.
- 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
Trilobite alimentary canal
- As in all but very primitive panarthropods, the mouth was ventral and opened into an expansion stomach that filled the forehead-like glabella.
Schozochroal eye from Trilobites.info
- 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 and one minor type of eye are known:
- Holochroal: Similar to the compund eyes of insects. Lens elements are contiguous and focus onto a point. A single cuticle "cornea" covers the entire eye.
- Schizochroal: (Right) Unique among animals to the phacopid trilobites. Lens elements are:
- separated by thick cuticle
- consist of two subunits with slightly different indices of refraction, creating an aplanatic correcting lens such as described by Huygens and Descartes
- equipped with their own distinct cornea
- A third type, Abathochroal: Unique among animals to the phacopid trilobites. Simple lens elements are separated by thin cuticle. A single cuticle "cornea" covers the entire eye.
- Of course, some have no eyes.
Trilobite ecology: The diversity of eye morphology and degrees of visual acuity in trilobites prefigures their great range of ecological specializations. While most were benthic deposit feeders, some developed the ability to capture larger prey while others got up into the the water column. To scratch the surface:
Trilobite attacks prey
- Predation/scavenging: 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), "elbowing" particulate food into their mouths. Indicated by modest gnathobase spines and reduced hypostomes.
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.
During the Ordovician, several groups gave rise to small nektonic plankton eaters. These are characterized by reduced cuticles, and enlarged eyes and limbs.
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. Some suggest that these are not actually trilobites. Owing to their highly derived, arguably paedomorphic, nature and our lack of detailed knowledge of the anatomy of other trilobites at similar developmental stages, this has been difficult to assess. Right now, opinion is divided.
Trilobite systematics: Here we have a problem. Being well known for close to two centuries and principle index taxa for the early Paleozoic, their complex, traditional Linnean taxonomy is studied by a great many professional biostratigraphers. This includes some groups that are probably paraphyletic (See the Prothero text for a full review.) Consequently, few phylogenetic investigations have been conducted of the relationships within Trilobita. This is bad because trilobites have been used in the study of evolutionary patterns and processes despite our not really knowing their real evolutionary pattern.
To date, most rigorous phylogenetic analyses have aimed at:
- The position of trilobites in the broader arthropod context
- The phylogenies of specific lower-order taxa. (See Lieberman and Karim, 2009 for more info.)
- Olenellida: Whose members retain a telson and lack facial sutures.
- Agnostida: Whose tiny members lack even a hypostome. But note: Some analyses place agnostids well outside of Trilobita!
Finally: additional trilobite information.