Arthropoda (crown group) is united by the following synapmorphies: Jointed appendages In primitive forms most limbs are similar, but in most various limbs are highly specialized Ancestrally limbs seem to be biramous: one branch for locomotion, the other (dorsal) branch being a gill Various derived forms lose either the dorsal or ventral ramus 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 (AKA "tagmatization". Specialization and fusion of body segments and their associated limbs.) Compound eyes

Marella splendens of Burgess Shale.

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Internally 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

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Arthropod phylogeny at a glance: We have reviewed creatures on the arthropod stem. Crown group Arthropoda breaks down into two groups:

• Arachnomorpha: Trilobites, Chelicerates, and their kin.
• Mandibulata: Centipedes, crustaceans, insects, and their kin.

Contains:

• Trilobitomorpha: Trilobite and relatives
• Cheliceramorpha: Chelicerates (horseshoe crabs, sea scorpions, arachnids) and relatives

Trilobita: Trilobites. (Cambrian - Permian) Major features: 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. Konzervatlagerstätten: Chengjiang, Sirius Passet, Burgess Shale.) These relatives include minor groups such as: Naraoida: Similar to trilobites without calcification or segmentation of thorax Tegopeltida: Similar to trilobites without distinct cephalon Strictly marine.

From The Virtual Fossil Museum

• 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 their gills)

• The mouth was ventral and opened into a stomach that filled the forehead-like glabella.
  
  
  

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• 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: 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
    Thus, each element creates a sharper image and transmits more light.
  • 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.

Very diverse feeding habits, including: 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.

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.

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.

Trilobites being arthropods, we have good growth series for many of them, This enables us to identify agnostids as paedomorphic.

Finally: additional trilobite information.

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Olenellus a trilobite From www.trilobites.info

Cyamocephalus a chelicerate From Palaeos

Note, for example the post-anal telson (spine), a feature soon lost in trilobites but retained and elaborated in chelicerates.

Cheliceramorpha

Characteristic body segmentation:

• Anterior segments fused to form prosoma (bearing the head and thorax, with six pairs of appendages)
• posterior form opisthosoma (primitively segmented); often followed by a
• telson, or tail spine

Tagmosis: NO antennae Chelicerae: shred food and transfer to mouth Pedipalps: Second par of limbs often specialized for various functions include seizing prey, walking, and copulation Four pairs of walking legs Breath by book gills - specialized psoterior biramous appendages. (book lungs in the terrestrial arachnids, and even trachaea in some specialized small-bodied arachnids) See xiphosuran Limulus polyphemus (right) An early representative: Sanctacaris of the Burgess shale. With six major prosomal limbs and numerous minor limbs of the opistosoma.

Several major groups:

An early representative: Sanctacaris of the Burgess shale. With six major prosomal limbs and numerous minor limbs of the opistosoma.

Pycnogonida (sea spiders) (Dev. - Rec.) Have chelicerae, but pedipalps ambiguous. Males have specialized limbs behind chelicerae called ovigers for grooming and transporting fertilized egg mass that may be homologous. Very sparse fossil record

Xiphosura (horseshoe crabs) (Sil. - Rec.) Aquatic (most marine, but a few fossil brackish and freshwater forms), but can come onto land for short periods of time Fossil record from Silurian onward Early forms and larvae of modern forms are VERY similar to the earliest trilobites. Tagmosis: Chelicerae Five pairs of walking legs - i.e. pedipalps not differentiated.

Eurypterida (sea scorpions) (Ord. - Per.) Ordovician through Middle Permian, but heyday is Silurian and Devonian Predatory: Either chelicerae, pedipalps, or first walking legs could be developed as powerful claws Anteriormost six segments of opisthosoma typically very broad: form mesosoma - Derived book gills enclosed in gill chambers superficially resembling sternites. These chambers have, on their roof, vascularized gill tracts that apparantly functioned to retain water when the creature was on land. Opisthosoma flexible dorsoventrally, with tergites (half-bands) of cuticle above and sternitesbelow Sixth appendage pair modified as paddle. Based on joint reconstructions thistended to be used in rowing rather than subaqueous flight. Extimated top speed is about 2.5 X body length per second. Reached large sizes (more than 2 m long) Traditionally lumped with Xiphosaura into "Merostomata", but this is probably a paraphyletic assemblage (essentially "marine chelicerates" or "non-arachnid chelicerates") Seem to have been more heavily mineralized than xiphosurans or arachnids, possibly because of larger body size

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Walking: Seem to have been euryhaline. Depositional settings of eurypterid fossils indicate that they ranged from fresh to marine water, that individual animals seemed to be tolerant of a broad range of salinity, and that different groups tended to have preferences for a particular environment. Some have limbs that seem robust enough to allow excursions onto land.

We have trackways that appear to have been made by brackish-fresh water eurypterids, indicating that, like crabs, they could emerge from the water.

Eurypterid phylogenies have been proposed, but we should be cautious, as there is no guarantee that Eurypterida is monophyletic with respect to the next group...

Arachnida (arachnids: scorpions, whip scorpions, pseudoscorpions, mites, ticks, daddy longlegs, spiders, and their extinct allies) (Sil. - Rec.)

• Terrestrial chelicerates with book lungs (although early scorpions had gills)
• Silurian (scorpions) onward: among the first terrestrial animals
• Specialized pedipalps; posterior four pairs of limbs are walking limbs
• Major groups:
  • Scorpiones (Scorpions Sil - Rec.)
    • Predators with large raptorial pedipalps.
    • Postabdomen and telson modified for stinging.
    • Ventral sensory appendages called pectines
    • discovery of flourescent cuticle has facilitated discovery of new species, some of which inhabit shoreline.
    • Silurian representative with gills
  • Aranae (Spiders Dev - Rec.)
    • consoldiation/ loss of segmentation of abdomen,
    • Abdomen and prosoma connected by narrow pedicle
    • Use webs to various degrees in prey capture.
    • Chelicerae modified as poison injecting fangs and for slurping fluidized tissues of prey.
    • Pedipalps specialized for walking and prey manipulation in all, but as intromittant organs in males.
    • Some mid-Paleozoic forms show origins of spiders (prior to fusion of opisthosoma segments)
  • Acari (Ticks and mites Dev. - Rec.) (OK, seriously)
    • consolidation of prosoma and opisthosoma beneath a single caprapace.
    • like spiders, they injest only fluids, which they pump in with a muscular "pharynx".
    • Ecologically diverse: detritivores, parasites, predators, herbivores.
    • Being very small, they can inhabit ecosystems as small as mammalian eyelashes.

  • Others, briefly mentioned:
    • Opiliones - daddy longlegs - predators
    • Solpugidae - solifuges "sun spiders" - fast-running predators
    • Mastigoproctu - whip scorpions - predators
    • Pseudoscorpiones - pseudoscorpions - parasites, detritivores
    • Amblypygi - whipless whip scorpions (AKa whip spiders) - predators

  A creepy thought:

  Problems with Arachnid monophyly: All of this rasies some creepy questions. Everyone knows that arachnids are the "land-chelicerates," but strictly speaking, Arachnida is diagnosed as monophyletic by only a handful of rather subtle morphological synapomorphies including:

  • External digestion (yuck)
  • Slit sensory organs
  These would be hard things to identify in eurypterids even if they were there. Indeed, some workers claim to have seen slit sensory organs in some. This, combined with the general similarity of scorpions and eurypterids inclines us to be suspicious of whether Arachnida is really monophyletic. Could scorpions and their relatives actually be kinds of eurypterid? (Indeed, the same large eurypterids that seem frequently to have come out of the water also had hollow telsons. Could this be for venom transmission?) Possibly. Alternately, virtually no one is confident that eurypterids are monophyletic. Perhaps some (E.G. the ones with the hollow telson "stingers") are actually nested inside Arachnida. The issue is unresolved and likely to get muddier before it clears up.

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