Ecdysozoa II: Chelicerata and Mandibulata
Trilobites and chelicerates are united in the monophyletic group Arachnomorpha: ("Artiopoda" of some researchers.) Synapomorphies are too technical to discuss here, however a comparison of very early trilobites and chelicerates shows their general similarity to their last common ancestor. For a thorough review, see Legg et al. 2013.
Olenellus a trilobite
Cyamocephalus a chelicerate
Note, for example the post-anal telson (spine), a feature soon lost in trilobites but retained and elaborated in chelicerates.
We have considered Trilobitomorpha previously. Now we turn to Cheliceromorpha - Chelicerata and its closest relatives.
The chelicerate body plan:
- 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
- NO antennae
- Six pairs of uniramous prosomal apendages (derived from inner ramus of ancestral biramous limb):
- Chelicerae: shred food and transfer to mouth
- Pedipalps: Second pair of limbs often specialized for various functions include seizing prey, walking, and copulation
- Four pairs of walking legs
- Opisthosomal segments are either without limbs or bear book gills - specialized appendages used for gas exchange.
- Note: All limbs are uniramous, however the prosomal appendages seem to be derived from the walking ramus of ancestral biramous limbs (endopod), and the opisthosomal appendages from the breathing ramus (exopod).
- This is the limb pattern in all living chelicerates and most fossils, however the Briggs et al. 2012 description of biramous limbs in a Silurian stem xiphosuran reveals that the earliest chelicerates retained the ancestral arthropod condition.
The living chelicerates:
Pseudopallene pachycheira from Port Phillip Bay
- Trunks are greatly reduced, with gut tube and other organ systems occupying the proximal limb segments.
- The opisthosoma is reduced to a small nub.
- Tagmosis: Almost the entire body is prosomal. Limbs:
- Chelifores homologous with chelicerae
- "Palps" - pedipalp homologs?
- Ovigers" Males have specialized third limbs for grooming and transporting fertilized egg mass.
- Four pairs of walking limbs
Haliestes dasos (Sil.) from BBC News
Living pycnogonids lack many features we associate with arthropods, E.G.:
- Compound eyes
- If we exclude fossil arthropods from analyses and polarize analyses with outgroups like priapulids, pycnogonids show up as the sister taxon of other arthropods.
- The inclusion of fossil arthropods clarifies that these losses in pycnogonids are secondary and allows them to be the sister taxon of other chelicerates.
- Small opisthosomae
- In some - telsons (see Devonian Flagellopantopus blocki.)
Limulus polyphemus from Wikipedia
- Marine, but a few fossil brackish and freshwater forms. Living members can tolerate brackish water and come onto land for short periods of time to spawn.
- Opisthosoma is reduced and, in more recent forms fused into an inflexible opisthosomal shield.
- Retain compound lateral eyes and simple mid-line ocelli. (Ancestral for Euarthropods.)
- Five pairs of walking legs - i.e. pedipalps not differentiated.
- Although some are adapted to life in or near water, all are air-breathing.
- The book gills are internalized into a book lung facilitating air breathing. (Some members also use a tracheole system similar to that of insects.)
- Compound eyes are significantly reduced or eliminated
- Hearing facilitated by slit sensilla in legs.
- Chelicerae, pedipalps, and walking legs are strongly differentiated
- Feeding systems are modified so that digestion is, to some extent, external. (M'mmmm!)
Major fossil groups:
Sanctacaris uncata from Life Before the Dinosaurs
- A fused prosoma
- Six pairs of prosomal appendages, including five robust grasping limbs and one antennate pair. Each grasping limb is biramous, with an antenna-like ramus and a stout one. None have pincers.
- Opisthosomal segments each bear a pair of swimming paddles.
Achanarraspis reedi from Palaeontology Online
- The final pair of walking limbs modified as paddles.
- Opisthosoma flexible dorsoventrally, with tergites (half-bands) of cuticle above and sternites below.
- Opisthosoma divided into anterior preabdomen and posterior postabdomen
- Book gills are limited to the preabdomen and concealed in pouches.
- The preabdomen has three segments, the postabdomen has nine.
Eurypterus by Dimitris Siskopoulos from Wikipedia
- Tagmosis resembles that of chasmataspidids, but preabdomen has seven segments and postabdomen only five. (See comparison)
- Reached large sizes (more than 2 m long)
- Seem to have been more heavily mineralized than xiphosurans or arachnids, possibly because of larger body size.
- Feeding ecology: Ranged from xiphosuran-style hunting and scavenging to attacking large prey. In derived forms, either chelicerae or first walking legs could be developed as powerful claws
- 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.
- Anteriormost six segments of opisthosoma typically very broad: form preabdomen - 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 (a development of the simpler arrangement of chasmataspidids.)
- The telson could be broadened into a paddle or remain slender. In some, it is hollow - conceivably used to transmit venom as in scorpions.
Eurypterid Systematics: Traditionally Eurypterida, Chasmatapsidida, and Xiphosaura were lumped into a "Merostomata" that was blatantly paraphyletic with respect to arachnids. Now frowned upon, but you can expect to encounter the term.
- Changing hypotheses of appropriate outgroup choices
- Failure to anchor phylogenies by the inclusion of representatives of Arachnida.
Eurypterid convention dominated by Pterygotus (left) and Stylonurus (right)
in classic image by Charles R. Knight from Wooster Geologists
- Retention of four pairs of robust walking legs in taxa like Stylonurus (above right).
- Internalization of book gills should protect from desiccation.
- Silurian-Devonian trackways actually attributable to eurypterids.
Centruroides sculpturatus from Orange Pest Control
Arachnida:(Scorpions, whip-scorpions, pseudoscorpions, whipless whip-scorpions, mites, ticks, daddy longlegs, spiders, sun-spiders, and their extinct relatives.) (Sil. - Rec.) General characteristics (review):
- The book gills are internalized into a book lung, (Schematic)
- Compound eyes are significantly reduced
- Hearing by slit sensilla in legs.
- Chelicerae, pedipalps, and walking legs are strongly differentiated
- External digestion (Yuck!)
Major living groups:
Palaeophonus (Sil) from solpugid.com
- Compound eyes are reduced.
- Chelicerae are unspecialized
- Pedipalps large and raptorial (in contrast to eurypterids).
- Postabdomen and telson modified for stinging (or maybe this is a plesiomorphy?).
- Ventral sensory appendages called pectines
- discovery of flourescent cuticle has facilitated discovery of new species, some of which inhabit shoreline.
- Opisthosoma and prosoma connected by narrow pedicle
- Segmentation of opisthosoma is reduced (for most, it is completely absent)
- Silk glands secrete silk used to various degrees in prey capture.
- Chelicerae modified as poison injecting fangs and for slurping fluidized tissues of prey, but lacking chelae (pincers).
- Pedipalps specialized for walking and prey manipulation in all, but as intromittant organs in males.
Deer tick from LiveScience
- Differentiation of prosoma and opisthosoma lost
- Loss of limb pairs
- Loss of eyes
- Loss of heart (literal).
- In some, chelicerae specialized for stabbing and sucking.
- Single pair of eyes
- Specialization of the second walking leg pair as "antennae"
- Direct internal fertilization
- Known for long legs, but there are short-legged forms
- Book lungs lost. Breathing is by means of a tracheal system.
Solpugid from New Mexico Pest Management Services
- Solpugida (AKA "Solifugae") - solifuges "sun spiders" - fast-running predators (right)
- Thelyphonida - whip scorpions - predators
- Schizomida - short-tailed whip scorpions
- Palpigradi - micro whip scorpions
- Pseudoscorpiones (Dev. - Rec.) - pseudoscorpions - parasites, detritivores
- Amblypygi - whipless whip scorpions (AKA whip spiders) - predators
- Ricinulei - hooded tickspiders
Trigonotarbid from University of Aberdeen - The Rhynie Chert
- Broadly resemble spiders but have segmented opisthosoma in which tergites are divided into three longitudinal sections.
- Chelicerae are blade-like but not modified for venom injection. Nevertheless, the presence of filtration hairs around mouth indicates that food was digested externally
- Pedipalps unspecialized (but some bear a small claw.)
- Bear two median simple eyes and two lateral eyes that often contain clusters of ommatidia
- Primarily known from specimens from the Rhynie Chert (Carb.)
Recently, the gait of trigonotarbids has been digitally reconstructed, resulting in a viral video.
Plesiosiro madeleyi from Wikipedias
- Haptopoda (Late Carb.): Poor known from a single genus (right). Broadly resemble fossil opliones.
- Uraraneida (Devonian - Permian.): Spider-like, but:
- possessing a rudimentary flagellum (as in Thelyphonida) - plesiomorphic?.
- possessing silk glands without proper spinnerets.
- Idmonarachne: (Late Carboniferous) Almost a spider. Like Uraraneida without the flagellum.
After Schultz, 2007
Arachnid phylogeny:Here be dragons. Schultz, 2007 underscores the basic problem in a morphological analysis. He (and others) identifies one likely clade - Tetrapulmonata, containing spiders and the various whip scorpion groups. however he can't resolve other relationships near the base of Arachnida.
Sharma et al.'s 2014 phylogenomic analysis suggests that very rapid evolution of portions of the arachnid genome swamps phylogenetic signal. Correcting for this by using only slowly evolving portions of the genome, they reach the surprising conclusions that:
Is our assumption that chelicerates became terrestrial only once valid at all? Stay tuned!
- Derek E. G. Briggs, Derek J. Siveter, David J. Siveter, Mark D. Sutton, Russell J. Garwood, and David Legg. 2012. Silurian horseshoe crab illuminates the evolution of arthropod limbs. Proceedings of the National Academy of Sciences 109(39): 15702Š15705.
- James C. Lamsdell, Simon J. Braddy, and O. Erik Tetlie. 2010. The systematics and phylogeny of the Stylonurina(Arthropoda: Chelicerata: Eurypterida). Journal of Systematic Palaeontology 8(1): 49-61.
- David A. Legg, Mark D. Sutton & Gregory D. Edgecombe. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4(2485).
- Jerome C. Regier, Jeffrey W. Shultz, Andreas Zwick, April Hussey, Bernard Ball, Regina Wetzer, Joel W. Martin, and Clifford W. Cunningham. 2010. Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature 463, 1079-1083.
- Jeffrey W. Schultz. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150(2): 221-265.
- Prashant P. Sharma, Stefan T. Kaluziak, Alicia R. Perez-Porro, Vanessa L. Gonzalez, Gustavo Hormiga, Ward C. Wheeler, Gonzalo Giribet. 2014. Phylogenomic interrogation of Arachnida reveals systemic conflicts in phylogenetic signal. Molecular Biology and Evolution August 2014.
- O. Erik Tetlie and Michael B. Cuggy. 2007. Phylogeny of the basal swimming eurypterids (Chelicerata; Eurypterida; Eurypterina). Journal of Systematic Palaeontology 5(3).