Ecdysozoa III: Myriopoda and Pancrustacea


Recall that Mandibulata encompasses the living arthropods characterized by mandibles and two pairs of maxillae. This group encompasses over 75% of living animal diversity. With that much diversity, it is a wonder that with the application of phylogenetic techniques and the illumination offered by fossils, we understand its evolutionary history as well as we do. In this presentation we follow the results of Legg et al. 2013.

Living Mandibulate Diversity:

Falls into two major groups:

Scolopendra galapagoensis
Myriapoda: (Sil. - Rec.) Including


Sally lightfoot - Grapsus grapsus
Pancrustacea: (C. - Rec.) Including

Stem Mandibulata: Mandibulata seems to have originated during the Cambrian, with organisms on the mandibulate stem occurring in the earliest Cambrian. Stem mandibulates include: Note: All of the tiny bivalve taxa discussed above, along with living analogs, seem to develop from larvae possessing head appendages only, then to add segments and appendages posteriorly with each molt. This gives us confidence that their anterior head appendages are antennae. For our purposes, the presence of these appendages is a synapomorphy of total-group Mandibulate.

Myriapoda: (Sil. - Rec.)

Mandibulate arthropods with honomous trunks. Whether they represent the first fully terrestrial animals depends on the resolution of claims that some early scorpions were actually aquatic. They were major components of the Silurian and Devonian land faunas, achieving large size as predators and as detritivores and grazers.

Potential synapomorphies:

Diversity Four monophyletic groups are known: Most recent phylogenetic analyses (E.G.: Shear and Edgecomb, 2010) reveal the phylogeny at right. Symphyla and Pauropoda have no fossil record except in Neogene amber, so we focus on the other two groups.


Narceus americanus from Wikipedia
Diplopoda (millipedes: mostly herbivores and detritivores. Sil - Rec.)
Fossil diplopodans:


Scutigera coleoptrata from Dirk's Pest Management Specialists
Chilopoda: (centipedes; primarily predators. Sil. - Rec.) Despite having a homonomous trunk, centipedes achieve a degree of biomechanical diversity, with some specialized for fast running and compact bodies (Scutigera right) and others being more serpentine.

Tagmosis: Centipedes have a single pairs of limbs, spiracles, etc. per body segment (plesiomorphic). Head appendages are plesiomorphic for Myriapoda:

Chilopod synapomorphy: The first trunk limbs are modified as poison-injecting forcipules and co-opted into the feeding apparatus.

Chilopod fossil record:


Pancrustacea:

Pancrustacea (equivalent to "Tetraconata" of some authors) contains all traditional crustaceans and hexapods. Work of the last decade has produced the growing consensus that "crustacea" is paraphyetic with respect to Hexapoda, but there is no clear consensus of exactly where, within Pancrustacea, hexapods go. For now, we follow the morphology-based analysis of Legg et al. 2013., but interested students should also review the molecular result of Regier et al., 2010.


Nauplius larva from Vatten-Kikaren
Synapomorphies of Pancrustacea, involved soft tissue characters of the brain and the shape of ommatidia, and exceed the technical limits of this course. Indeed, the whole topic is so complex that we can only scratch the surface. Beyond possessing the synapomorphies of Mandibulata, Pancrustacea displays:


Pancrustacean head comparison from The Snodgrass Tapes
Aquatic "crustaceans" are the only mandibulates to retain: but these are synapomorphies of the total-group Mandibulata. Independently lost in Hexapoda and Myriapoda.


Pancrustacean tagmata from Crustacea - The University of Bristol
Crustaceans ancestrally display tagmosis of the post-cepalic body into:


Major Pancrustacean Groups

Before discussing pancrustacean phylogeny, an inventory of major monophyletic groups:


Chrissia formosa from Ostracod Research at the Lake Biwa Museum
"Ostracoda" (ostracods) (Ord. - Rec.): Tiny bivalve planktonic crustaceans. Characteristics: Because they evolve rapidly, are widespread, and their valves are robust fossils, ostracods are classic index fossils.

Until recently, Ostracoda and Thecostraca (the barnacles) would have been united in the clade Maxillopoda however recent analyses do not find this to be so.


Tetraclita milleporosa
Thecostraca (Barnacles (Cirripedia) and their kin.) (Cam. - Rec.)

Characteristics:

Two major groups, each with its own pattern of adult plates: Because they can form, in essence, barnacle "reefs" they have a very good record. The earliest putative barnacle: Priscansermarinus barnetti - a possible lepadomorph from the Burgess Shale.


Tanymastix stagnalis by JFCART from Flickriver.com
Branchiopoda ("sea-monkeys," water fleas, and clam shrimp) (Poor record - Cam. Rec.)

Characteristics:

Fossil record: The earliest branchiopod is Rehbachiella from the Late Cambrian of Orsten. (Reconstruction) Although originally marine, branchiopods today mostly occur in fresh-water. Indeed, brine shrimp (sea monkeys) have developed a life cycle in which eggs are able to withstand dessication for long intervals. Thus, new generations can repeatedly colonize ephemeral pools of water.


Copepopd from Florida Medical Entomological Laboratory
Copepoda (Minute marine and fresh-water plankton) (Poor record - Carb. - Rec.)

Characteristics:



Cephalocarid from UC San Diego - Marine Invertebrate Phylogenetics Lab
Cephalocarida (No fossil record): Horseshoe shrimp. A minor group with only twelve species. Strictly shallow-marine inhabitants of intertidal zone. Morphologically simplified, with no pleopods, and second antenna shifted posterior to mouth in adults.


Panulirus argus from NOAA Fishwatch
Malacostraca (Cam. - Rec): The more familiar crustaceans.


Spelonectes sp. from UC Berkeley Museum of Paleontology
Remipedia: (Carb. - Rec.), Discovered in 1981, these morphologically, are the most primitive looking crustacean group, with homonomous bodies, but for the characteristic crustacean tagmosis of cephalic appendages. Most live in submerged caves and places of permanent shade in brackish-saline aquifers. Thus their eyes are (presumably) secondarily reduced. Characteristics: Remipedes were discovered as the fossil Tesnusocaris goldichi (Carboniferous).


Silverfish from UC Berkeley Museum of Paleontology
Hexapoda (Dev. - Rec.):

Proper insects and their close relatives.

See below for more information. But there are significant differences. Alas, insect cuticle is not calcified. Thus, their fossil record is poor compared to that of crustaceans (though not as bad as you might think). what there is comes largely from Konzervat-Lagerstätten



Pancrustacean Phylogeny:


The current state of the art is unsettled, with large areas of disagreement between molecular and morphological results:

Points of Agreement:

Significant Points of Disagreement:

We await a future consensus.

Malacostraca:


Eocarid Anthracaris gracilis from Mazon Creek Fossils
Malacostraca (Shrimp, lobsters, krill, isopods, mantis-shrimp, crabs) (Cam. - Rec.)

Characteristics:



Hoplocarid Odontodactylus scyllarus from Wikipedia
Major malacostracan groups:

Hexapoda:


Bristletail from Flickr
The weird part: Hexapoda (Dev. - Rec.): Proper insects and their close relatives. Closely related to Remipedia in current results. Ancestrally each group is adapted to life underground, the remipedes in saline aquifers, the hexapods in moist soils.

Alas, insect cuticle is not calcified. Thus, their fossil record is poor compared to that of crustaceans (though not as bad as you might think). What there is comes largely from Konzervat-Lagerstätten



Insect head appendages from Scioly.org
Hexapod characteristics: Hexapods are significantly different from other pancrustaceans in their tagmosis.

Hexapod Phylogeny: (Taken from Tree of Life.)

Basal hexapods are sometimes termed "Parainsecta" and include creatures like:

All are soil-inhabiting detritivores and predators.

The basal proper insects form part of a paraphyletic grade group - "Apterygota," wingless insects, including:

These are distinct from basal hexapods in being larger and spending time on the surface. Insecta is diagnosed by a list of technical anatomical synapomorphies.


Doomed adult mayfly
Within Insecta is Pterygota, the winged insects (first organisms to fly). In most, a pair of thoracic wings (arguably derived from the exopodite ramus) is matched with the posterior two limb pairs. In some extinct forms, there was a third small pair anteriorly, making a wing pair corresponding to each leg pair. The evolutionary origin of wings is enigmatic, but a leading hypothesis holds that they are homologous to the dorsal ramus of ancestral biramous appendages.

Pterygota incudes:



Strudiella devonica from Nature - Garrouste et al., 2012
The hexapod fossil record:

Outstanding issues:

Dissertation, anyone?

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