Zoology 250 Phylogenetic Trees (2017)

(= arthropods and nearest relatives;

see also an alternative molecular phylogeny)

(tree and traits modified from
Ruppert et al. 2004 p. 540, and
Brusca & Brusca 2003 p. 501, 875)


                    ===========O=== Onychophora (velvet worms; fossil Hallucigenia)
<<=======P==========|      ====T=== Tardigrada (water bears)
                           ====A=== TO ARTHROPODA ====================>

Back to Zool 250 tree for animal phyla or Protostomia

TRAITS SUPPORTING EACH CLADE (** plesiomorphic- a primitive state, not unique to clade):
a) elongate body of multiple similar segments**
b) ventro-lateral legs (lobopods) with hooked tips
c) must molt to grow
d) chemical makeup of cuticle (alpha-chitin and non-collagenous protein)
e) epidermal cilia lost
f) extensive hemocoel ('open' circulatory system)
g) coeloms reduced to nephridia & gonads
h) heart with openings to hemocoel (ostia)
i) panarthropodan sensilla
j) paired, segmental, saccate nephridia
a) external segmentation suppressed
b) unique oral papillae
c) possess tufted & unbranched tracheae
a) tri-partite cerebral ganglia (proto-, deuto-, & tritocerebrum) (Harzsch 2004 Evol. & Dev. 6:303)
b) loss of circular muscle layer
a) loss of heart
b) loss of nephridia
c) 4 pair stubby, uniramous, unjointed or weakly jointed legs
d) claws of anterior legs modified into stylets & stylet supports
e) tri-radiate, myo-epithelial sucking pharynx
f) eutelic development
a) jointed appendages with intrinsic muscles
b) 1 pair lateral compound eyes
c) 4 pair median ocelli
d) two tagmata (head, trunk)
e) calcification of cuticle
f) fully segmental, articulated sclerites (body-wall cuticular plates)
g) each body segment has 2 sclerites (dorsal tergum, ventral sternum)
h) two-part appendages: basal protopod & distal, multisegmented part (telopod)
i) complete loss of motile somatic cilia/flagella (some sperm retain flagella)
j) unique hemocyanin respiratory pigment
k) larva with 3 pair of appendages
l) 6 pair of saccate nephridia (4 in head, 2 in trunk)

(pycnogonid sea spider)

(horseshoe crab)

(true spider)
(see also an
alternative molecular phylogeny)

(tree and traits modified from
Ruppert et al. 2004 p. 540, 720, and
Brusca & Brusca 1990 p. 485, 538, 589, 598, 659, 683, 691;
taxa in FULL UPPER CASE indicate the four arthropod subphyla)

(diplopod myriapod)

(chilopod myriapod)

(extinct trilobite)

                                            ===P=========== Pycnogonida (sea spiders)
   ==============CHELICERATA================|    ==X======= Merostomata (horseshoe crabs)
   |                                        ==E==|
   |                                             ==A======= Arachnida (spiders, mites, scorpions, etc.)
<<=|   ==========TRILOBITOMORPHA=========================== (extinct trilobites)
   |   |
   |   |                                         ========== Symphyla (symphylans)
   |   |                                    ==S==|
   |   |                                    |    ========== Diplopoda (millipedes)
   |   |                     ===Myriapoda===|
   =T==|                     |              =============== Chilopoda (centipedes)
       |                     |
       |  =======UNIRAMIA====|           =====Entognatha=== Collembola (springtails)
       |  |                  |           |
       |  |                  ==Hexapoda==|               == Thysanura (silverfish and firebrats)
       |  |                              =====Ectognatha=|
       =M=|                                   (Insecta)  == to Pterygota (winged insects) ===>
          ======= TO CRUSTACEA =======================================================>

Back to Zool 250 tree for animal phyla or Protostomia

TRAITS SUPPORTING EACH CLADE (** plesiomorphic- a primitive state, not unique to clade):
a) two tagmata (cephalothorax=prosoma, no distinct head; abdomen=opisthosoma)**
b) cephalothorax= acron + 7 segments with legs
c) seg.#1 lost (no antennae)
d) mouth lies before segment 2 embryologically
e) seg.#2 limbs= chelicerae (pincer-like or fangs)
f) seg.#3 limbs= palps or pedipalps
g) gonopores on abdomen segment #2
h) compound eyes
P= Pycnogonida:
a) pre-oral proboscis
b) ovigers (unusual 3rd pair limbs)
c) abdomen (=opisthosoma) reduced or absent
d) long, 9-segmented walking legs
e) multiple pairs of gonopores on some/all legs
f) compound eyes lost; 4 median ocelli on turret
a) cephalothorax has a carapace shield
b) 1st or 2nd abdomen segment= genital somite
c) 2 median eyes
X= Xiphosura:
a) enlarged cephalothorax with large carapace
b) abdominal limbs modified as book gills
c) long, spiked telson
d) segmental nephridia share a common pore
A= Arachnida:
a) abdominal limbs reduced, lost, or modified as spinnerets
b) 2nd prosomal segment legs= pedipalps
c) compound eyes lost (living spp.); modified to 5 pairs of single eyes
a) mouth lies before segment 2 embryologically
b) segment #1 limbs= antennae
c) 4 pair post-oral head segments with biramous limbs similar to thoracic limbs
a) 3 tagmata (cephalon, thorax, pygidium)
b) body dorso-ventrally flattened
c) limbs of post-oral head segs. similar to limbs of thorax (all with 2 branches, biramous?)**
a) 2 tagmata (head, trunk of many similar segs.)**
b) 5 pair head appendages (2 pair antennae, 1 pair mandibles, 2 pair maxillae)
c) body segment cuticle of 4 sclerites (tergum, 2 lateral pleura, sternum)
d) mouth anterior to segment 3 in adult
e) ommatidium has crystalline cone & cornea
UNIRAMIA= Tracheata, = Atelocerata:
a) second head segment and second antennae lost
b) mandible lacks palps
c) paired segmental trachea & spiracles associated with terrestrial life
d) uniramous limbs (exopodite lost)
e) 2 pair segmental nephridia (segs. #4 & #5) or lost
a) 2 tagmata (head, trunk of many similar segs.)**
b) no median ocelli; compound eyes loosely spaced
c) loss of palps on first & second maxillae
d) repugnatorial glands
a) maxillae have no palps
a) second maxillae fuse as labium
b) 3-segment thorax with limbs
c) 11-segment abdomen, no limbs
a) mouthparts sunk in a pouch
a) mouthparts not sunk in pouch
a) possess wings on 1 or 2 thoracic segments

$ new evidence suggests Myriapoda branched off before Hexapoda & Crustacea diverged (e.g., these 2 groups both have a 4-part crystalline cone in their ommatidia but myriapods do not (Regier et al. 2010), so Myriapoda may be a separate subphylum (i.e., subphylum Uniramia is rejected); see alternative molecular phylogeny.

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Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)