Phylogeny of Arthropods Phylogeny of Echinoderm Monophyletic Theory Polyphyletic Theory Asterozoans Hypothesis Cryptosyringed Hypothesis

1. NAME: ANSARI KAUSAR MOHD KHALID
ID NO: 7763
ROLL NO: 08
CLASS: MSC-I (2022-23)
SUBJECT: PAPER-1 (NON-CHORDATES)
DEPARTMENT OF ZOOLOGY

2. 1
➢ PHYLOGENY OF ARTHROPODS:
▪ MONOPHYLETIC THEORY
▪ POLYPHYLETIC THEORY

3. 1) Traditionally, arthropods have been included in a single phylum of metameric,
coelomate animals.
2) Arthropods share many features with annelids, such as metamerism and a nervous
system consisting of supra oesophageal ganglia, nerves encircling the esophagus,
and a ventral series of segmental ganglia.
3) Such similarities led to claims that the two phyla are related and that Arthropods
likely evolved from annelid like ancestors, but this idea is not supported by current
research.
4) Annelids, molluscs, and several minor phyla, all sharing similar larval types and
protostome development, are now placed in superphylum Lophotrochozoa.
5) Based on 18S ribosomal RNA sequence data, phylum Arthropoda is considered a
member of superphylum Ecdysozoa, i.e., animals that molt, along with Nematoda
and other smaller phyla such as Nematomphora, Onychophora (wormlike tropical
and subtropical organisms), and Tardigrada (water bears).
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➢ PHYLOGENY OFARTHROPODS:

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5. ➢ MONOPHYLETIC THEORY:
▪ Based on observation that many features such as the exoskeleton, open
circulatory system, hemocoel, etc. are shared by nearly all taxa within the
group and appears to be homologous(i.e. Have the same evolutionary
origin).The theory supports the strong evidence that all groups like
Myriapods, Crustaceans, And Chelicerates must have evolved from a
common ancestors.
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6. ▪ The monophyletic classification scheme, proposed by
BOUDREAUX, recognizes three evolutionary lineages
within the Phylum Arthropod: Trilobita, Chelicerate,
And Mandibulata. (See clad:2)
▪ BOUDREAUX'S classification scheme excludes
Onychophorans because they lack a true exoskeleton,
but other workers disagree. MEGALITSCH AND
SCHRAM(1991) regard the Onychophora as Closely
Related To Myriapods (See clad:3). In their
classification, Crustacea And Chelicerate are the most
primitive groups. Insects, Myriapods And Onychophora
are grouped together in the Super Class Uniramia
because of similarities in leg structures and locomotion.
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7. ➢ THE POLYPHYLETIC THEORY:
If we embrace the idea of polyphyletic origin, then arthropods are represented by as many as
four phyla each of which is presumed to have evolved separately from primitive annelid
ancestors:
1)Trilobita: 4,000 sps.+ including all trilobites (extinct marine organisms
we're abundant during the Palaeozoic era.)
2)Chelicerata: 70,000 sps. - including spiders, scorpions, mites, ticks,
horseshoe crabs, and seaspiders
3)Crustacea: 30,000 sps. - including shrimps, crabs, lobsters, woodlice, bamacles,
amphipods, branchiopods, and copepods
4)Uniramia: 1.2 million sps. - including onychophora, millipedes, centipedes, pauropods,
symphylans, and insects.
Embryological development of head and mouth parts has also been offered as evidences to
support the polyphyletic hypothesis. In myriapods and insects, the head is a separate
functional region. But in the crustacea and chelicerate, the head and thorax develop together
as a single body region, the cephalothorax. Furthermore, within the myriapods and insects
there is evidence that additional segments are added to form mouthparts, suggesting that
mouthparts of chelicerates crustaceans and other arthropods are not homologous
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8. ➢ PHYLOGENY OF ECHINODERMS:
▪ ASTEROZOAN HYPOTHESIS
▪ CRYPTOSYRINGID HYPOTHESIS
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9. ➢ PHYLOGENY OF ECHINODERMS:
Echinoderm phylogeny leads us to recommend the following changes
to the higher level classification of echinoderms:
1) The phylum Echinodermata includes only those groups with radial
symmetry.
2) The more traditional view is that the first echinoderms were
sessile, became radial as an adaptation to that existence, and then
gave rise to the free moving groups.
3) All evidence indicates that crinoids are the most basal.
4) Urchins and sea cucumbers are generally considered to form a
clade of the most derived.
5) It remains unclear whether the brittle stars group more closely with
this clade or with starfish, due to conflicts between molecular,
morphological, and embryological evidence.
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11. Several competing hypotheses to explain the
PHYLOGENY OF ECHINODERMS have been
developed over the years, including two contrasting
hypotheses for relationships among the five living
classes. In one, called the Asterozoan Hypothesis,
Ophiuroidea and Asteroidea are sister taxa and form a
clade called Asterozoa, whereas Holothuroidea and
Echinoidea form a clade called Echinozoa (with
Crinoidea sister to Asterozoa + Echinozoa). In the
second, called the Cryptosyringid Hypothesis,
Ophiuroidea is placed as the sister taxon to
Holothuroidea + Echinoidea. Recent molecular
phylogenetic analyses have supported the Asterozoan
hypothesis, which we similarly favor here.
➢ ASTEROZOAN HYPOTHESIS AND CRYPTOSYRINGID HYPOTHESIS:
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12. ➢ REFERENCE:
▪ INVERTEBRATE ZOOLOGY BY P.S. DHAMI AND J.K. DHAMI
▪ https://www.slideshare.net/MuznaKashaf1/arthropod-phylogeny-and-
classification
▪ https://www.slideshare.net/bhumikakapoor12/evolutionary-history-
of-insects
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13. THANK-YOU ☺
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