Origins Flashcards
Impact on humans
Provides food and ecosystem services such as pollination. They do however also spread disease and damage crops.
Why so many arthropods?
Success is probably due to small size allowing them to occupy a greater variety of ecological niches and smaller actual physical niches. Also due to water conservation techniques, water conserving excretory system and tracheal breathing.
Diversity of Arthropoda
Non insect arthropods make up around 20% of all arthropods. Arthropods mainly consists of beetles, flies, butterflies, moths, bees, wasps, spiders and crustaceans.
Six Arthropoda Phyla
Tardigrada Pentastoma Onychophora Chelicerata Uniramia Crustacea
Lobopod arthropods
Soft bodied, non jointed legs with no exoskeleton. Have fleshy appendages along their bodies.
Consists of the phyla tardigrada, pentagonal, Onychophora.
Tardigrada and Onychophora
May be survivors of the lobopod proto arthropods of the Cambrium.
Onychophora have bits of chitin embedded in their skin, and Tardigrades have external cuticular plates.
Pentastoma
Lobe limbs may be secondary, and they have been considered a degenerate Crustacea.
Controversy of the phylogeny of Arthropoda
The worms and arthropods shared a common ancestor in Cambrian. Both bodies would have had a number of serially repeated legs.
At one stage Onychophora were thought to be and intermediate form between the worms and the Arthropoda, but molecular evidence suggests that they are a sister group to Arthropods.
Cambrian
550 million years ago
Onychophora Features
It has a segmented body, with a soft cuticle and chitin similar to that of arthropods. It moult and has tarsal claws, antennas, a haemocoel, mandibles and a trachea which are all common in arthropods as well. However the features it shares with annelids are a segmented body, soft cuticle and paired nephridia.
Nephridia
A tubule open to the exterior which acts a s an organ of excretion or osmoregulation. It typically has ciliated or flagellated cells and absorptive walls.
True Arthropods
Chelicerata
Uniramia
Crustacea
Panarthropods (True Arthropods and Lobopods)
Mostly a monomeric body plan, haemocoelic body cavity, but with serially repeated legs ending in claws, and usually a hard exoskeleton and ventral nerve cord. Onychophora and Tardigrada have body designs intermediate to that of Annelids and true Arthropods.
Metameric
Consisting of several similar segments.
Monomeric
Comprising of a single segment.
Basic body design of an arthropod
The metameric part of the body contains segmental arrangement of all of the organ systems associated with the limbs. Non leg associated organs (excretion, and reproduction) are not segmentally arranged or repeated. This basic pattern has undergone considerable change in the various groups, by means of fusion of segment, specialization of segments and loss of appendages etc. Apart from the cuticle, arthropods have few common features (compound eyes absence of cilia). This suggests that the arthropod condition might represent a type of body design achieved by unrelated organisms, not a monophyletic assemblage.
Jointed hard exoskeleton and moulting
Characteristic of true arthropods. Exoskeleton is made of chitin and protein and sometimes impregnated with calcium carbonate. It restricts growth, thus moulting is required, with rapid growth in between. From a phylogenetic perspective the exoskeleton probably arose as bands of chitin across a body in an organism which had a hydrostatic skeleton such as in Tardigrades. Some arthropods still use a haemocoel to extend the legs.
Moulting in true Arthropods
The old exoskeleton is partially reabsorbed before the moult, then is shed. A soft animal emerges which is vulnerable and thus grows a new exoskeleton beneath the old one. The new skeleton takes a while to harden allowing the organism to grow in size.
Ancient arthropods
In the Cambrian there were many diverse Lobopods and a range of bizarre Arthropods. Few of these survived that period, and only five major groups survived beyond the Devonian.
Devonian
350 million years ago.
Hallucinogenia
One of the strangest animals to have existed, only known from Cambrian fossils that are missing heads. Reconstruction was made after event discovery of fossils with heads.
Trilobita
Survived for 300 million years, but went extinct in the Permian.
Classes of Chelicerata
Merostomata
Arachnida
Pycnogona
Orders of Arachnida
Scorpiones Solpugida Opiliones Acariformes Araneae
Phylum Chelicerata
The body is divided into the anterior prosoma, which is covered by a dorsal carapace. The posterior opisthosoma lacks legs, and generally also any appendages. Appendages are uniramous, and compound eyes acre typically absent. Chelicerae form mouthparts but parts of other limbs bases near mouth may assist in crushing food. It has a non calcareous exoskeleton.
Opisthosoma
Abdomen
Prosoma of Chelicerates
Acron and six appendage bearing segments.
Horseshoe Crabs
Probably the ancestors of the remaining Chelicerata. They have external gills on appendages of the opisthosoma.
Chelicerata Book lungs
Resembles external gills that have been invaginated. Resembles a stack of plates, but in some they are tube like and form sieve like/tube trachea.
Debate on the evolutionary relationships between the Hexapoda, Myriapoda, Crustacea and Chelicerata
Many do not support the phylum uniramia, and the most recent evidence suggests that Hexapoda and Crustacea may be sister groups and form the Pancrustacea. In this system, the Myriapoda are either related to the Pancrustacea or the Chelicerata.
Mandibulata model vs Paradoxopod model
Both the Mandibulata and paradoxopoda models propose a pancrustacean Claude consisting of Hexapods and Crustaceans as closely related sister groups. They differ in their treatment of the Myriapods and Chelicerates. Paradoxopoda has Myriapods and Chelicerates as closely related sister groups whilst Mandibulata has Myriapoads as a sister group to the Pancrustaceans and the Chelicerates as a distant, early branching Claude of arthropods. While the Mandibulata is supported by taxonomy (crustaceans, Hexapods, Myriapods have mandibles and Chelicerates have chelicerae), Paradoxopodia is primarily supported by recent mitochondrial molecular phylogenetics. Also under contention is the relationship between crustaceans and Hexapods within the increasingly well supported Pancrustacean clade. It has not yet been settled if the two are distinct sister groups or if the Hexapods are nested wishing the Crustaceans. The latest studies which used a wider number of species in the analysis, plus a greater number of genes supports the hypothesis that Hexapoda are nested within Crustacea.
