Evolutionary History of the Arthropods

The phylum Arthropoda is closely linked with two other phyla, the velvet worms, or Onychophora (see diagram below), and the water bears, or Tardigrada.  A nonliving cuticle coverers members of both phyla, and they have appendages.  However, the cuticle is not hard, and the appendages are not jointed.  They resemble worms in some ways – they are elongate, and their body regions are not highly differentiated.  Examine the specimens of velvet worms on display.  Living velvet worms are terrestrial, found in moist environments in the tropics.  They are carnivores, tangling up their prey in a kind of sticky goo they secrete.
Fossils of animals that can be identified as arthropods are abundant in deposits dating back at least to the Cambrian period.  These fossils, and the most basal members of living arthropods groups show the ancestral condition of the body in arthropods:  similar body segments and similar appendages.  That condition can be illustrated by this cartoon:

However, two major evolutionary trends have occurred in the arthropods:  tagmosis, or the fusion of segments into different body regions (or tagma), and the specialization of appendages in the different body regions.  Here is a cartoon illustrating a derived state within the arthropods:

This illustration shows an arthropod like an insect, with three tagma:  head, thorax, and abdomen.  The appendages on the head are specialized for sensation (like the antennae) or for manipulation of food.  The segments on the head are highly fused and compressed together.  The segments in the thorax are enlarged, serving as a solid fulcrum for the action of the more elongated walking legs.  The abdomen houses internal organs, and the appendages on the abdomen are often highly reduced.

Examine some of the examples of tagmosis and specialization of appendages shown in the displays.  The patterns of tagmosis and specialization of appendages differ among groups of arthropods.  Some groups do not have separate head and thorax regions, but instead have a cephalothorax, but the functions of the head and thoracic regions even in these arthopods maintain their distinctiveness.

Among other things, the patterns of tagmosis and the types of appendages (especially on the head) serve to distinguish the major groups of arthropods.  Here is a table that you can use as reference in distinguishing the living groups of arthropods:

 

Chelicerata

Crustacea

Myriapoda

Hexapoda

Head Appendages

Chelicrae       Pedipalps

 

 

2 pairs antennae Mandibles

1st & 2nd Maxillae

 

1 pair of Antennae Mandibles

1st & 2nd Maxillae

 

1 pair of Antennae Mandibles

1st & 2nd Maxillae

 

Tagma

Cephalothorax

Abdomen

Variable (often a cephaplothorax is present)

Head, thorax, and abdomen, but not strongly differentiated

Clearly differentiated head, thorax, and abdomen

Appendages

Uniramous (primitively biramous)

Biramous

 

Uniramous

 

Uniramous

 

Other Features

 

Calcium carbonate in exoskeleton

 

Wings in most insects

 
The phylogenetic relationships among these four groups is still subject to debate.  Some phylogenies link the crustacea with the chelicerates, in part because both groups at least primitively had biramous appendages.  The myriapods and hexapods were linked in a group called the “Uniramia,” because of their uniramous appendages, and members of the uniramia also have similar head appendages.  More recent phylogenies based on molecular data have linked the crustacea and the hexapoda, with the myriapods branching off earlier, and the chelicerates basal to all the others.  Try drawing the cladograms for these two scenarios, and see how you would account for the evolution of mandibles in each.