GEOL 331 Invertebrate Paleontology

Fall Semester 2008
Arthropoda II Crustacea sensu lato

This lecture used to be known as "Arthropoda II Mandibulata." Mandibulata was the name given to a clade containing Myriapoda, Hexapoda, and Crustacea; the arthropods with mandibles (jaw-like mouthparts). More recent analyses placing Hexapoda within Crustacea render this term obsolete.

Synapomorphies of Crustacea, as currently defined would exceed the technical limits of this course. For practical purposes we will continue to address ancestrally aquatic crustaceans - "Crustacea" in the traditional sense, and their terrestrial descendants, Hexapoda, separately.

Aquatic Crustacea. Extremely morphologically diverse, but with distinct trends:

United by the following morphological synapomorphies:

Crustaceans ancestrally display tagmosis of the post-cepalic body into:


Crustacean Phylogeny: (Taken from molecular phylogeny of Regier et al., 2005. Note, two authors, Schultz and Kambic are from UMD. Kambic is a survivor of GEOL331!)

"Ostracoda" (ostracods) (Cam. - Rec.): Tiny bivalve planktonic crustaceans. Characteristics:
  • Small: Typically around 2 mm. Largest living species up to 30 mm. Largest fossil on record was 80 mm.
  • Lives within an expanded bivalve carapace. This is homologous to the carapace of other crustaceans, but with a mid-line hinge, allowing to to be closed over the rest of the animal.
  • The posterior body is greatly reduced.
  • Seven pairs of appendages are present, of which the first five are the antennae and mouthparts. These do double-duty as swimming organs.
Because they evolve rapidly, are widespread, and their valves are robust fossils, ostracods are classic index fossils.

Until recently, Ostracoda and Cirripedia (the barnacles) would have been united in the clade Maxillopoda. Regier et al find them to be:

  • Far from Cirripedia at the base of Crustacea
  • Paraphyletic, to boot.

It will be interesting to see if subsequent morphological analyses confirm their result.


Remipedia (no fossil record), 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. Thus their eyes are (presumably) secondarily reduced. Morphologically adapted from swimming on their backs.


Cephalocarida (No fossil record): Horseshoe shrimp. A minor group with only nine species. Strictly shallow-marine. Morphologically simplified, with no pleopods, and second antenna shifted posterior to mouth in adults.


Branchiopoda ("sea-monkeys," water fleas, and clam shrimp) (Poor record - Sil. Rec.)


  • Small, <10 mm.
  • Variable number of thoracic limbs
  • Basal section of thoracic limbs developed at paddles. Hence the name "gill-footed."
  • 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.


Cirripedia (barnacles) (Ord. - Rec.)


  • Begins life normally enough, as small crustacean with bivalve carapace. Soon attaches by the head to hard substrate, sheds carapace and secretes a set of distinct calcareous plates that are non-homologous to those of other arthropods.
  • Thoracic limbs modified to form suspension-feeding cirri.
Two major groups, each with its own pattern of adult plates:
  • Balanomoprha: Acorn barnacles.
  • Lepadomoprha: Goose-neck barnacles. The adult plates are non-homologous with those of balanomorphs (?) and the creature is anchored to the substrate by means of a fleshy peduncle.
Because they can form, in essence, barnacle "reefs" they have a very good record.


Copepoda (Cretaceous - Rec. but almost no record): Tiny planktonic crustaceans. Feed on unicellular algae, although some are parasitic on other animals. The Cretaceous record is of a copepod parasite on the gills of a fish.


Malacostraca (Shrimp, lobsters, krill, isopods, mantis-shrimp, crabs) (Cam. - Rec.)


  • The common ancestor was probably a shrimp-like creature.
  • Eight pairs of thoracic appendages broken down into:
    • three pairs of maxillipeds for food manipulation
    • five pairs of walking limbs. Note: these remain biramous, even though the gills are typically enclosed by the carapace.
    • up to three anterior pairs of walking limbs may bear chelae (lobster pincers are robust examples)


Malacostracan Phylogeny: (Taken from Tree of Life.)

Major malacostracan groups:


The weird part: Hexapoda (Dev. - Rec.) - the putative sister taxon of Branchiopoda:

Proper insects and their close relatives. The branchiopod connection is not crazy.

  • Ancestrally, both groups are very small and are adapted to life in and around small bodies of water.
  • Like crustaceans, the insect body is tagmatized into a cephalon, thorax, and abdomen

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


Hexapod characteristics:
  • Tagmatization:

    • The head bears:
      • Compound eyes and ocelli .
      • One pair of antennae
      • Mandibles
      • First maxillae
      • Fused second maxillae form a labium or lower lip .
    • the head is set off from the thorax by a flexible joint, unlike in aquatic crustaceans
    • The thorax bears:
      • three pairs of uniramous walking legs
    • The abdomen bears:
      • eleven segments, each with styli
      • Sensory cerci posteriorly
      • Some have a median abdominal filiment (silverfish, mayflies)
    • Ventilation is exclusively by means of a tracheal system, and which tracheae take in air through spiracles and deliver it directly to the tissues. The tracheae are lined with cuticle that is shed during ecdysis. Note that by this means, insects are able to achieve aerobic scopes approaching 300 (compare to 10 for mammals).


Hexapod Phylogeny: (Taken from Tree of Life.)

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

All are soil-inhabiting detritivores.

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.

  • Within Insecta is Pterygota, the winged insects (first organisms to fly). In most, a pair of thoracic wings 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. Pterygota incudes:

    • Paleodictyopteroidea: Extinct group of large bodied insects (largest with 50 cm. wingspans.) with three wing pairs and piercing/sucking mouthparts.

    • Ephemoptera: (Mayflies) Spend most of life as fresh-water aquatic opportunistic feeders. They breathe using gills derived from abdominal styli (I.e. the plesiomorphic arthropod pattern.) Mayflies metamorphose into winged adults for a brief life of mating. As adults, they have no functional mouthparts.

    • Odonata: (Dragonflies and damselflies): Extremely skillful fliers and aerial predators. Spend their youth as aquatic predators (using specialized protrusible labium to snare prey), then metamorphose into winged adults. Unlike mayflies, they spend months as adults feeding and mating. Soem odd things: It is tempting to assume that odonatans ancestrally had life cycles like mayflies, and yet:
      • Although fossil adult odonatans were around in the Pennsylvanian, we find no fossils of their aquatic nymphs until the Jurassic.
      • The most phylogenetically basal odonatans lay eggs and spend their youth in moist soil and leaf litter, not in bodies of water.
      • Different structures perform the function of gills in different odonatan groups.
      Is it possible that the ancestral odonatan had no aquatic stage?

      Note: before considering the last pterygote group, consider what the life cycle of these "primitive pterygotes" tells us: Unlike apterygotes, pterygotes undergo a partial metamorphosis from and aquatic nymph to a terrestrial imago (adult).

  • Neoptera: (Other winged insects)

    • Evolved a hinged articulation by which the wings could be folded over the back. An obvious advantage if, (unlike dragonflies) you planned to spend time on the ground. This made possible a striking radiation of flying/walking insects. The real breakthrough came when Coleoptera (beetles) transformed their forewings into elytrae (wing covers) to protect the hindwings while on the ground. This made possible behaviors like burrowing and the invasion of microhabitats that might otherwise damage wings. Thus, beetles make up roughly 50% of animal diversity.

    • Neopterans also undergo a complete metamorphosis from a totally dissimilar larva (Latin for "mask") into an imago (Latin for "image") via a transformational pupa stage.

  • Japanese beetle larva.

    Japanese beetle pupa.

    Japanese beetle imago.

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