Gill chambers in basal chordates and vertebrates, pt. 1

Adding taxa without skulls
to the the large reptile tree (LRT, 1611+ taxa) would seem to bring with it a slew of problems. As a solution, the scores “skull absent”, “orbit absent” and “jaws absent” were added last weekend to several of the traits created eight years ago.

A primitive – hypothetical – chordate,
just developing a coelum (middle tissue between skin and intestine) is shown here (Fig. 1). It is not a taxon in the LRT, but serves as a zero point from which derived traits can be added in the present report. Think of it as a worm stiffened longitudinally with a notochord.

No one knows the size of its gill chamber. Early members of this clade likely lacked gills. Oxygen would have been absorbed both externally and internally on this tiny wriggling worm made up of not much more than skin over intestine with a notochord ventral to the dorsal central nerve chord. Tiny food particles would have been processed sometime during the trip from mouth to anus/cloaca. Few to no sensory organs appeared near the oral opening (not quite a mouth yet).

Figure 1. Hypothetical chordate ancestor to known chordates. This is the starting point for looking at gills and throats in basal chordates and vertebrates.

Figure 1. Hypothetical chordate ancestor to known chordates. This is the starting point for looking at gills and throats in basal chordates and vertebrates. The ‘actual size’ is also hypothetical.

A primitive extant chordate,
the lancelet (genus: Branchiostoma; Fig. 2) documents the next stage in chordate evolution and serves as the new outgroup taxon for the LRT. Still lacking a head or anterior sensory organs, Branchiostoma has ring sets of cilia both outside and inside the oral cavity. These are followed by a large atrium / gill chamber lined with slender gill bars for gas exchange. Ventrally a stiff rod, the endostyle, creates a mucous strand that captures food and, using microscopic cilia, carries the particles posteriorly to the simple, linear intestine which terminates in an anus / cloaca, no longer at the tip of the tail (see Fig. 1). Water from the atrium is expelled from a single ventral opening, the atriopore, anterior to the anus/cloaca. The swimming muscles that envelope Branchiostoma from tip to tip evolve to a chevron shape.

Figure 2. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes oxygen, water + food, sends the food into the intestine and expels the rest of the water. It also lacks a head and anterior sensory organs.

A Middle Cambrian ‘lancelet with eyes,’ aka primitive fish,
Metaspriggina (Fig. 3), apparently loses the cilia and atriopore of the lancelet (Fig. 2) and develops seven gill openings lateral to the relatively smaller gill chamber. Those tiny eyes both direct this tiny predator to tinier prey and serve to locate predators, setting off alarms that spur the tail to wiggle seeking a hiding place or to put distance between it and any approaching marauder.

Figure 1. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the placement of the eyes here with Birkenia in figures 2 and 3.

Figure 3. An early jawless, finless, lancelet-like fish from the Cambrian, Metaspriggina. Compare the placement of the eyes here with Birkenia in figures 2 and 3.

Evolving in a different direction
the tunicate (Fig. 4) is mobile only as a juvenile. It become sessile (attached to the seafloor) as an adult. The tail is resorbed and the gill chamber takes over the majority of the body. Traditional workers consider this stage primitive to the lancelet, but they don’t employ a simple worm-like chordate as an outgroup taxon. Tunicates are quite derived relative to lancelets.

FIgure 4. A tunicate diagram turned on its side to replicate the morphology of the lancelet (Fig. 2). Here the atriopore enlarges to become a exit siphon. The cilia are reduced. The gill chamber is greatly enlarged.

FIgure 4. A tunicate diagram turned on its side to replicate the morphology of the lancelet (Fig. 2). Here the atriopore enlarges to become a exit siphon. The cilia are reduced. The gill chamber is greatly enlarged.

Sessile tunicates gave rise to
barrel-shaped, planktonic  tunicates, otherwise known as salps (Fig. 5). They seem simple, but that’s because they have gotten rid of nearly every body part but the atrium / gill chamber. Salps alternate between sexual and asexual generations, each distinct in morphology. The atrium comprises the entire animal. Gonads, an endostyle, simple brain and digestive organs all migrate nside the atrium. The atriopore (exit siphon) rotates opposite to the entrance siphon, creating a little jet engine. Several morphologies have evolved (Fig. 5).

FIgure 5. Salp variety. Here the atrium is the animal with other organs inside the atrium.

FIgure 5. Salp variety. Here the atrium is the animal with other organs inside the atrium. Color helps link homologous elements.

Traditional cladograms
of chordate relationships (Fig. 6) nest tunicates basal to fish. Here both fish and tunicates are derived from lancelets, each evolving in different directions (mobile vs. sessile + planktonic). Tunicates have simplified and degenerated with fewer parts, distinct from the general trend in vertebrate evolution.

Figure 3. Traditional cladogram from Lingham-Soliar 2014.

Figure 3. Traditional cladogram from Lingham-Soliar 2014. Missing from this cladogram are the lancelets. The LRT does not agree with this tree topology.

The above boneless, headless and finless taxa
are all filter feeders with large gill chambers, as are several primitive fish (= armored and bony lancelets) in the LRT. We’ll look at those in future blogposts.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.