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).
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.
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.
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.
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).
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.
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.