Gill chambers in basal chordates and vertebrates, pt. 2

Yesterday we looked at several basal chordates
that use their large atria / gill chambers to capture planktonic prey. Today, we’ll peek at a few morphologies that modify the gill chamber in diverse ways and take planktonic feeding to new levels in prehistoric and extant taxa.

We’ll start with a taxon we looked at yesterday,
the Middle Cambrian first fish: Metaspriggina (Fig. 1). It had relatively few body parts. The torso was dominated by swimming muscles, followed by the much smaller gill chamber, liver, gut, heart and eyes, in that order.

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 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.

Ordovician Arandaspis
took body armor to turtle-like heights with a dorsal carapace and ventral plastron along with armored gill openings (Fig. 2) surrounding this basal fish / advanced lancelet. Compared to Metaspriggina (Fig. 1, the gill chamber of the Panserfische, Arandaspis, was several times larger, still servicing a small oral cavity. The eyes were still tiny and faced only forwards. Evidently Arandaspis wasn’t concerned with prey sneaking up from behind. If swallowed, it might have been too tough to bite and digest. No predatory arthropods could dismantle it. Like ancestral lancelets, perhaps Arandaspis burrowed into sandy sea floors.

FIgure 3. Arandaspis lived inside its gill chamber shell and armored tail.

FIgure 2. Arandaspis lived inside its gill chamber shell and armored tail.

Late Silurian Poraspis
 (Fig. 4) had fewer and larger tail plates. All gill plates were fused into one with a single lateral atrial water exit on the sides. The skull plates remained large and solid, protecting the head, still dominated by the enormous atrium / gill chamber. With the first appearance of a rostrum since the lancelet, Branchiostoma, the eyes moved laterally, protected by knight-like armor. Lateral line sensory canals first appear in such taxa.

Figure 4. Poraspis fuses the lateral gill plates together for greater armor, leaving only a slender single common opening for the exiting water. This was probably a sedentary taxon due to its inability to respire at a great rate.

Figure 3. Poraspis fuses the lateral gill plates together for greater armor, leaving only a slender single common opening for the exiting water. This was probably a sedentary taxon due to its inability to respire at a great rate.

Early Devonian Drepanasipis
evolved a flattened armor to protect its huge gill chamber with posterior atrial openings. 

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Distinctly different was Middle Silurian Birkenia
(Fig. 5). This late survivor from an early undocumented Ordovician radiation did not have stiff, plate-like armor like AndraspisPoraspis and Drepanaspis (above). Instead Birkenia was surrounded, supported and protected by hundreds of splinter-like, interwoven cartilage / bone, permitting much greater flexibility for this more mobile taxon. The tail fin was new, improving the efficiency of the swimming muscles. Small, barely mobile pectoral fins first appeared here. The eyes were larger, still protruding dorsally like those of Metaspriggina (Fig. 1) or a mudskipper like Periophthalmus. The rostrum of Birkenia extended anteriorly beyond the eyes. The oral cavity remained lancelet-like and lancelet-sized, with cilia surrounding the ventral opening. Likewise, the atrium / gill chamber remained relatively small. Nine gill openings were retained, a few more than in Metaspriggina (Fig. 1).

Figure 2. Birkenia is basically an armored Metaspriggina with a tail fin.

Figure 5. Birkenia is basically an armored Metaspriggina with a tail and pectoral fin.

These newest members of
the large reptile tree (LRT, 1611+ taxa; Fig. 6) nest at the base, helping us understand relationships among major groups, which have shifted slightly since their addition.

Figure 4. Subset of the LRT with the addition of several jawless taxa.

Figure 6 Subset of the LRT with the addition of several jawless taxa.

Metaspriggina walcotti (Simonetta and Insom 1993; Morris and Caron 2014; Cambrian, 500 mya; up to 10cm) is an early chordate, naked, jawless and finless, but with two anterodorsal eyes. The orbits comprise the proto-skull. The swimming muscles are larger, so this was probably a mobile feeder, using its eyes to seek prey and avoid predators. Not sure if an atriopore is present here, or if gill slits opened directly.

Arandaspis prionotolepis (Ritchie and Gilbert-Tomlinson 1977; Ordovician, 465mya; 15 cmlong) is a member of the Arandaspididae that looked like a large, armored tadpole. This jawless, finless filter feeder was basically an armored lancelet, like Metaspriggina, with a much larger gill chamber and a tail to improved propulsion. Both eyes and nostrils faced forward above the jawless mouth. Several armored gill slits appeared between the dorsal carapace and ventral plastron.

Poraspis brevis (Kiaer 1930; Late Silurian to Early Devonian, 410mya) is traditionally considered a member of the Heterostraci.

Drepanapis gemuendenensis (Schlüter 1887; Gross 1963; Early Devonian 405mya) was a flattened arandaspid with a superficially ray-like armored body. The common gill opening exited posteriorly. This bottom feeder with widely-spaced eyes is traditionally considered a member of the Heterostraci. This skull differs from diagrams produced by Gross 1963.

Birkenia elegans (Traquair 1899; Middle Silurian; up to 10cm) is a jawless, finless chordate with scales, twin nostrils and a hypocercal tail. Bony hooks top the dorsal region. Cilia line the permanently open jaws. Gill bars and gill openings are present. Birkenia is ancestral to Hemicyclaspis and the sturgeon Pseudoscaphirhynchus, along with all other extant fish and tetrapods.

Tomorrow,
we’ll look at several more taxa dominated by their gill chambers.


References
Kiaer J and Heintz A 1935. The Downtonian and Devonian vertebrates of Spitsbergen. V. Suborder Cyathaspida. Part I. Tribe Poraspidei, Family Poraspidae Kiaer. Skrifter om Svalbard og Ishavet 40:1-138.
Ritchie A and Gilbert-Tomlinson J 1977. First Ordovician vertebrates from the Southern Hemisphere. Alcheringa 1:351-368.
Simonetta AM and Insom E 1993. New animals from the Burgess Shale (Middle Cambrian)and the possible significance for the understanding of the Bilateria. Bolletino Di Zoologia 60:97–107.
Traquair RH 1899. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.

wiki/Branchiostoma
wiki/Metaspriggina
wiki/Arandaspis
wiki/Poraspis
wiki/Birkenia
wiki/Drepanaspis

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