Promissum, a ‘giant’ conodont, enters the LRT with primitive soft skull bones

Between Metaspriggina
and lampreys (Pteromyzon) is where conodonts nests in the large reptile tree (LRT, 2165 taxa). Promissum pulchrum (Kovacs-Endrody 1986, Late Ordovician, est 40cm in length, Figs 1, 2) is the ‘giant’ conodont. It is one of the very few specimens preserving a body outline. Most other conodonts are known only from scattered complex, sometimes microscopic, feeding apparatus structures. These are both common enough and distinct enough to identify strata and formations. What these structures belonged to was a major paleo question until recently.

Figure 1. Promissum, a conodont, compared to Lasanius, a birkeniid, to scale and to length.

According to Gabbot, et al 1995, who described Promissum, the ‘giant’ conodont,
“Conodonts are extinct Cambrian/Triassic vertebrates, soft bodied apart form the phosphatic elements of their feeding apparatuses.” Their cladogram nested conodonts between hagfish + lampreys and anaspids + ostracoderms. That is slightly different from the LRT, which employs generic taxa, not suprageneric taxa. Gabbot et al were unable to see or score skull traits (Fig 2).

Figure 2. The skull of Promissum as originally traced by Gabbot et al 1995 DGS colors added here. Note the origin of vertebrate skull bones at this grade. Blue dots are homologous with gill openings.

We looked at the feeding apparatus
of Promissum (Fig 3) earlier here without entering this taxon into the LRT. Comparative anatomy became possible only by including more basal vertebrates and by finally applying DGS methods to the Promissum data (Figs 1, 2).

Figure 3. Conodont feeding mechanism in dorsal and anterior views, retracted and extended.

Very few vertebrates are known from the Ordovician.
Even so, conodonts like Promissum are not outliers. After DGS tracing and analysis in the LRT Promissum looks like precursors and descendants. It fits right into a slot between better known Metaspriggina (Fig 1) and lampreys. Given this tight fit, the morphological gamut of basal vertebrates in the LRT is not expanding, suggesting that most, if not all basal vertebrate Bauplans are already known. All are variations of each other.

Figure 4. Birkenia skull for comparison to Jamoytius. Note the oralcavity has several mouth parts here identified with tetrapod homologies, but this was done at a time before many bssal vertebrates were added to the LRT. Perhaps these are closer in homology to conodont feeding elements. In any case, the skull elements here find closer homologs in more primitive soft conodont tissues.

Birkenia skull elements
(Fig 4) provide homologs for similar conodont skull elements. Generally that includes the deep short rostrum, dorsal orbits and long line of gill openings and specifically the individual skull bones. Smaller, simpler homologs of the complex conodont feeding apparatus elements (Fig 3) are also be present in Birkenia, shown here as slender struts mislabeled Vo, Pa, Ect and Pt, plus the stiff circum-oral elements mislabeled cilia.

Figure 5. The oral cavity of three lampreys.

Lamprey circum-oral rasping ‘teeth’
(Fig 5) likewise provide homologs to vertebrates at this grade of morphology, but not to gnathostome teeth.

Figure 6. Medial section of Acipenser (sturgeon) larva with temporary teeth from Sewertzoff 1928.

Conodonts and the LRT answer the enigma of baby sturgeon teeth
(Fig 6) which represent horny (not dentine nor enamel) vestiges of this ancestral condition in conodonts and likely sturgeon ancestors like Early Cambrian Haikouichthys (Fig 7).

Figure 7. Early Cambrian Haikouichthys nests basal to sturgeons, like baby Acipenser, in the LRT.

References
Gabbot SE, Aldridge RJ and Theron JN 1995. A giant conondont with preserved muscle tissue from the Upper Ordovician of South Africa. Nature 374:800–803.

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