Anchignathodontidae

Skeletal plan of Hindeodus parvus, from Zhang et al. (2017).

Belongs within: Spathognathodontidae.

The Anchignathodontidae are a lineage of conodonts found from the early Carboniferous to the earliest Triassic (Sweet 1988).

Galba among the conodonts
Published 17 January 2024

The end-Permian extinction was the most devastating event to impact life on this planet that we know of. Wiping out over four-fifths of genera alive at the time, it was referred to by Michael Benton as “when life nearly died”. But for at least one lineage of animals, the events of the end-Permian crisis proved not a curse but a blessing, briefly catapulting them to a position of much greater prominence than they had held before. And no, I’m not talking about lystrosaurs.

Apparatus elements of Hindeodus cristulus, from Sweet (1988). Element identities from left to right: P2, P1, M, S3 or S4, S1 or S2, S0 (because some S element pairs are not distinguishable in isolation, Sweet reconstructed the apparatus as only including six element positions).

The first members of the conodont family Anchignathodontidae had first appeared much earlier, in the early days of the Carboniferous period (Sweet 1988). Conodonts are typically found as isolated elements of the dental apparatus (which were the only mineralised parts of the animal in life), and descriptions of the apparatus can soon become daunting to the uninitiated (such as yours truly). Let it suffice to say that anchignathodontids have been reconstructed with a fifteen-element apparatus, including an unpaired anteriormost element followed by seven pairs (Jiang et al. 2011). The anteriormost (S0) element lacked a posterior process, the second-to last (P2) pair of elements were angled with relatively short arms, and the posteriormost (P1) pair of elements had a ‘carminiscaphate’ morphology with a relatively straight longitudinal axis, a narrow anterior process, and a broad posterior process that was hollowed out underneath (Sweet 1988). The overall apparatus suggests (to me) a fairly generalist predator with the anterior elements suited for grasping prey and the posterior ones for slicing it up, without the crushing plates found in some other conodonts.

For most of their roughly 100 million year existence, anchignathodontids do not seem to have been a spectacularly diverse lineage. Most species have been assigned to a single genus Hindeodus whose stratigraphic range coincides with that of the family. Lai et al. (2001) interpreted Hindeodus as a pelagic genus with some species found worldwide at the end of Permian. One species, H. parvus, has been used as the global marker for the start of the Triassic. Hindeodus species may be found in both near-shore, shallow-water deposits and offshore, deep-water locales.

At the end of the Permian, Hindeodus populations seem to have been relatively unaffected by the devastation surrounding them. The exact causes and events of the end-Permian extinction remain poorly understood but part of the process seems to be the development of anoxic regions in deeper sea waters. The surface-dwelling Hindeodus seem to have been better positioned to weather this challenge whereas more benthic species found themselves choked out. Hindeodus populations may even have benefited. We know from recent experience that warm ocean temperatures, high nutrient levels and low oxygen may lead to a spike in jellyfish populations. Such a spike in the end-Permian is unlikely to have left any obvious trace but may have provided a buffet experience for a pelagic predator such as Hindeodus.

Typical P2 element of Isarcicella, from Sweet (1988).

Following the first wave of extinction during the Changhsingian stage of the latest Permian, the anchignathodontids began a speciation spree that continued unabated into the beginning of the Triassic (Orchard 2007). Over two dozen species of anchignathodontid are known from either side of the Permian-Triassic boundary, including representatives of two distinctive new Triassic genera, Isarcicella and Sweetohindeodus. These genera are characterised by lateral broadening of the posterior process in the P1 element; in Isarcicella, there is often a large cusp on the lateral expansion. The remainder of the Isarcicella apparatus has not yet been reconstructed but there is no question that it had some formidable back teeth.

And then, somehow, it all collapsed. One might expect this sudden radiation within anchignathodontids to portend a long and storied apotheosis, but this was not to be the case. Anchignathodontids were the globally dominant radiation of conodonts for a few hundred thousand years after the end of the Permian, and then they were gone. They simply vanished from the fossil record, never to return, and without descendants to continue their genetic heritage. It would be the Ellisoniidae and Gondolellidae that would represent the conodonts in the remainder of the Triassic, before their own extinctions would end the lineage as a whole.

How the anchignathodontids became extinct is an unanswered question. Was there something about the end-Permian environment that particularly suited them, only for them to lose their advantage as global conditions returned to normal? Did the gondolellids find some way to muscle into their ecological territory, stripping them of their hard-won crown? Or were they somehow just unlucky, escaping the initial earthquake but caught off-guard by one of its aftershocks? I’m usually not that focused on questions of extinction—when it comes to extinct species, I usually find it more interesting to imagine how they lived rather than how they died—but when a spectacular collapse comes so hard on the heels of a spectacular rise, the question is hard to ignore.

Systematics of Anchignathodontidae

Characters (from Sweet 1988): Skeletal apparatus basically seximembrate, including carminiscaphate Pa elements, angulate Pb elements with relatively short processes, and alate Sa elements lacking any trace of posterior process.

<==Anchignathodontidae [Anchignathodontacea]
    |--Cudotaxis priceslingiS88
    |--Aethotaxis Baesemann 1973S88
    |    `--A. advenaS88
    |--Subbryantodus Branson & Mehl 1934S88
    |    `--*S. arcuatus Branson & Mehl 1934H62
    `--Hindeodus Rexroad & Furnish 1964 [incl. Anchignathodus]S88
         |--*H. cristulusS88
         |--H. crassidentatus (Branson & Mehl 1934)AS93 [=Subbryantodus crassidentatusS88]
         |--H. ellisoniS88
         |--H. eurypyge Nicoll et al. 2002FP12
         |--H. inflatus Nicoll et al. 2002FP12
         |--‘Anchignathodus’ isarcicusRR79
         |--H. julfensisS88
         |--H. minutusS88 [=Anchignathodus minutusCB86]
         |--H. parvus (Kozur & Pjatakova 1976)FP12
         |--H. penescitulusS88
         |--H. perlmutteriS88
         |--H. pisaiFP12
         |--H. praeparvusFP12
         |--H. scitulusS88
         `--H. typicalis (Sweet 1970)AS93 [=Anchignathodus typicalisS88; incl. Ellisonia teichertiS88]

*Type species of generic name indicated

References

[AS93] Aldridge, R. J., & M. P. Smith. 1993. Conodonta. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 563–572. Chapman & Hall: London.

[CB86] Chen D.-Q. & Bao H. 1986. Lower Permian ostracodes from the Chihsia Formation of Jurong and Longtan, Jiangsu Province. Acta Micropalaeontologica Sinica 3 (2): 107–132.

[FP12] Farabegoli, E., & M. C. Perri. 2012. Millennial physical events and the end-Permian mass mortality in the western Palaeotethys: timing and primary causes. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 719–758. Springer.

[H62] Hass, W. H. 1962. Conodonts. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt W. Miscellanea: Conodonts, Conoidal Shells of Uncertain Affinities, Worms, Trace Fossils and Problematica pp. W3–W69. Geological Society of America, and University of Kansas Press.

[RR79] Ross, C. A., & J. R. P. Ross. 1979. Permian. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A291–A350. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[S88] Sweet, W. C. 1988. The Conodonta: Morphology, taxonomy, paleoecology, and evolutionary history of a long extinct animal phylum. Clarendon Press: Oxford.

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