This page covers the stem group of the hagfishes (Order Myxiniformes, clade Cyclostomata), an extant clade of jawless fishes found in cold seawater, commonly buried in mud except for the tip of the head (Encyclopedia Britannica). Around 70 living species are known.
On the basis of phylogenetic analysis of morphological characteristics, Miyashita et al (2021) identified two stem-group hagfishes, as illustrated in the following phylogenetic time tree:
On the basis of phylogenetic analysis of morphological characteristics, Miyashita et al (2021) identified two stem-group hagfishes, as illustrated in the following phylogenetic time tree:
Figure 1. Phylogenetic time tree of the Myxiniformes stem group
The two stem-group hagfishes shown in the above tree were both found in the Late Carboniferous (Moscovian) Francis Creek Shale Member at Mazon Creek, Grundy County, Illinois, USA (Bardack, 1991; McCoy et al, 2016; Miyashita et al, 2021). These fossils are illustrated below (click on any image for a larger view):
Figure 2. Images of stem-group hagfishes
The second stem-group hagfish shown in Figure 2, Myxinikela siroka, is described by Miyashita (2020) as “a curious hybrid of hagfish- and lamprey-like features: a rather squat hagfish with branchial baskets, well-differentiated midline fin, and squinting eyes of a lamprey”. However, the species does share many features with the hagfish crown group (Miyashita, 2020). The primary differences between Myxinikela and the crown group are found in the size, shape and location of the eyes, the pigmentation of the eyes (not seen in the crown group), the short length of the branchial apparatus and the fact that this apparatus is not separated from the rest of the head by a lingual apparatus (Miyashita, 2020).
The oldest known fossil member of the crown-group hagfishes is the recently described Tethymyxine tapirostrum, found in sublithographic limestones of the Hâdjula Lagerstätte of Late Cretaceous age (Cenomanian) at a locality 10 km east of Byblos, Lebanon (Miyashita et al, 2019). This fossil is illustrated below:
The oldest known fossil member of the crown-group hagfishes is the recently described Tethymyxine tapirostrum, found in sublithographic limestones of the Hâdjula Lagerstätte of Late Cretaceous age (Cenomanian) at a locality 10 km east of Byblos, Lebanon (Miyashita et al, 2019). This fossil is illustrated below:
Figure 3. Crown-group hagfish: Tethymyxine tapirostrum (Late Cretaceous)
The ages of the fossils shown in Figures 2 and 3, taken by themselves, suggest that the transition from the stem-Cyclostomata to the hagfish crown group took place over a period of at least 206 million years, from the Late Carboniferous to the Late Cretaceous (Figure 1). However, the hagfish stem line includes a ghost lineage (shown as a blue bar in Figure 1), because the oldest known stem-group lamprey (of Late Devonian age; see page on lampreys) is significantly older than the stem-group hagfishes, which are of late Carboniferous age. Given that the two stem groups must have appeared at the same time, the hagfish stem-group transition must also have begun in the Late Devonian and would have lasted between 258 and 278 million years.
References
Miyashita, T. (2020). A Paleozoic stem hagfish Myxinikela siroka—revised anatomy and implications for evolution of the living jawless vertebrate lineages. Canadian Journal of Zoology, 98(12), 850-865.
Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Currie, P. J. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146-2151.
Miyashita, T., Gess, R. W., Tietjen, K., & Coates, M. I. (2021). Non-ammocoete larvae of Palaeozoic stem lampreys. Nature, 591(7850), 408-412.
Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Currie, P. J. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146-2151.
Miyashita, T., Gess, R. W., Tietjen, K., & Coates, M. I. (2021). Non-ammocoete larvae of Palaeozoic stem lampreys. Nature, 591(7850), 408-412.
Image credits - Hagfishes
- Header: A broadgilled hagfish (Eptatretus cirrhatus), in Island Bay, Wellington, New Zealand, May 2019: mattruglys / iNaturalist.org. License: CC By Attribution-NonCommercial
- Figure 2 (Gilpichthys greenei): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Myxinikela siroka, fossil): From Open Access article Gabbott, S. E., Donoghue, P. C., Sansom, R. S., Vinther, J., Dolocan, A., & Purnell, M. A. (2016). Pigmented anatomy in Carboniferous cyclostomes and the evolution of the vertebrate eye. Proceedings of the Royal Society B: Biological Sciences, 283(1836), 20161151.
- Figure 2 (Myxinikela siroka, life restoration): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3: From Open Access article Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Currie, P. J. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146-2151.