‘Enigmatic’ Protospinax enters the LRT between dogfish and sawfish + skates

Jambura et al 2023
describe an excellent skeleton (PBP-SOL-8007) of the Late Jurassic shark, Protospinax annectans (Woodward 1918-1919, Figs 1, 2).

From the abstract
“since its first description more than 100 years ago, its phylogenetic position within the Elasmobranchii (sharks and rays) has proven controversial, and a closer relationship
between Protospinax and each of the posited superorders (Batomorphii, Squalomorphii, and Galeomorphii) has been proposed over the time.”

By including more taxa and scoring no soft tissue, the large reptile tree (LRT, 2222 taxa, Fig 4) nests Protospinax (Figs 1, 2) between Squalus, the dogfish shark (Fig 5), and Pristis, the sawfish, a taxon basal to guitarfish + skates. Earlier the LRT invalidated the superorder Batomorphii (Batoidea) by splitting traditional members into a variety of distantly related clades.

Figure 1. PBP-SOL-8007 specimen of Protospinax annectans (Woodward 1918) in situ and outline graphic from Jambura et al 2023. Frame two with contrast boost added here.

The Jambura et al abstract continues:
“A data matrix with 224 morphological characters was compiled and analyzed under a
molecular backbone constraint.”

The LRT does not employ molecules. Readers are once again warned against using molecules in phylogenetic analyses because too often they recover untenable tree topologies.

It’s worth noting: in the Jambura et al. cladogram (Fig 3) no taxa are transitional between hybodontid sharks and traditional batoids.

By comparison, the LRT (Fig 4) documents several flattened taxa transitional to several rays and skates.

Figure 2. Skull of PBP-SOL-8007 specimen of Protospinax annectans (Woodward 1918) in dorsal view, in situ, under UV light. First color overlay from Jambura et al 2023. Second color overlay with tetrapod homology colors (Frames 4, 5) added here. Figure 5 indicates mandible elements largely hidden beneath the skull.

The Jambura et al abstract continues:
“the revision of our morphological data matrix within a molecular framework highlights the lack of morphological characters defining certain groups, especially sharks of the order Squaliformes, hampering the phylogenetic resolution of Protospinax annectans with certainty. Furthermore, the monophyly of modern sharks retrieved by molecular studies is only weakly supported by morphological data, stressing the need for more characters to align morphological and molecular studies in the future.”

By contrast, the LRT (Fig 4) completely resolves the Chondrichthyes by including more taxa, including a more primitive outgroup taxon, and avoiding molecular + soft tissue data.

Figure 3. Cladogram from Jambura et al 2023 dividing chondrichthys into traditional clades, including batoids, an invalid clade in the LRT.

Jambura et al report,
“Phylogenetic analyses are the foundation for many evolutionary studies, providing
vital information about evolutionary rates, origination, diversification, and extinction of
certain phylogenetic units of different ranks, and thus contribute to our understanding
of the inherent drivers of biological diversity.”

Agreed.

Figure 4. Subset of the LRT focusing on sharks and their kin with the addition of Protospinax (yellow). Here traditional members of the Batoidea are split apart invalidating that clade.

Unfortunately, Jambura et al concluded,
“For the moment, the phylogenetic position of Protospinax is best regarded as tentative and its use as a calibration fossil for the divergence time of modern sharks and rays is highly questionable until its phylogenetic position can be resolved.”

Colleagues: Please add taxa and avoid molecules, then report results, especially if they differ from the LRT (subset Fig 4), which had no trouble nesting Protospinax.

Figure 5. Skull of Squalus acanthi as with DGS colors added according to tetrapod homologies. Compare to Protospinax skull in figure 2.

Protospinax annectans
(Woodward 1919, Jambura et al 2023, Lower Tithonian, Late Jurassic, 90cm long) is traditionally nested close to angel sharks and some saw sharks, but here nests between Squalus and saw sharks + guitarfish + skates. This appears to be a novel hypothesis of interrelationships. If not, please send a citation so I can promote it here.

References
Jambura PL et al (12 co-authors) 2023. Systematics and Phylogenetic Interrelationships of the Enigmatic Late Jurassic Shark Protospinax annectans Woodward, 1918 with Comments on the Shark–Ray Sister Group Relationship. Diversity:15(311) 1-43.. https:// doi.org/10.3390/d15030311
Woodward AS 1919. On Two New Elasmobranch Fishes (Crossorhinus jurassicus, sp. nov., and Protospinax annectans, gen. et sp. nov.) from the Upper Jurassic Lithographic Stone of Bavaria. Proc. Zool. Soc. Lond. 1919, 13, 231–235.

More information on: Backbone constraint

wiki/Squalus
wiki/Protospinax