Stensioella is supposed to be a basal placoderm

Updated Feb. 27, 2020 and May 4, 2020
with a higher resolution image (Fig. 1) and 80 more taxa, Stensioella now nests closer to Guiyu the oldest articulated bony fish, another flattened sister to the coelacanths with large lobe fins.

Figure 1. Updated tracing of the Stensioella skull.

Figure 1. Updated tracing of the Stensioella skull.

Here,
in the large reptile tree (LRT, 1482 taxa, Fig. 2) Stensioella (Fig. 1) now nests with Guiyu another lobefin close to coelacanths.

According to Wikipedia
Stensioella heintzi (“Heintz’s Little Stensio“) is an enigmatic placoderm of arcane affinity. It is only known from the Lower Devonian Hunsrück slates of Germany, where the only specimens have been found. [It] had armor made up of a complex mosaic of small, scale-like tubercles.”

“Some paleontologists believe that there are very few concrete reasons for S. heintzi’s placement in Placodermi. The paleontologist, Philippe Janvier suggests that it was actually a holocephalid, and not a placoderm at all. However, if this is true, then the holocephalids (chimaerasiniopterygianspetalodonts, et al.) diverged from sharks before the Chondrichthyan Devonian radiation.

The LRT tested a wide gamut of vertebrates
including all prior candidate sister taxa (Fig. 2).

Figure 2. The AMNH specimen of Stensioella in dorsal view along with a diagram in ventral view.

Figure 2. The AMNH specimen of Stensioella in dorsal view along with a diagram in ventral view.

Stensioella heintzi (Broili 1933; Emsian, late Early Devonian) is widely considered the most basal placoderm, but here nests with Silurian lobefins. Enlarged pectoral fins on a flattened head and torso mark this taxon. Distinct from most sisters, the posterior portion is also preserved. Here specific bones with tetrapod names are identified for the first time. Stensioella would have had a lifestyle like Squatina, the extant angel shark.

Figure x. Newly revised fish subset of the LRT

Figure x. Newly revised fish subset of the LRT

As mentioned previously
basal taxa in all the above fish clades are/were bottom feeders. Open water feeders with more specialized heterocercal tails, develop by convergence in every clade, except the tetrapoda… that is, until reptiles returned to the sea and evolved into ichthyosaurs, whales, etc.


References
Broili F 1933. Weitere Fischreste aus den Hunsrickschiefern. Situngsbirechte der bayerischen Akademie der Wissenschaften, Mathematisch-Naturewissenschaftliche Klasse 2: 269–313.

wki/Stensioella

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