Little Koilops: perhaps a late-surviving juvenile Parmastega

Due to the wide gamut inclusion of 2338 taxa,
some time ago the large reptile tree (LRT) data set was split into several overlapping programmable segments of fewer then 1500 taxa, the maximum allowed by the software.

In this case fish were separated from tetrapods. Many taxa appeared on both lists.

Figure 1. Parmastega and Koilops now nest together in the LRT. Their great size difference and the juvenile-ish traits of Koilops (short rostrum, large orbit, small teeth) suggest an ontogenetic interrelationship, not just a phylogenetic one. ” data-image-caption=”

Figure 1. Parmastega and Koilops now nest together in the LRT. Their great size difference and the juvenile-ish traits of Koilops (short rostrum, large orbit, small teeth) suggest an ontogenetic interrelationship, not just a phylogenetic one.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/parmastega-skull-3views.jpg?w=236″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/parmastega-skull-3views.jpg?w=584″ class=”size-full wp-image-93208″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/parmastega-skull-3views.jpg” alt=”Figure 1. Parmastega and Koilops now nest together in the LRT. Their great size difference and the juvenile-ish traits of Koilops (short rostrum, large orbit, small teeth) suggest an ontogenetic interrelationship, not just a phylogenetic one.” width=”584″ height=”742″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/parmastega-skull-3views.jpg?w=584&h=742 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/parmastega-skull-3views.jpg?w=118&h=150 118w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/parmastega-skull-3views.jpg?w=236&h=300 236w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/parmastega-skull-3views.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Parmastega and Koilops now nest together in the LRT. Their great size difference and the juvenile-ish traits of Koilops (short rostrum, large orbit, small teeth) suggest an ontogenetic interrelationship, not just a phylogenetic one.

Relatively recently added Devonian Parmastega
(Fig 1) should have been one of those overlapping taxa. Now this 272myo nests with little Koilops (Fig 1), a younger, smaller, earliest Carbonifermous relative of Elpistostege, Tiktaalik and Spathicephalus, all forming a clade of their own basal to finned Panderichthys and fingered Tetrapoda. Phylogenetic bracketing in this clade  gives all the skull-only taxa fins, as in Tiktaalik and Elpistostege, rather than fingers.

In other words, Parmastega and Koilops occupy a branch that did not lead to extant tetrapods, but were only a short way off.

More to the point, if Koilops was a phylogenetically miniaturized adult, it did not nest at the base of any larger more derived taxa in the LRT.

According to Wikipedia-Koilops:
“Koilops is an extinct genus of four-limbed stem-tetrapod from the Mississippian (mid-Tournaisian) of Scotland. It contains a single species, Koilops herma, based on a mold of an 8.0 cm (3.1 in.)-long skull from the Ballagan Formation. A phylogenetic analysis in its original description places Koilops as a close relative of Tulerpeton and colosteids.”

In the LRT, Tulerpeton is closer to reptiles = amniotes. Colosteids are basal-most tetrapods = fish with fingers and toes.

See how phylogenetic analysis can give a skull-only taxa hypothetical fingers and toes?

That hypothesis is not supported by the LRT.

Figure 2. Juvenile and adult Eusthenopteron compared from Schultze 1984. The cranium of the juvenile appears convex here, but was likely flatter. ” data-image-caption=”

Figure 2. Juvenile and adult Eusthenopteron compared from Schultze 1984. The cranium of the juvenile appears convex here, but was likely flatter.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/06/eusthenopteron_juvenile_diagram.jpeg?w=273″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/06/eusthenopteron_juvenile_diagram.jpeg?w=584″ class=”size-full wp-image-46229″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/06/eusthenopteron_juvenile_diagram.jpeg” alt=”Figure 2. Juvenile and adult Eusthenopteron compared from Schultze 1984. The cranium of the juvenile appears convex here, but was likely flatter.” width=”584″ height=”641″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/06/eusthenopteron_juvenile_diagram.jpeg?w=584&h=641 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/06/eusthenopteron_juvenile_diagram.jpeg?w=137&h=150 137w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/06/eusthenopteron_juvenile_diagram.jpeg?w=273&h=300 273w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2020/06/eusthenopteron_juvenile_diagram.jpeg 620w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 2. Juvenile and adult Eusthenopteron compared from Schultze 1984. The cranium of the juvenile appears convex here, but was likely flatter.

Their great size difference
and the juvenile-ish traits of Koilops (short rostrum, large orbit, small teeth, Fig 1) suggest an ontogenetic interrelationship with Parmastega (Fig 1), not just a phylogenetic one, following patterns established earlier by juvenile and adult Eusthenopteron specimens (Fig 2).

Koilops herma
(Clack et al. 2016; NMS G. 2013.39/14) Tournasian, early Carboniferous ~375 mya) was traditionally considered a basal tetrapod. Here iit nests as a possible juvenile of Parmastega. These two nest with Elpistostege, Spathicephlaus and Tiktaalik, but was smaller and later than all three. In this small, neotonous taxon the nares wereventral to the rim of the short rorstrum. The pineal foramen was enormous.

Parmastega aelidae
(Beznosov et al. 2019) was recovered from 372mya strata, making it the oldest reconstructable tetrapod – if it is is a tetrapod. Originally recovered between Tiktaalik + Elpistostege and Ventastega, here it nests with Koilops, a taxon omitted from prior studies. All these taxa nest together in the LRT. A concave rostrum, elevated orbits and large size mark it as a derived taxon, different from others, despite its antiquity. Based on the large size of the pectoral girdle (taller than the skull!, Fig 1), and its other differences with basal tetrapods, Parmastega may represent an early experiment at growing limbs and fingers from lobe fins by convergence with the clade Tetrapoda.

Most early tetrapods had small weak limbs and girdles. For Parmastega to have such a large scapula so early in the Devonian (372mya) is unexpected and worthy of further study.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

References
Beznosov PA, Clack JA, Lufsevics E, Ruta M and Ahlberg PE 2019. Morphology of the earliest reconstructable tetrapod Parmastega aelidae. Nature 574:527–531.(971):29-101.
Clack et al. (14 other authors) 2016. Phylogenetic and environmental context of a Tournaisian tetrapod fauna. Nature ecology & evolution 1(0002):1-11.

wiki/Koilops
wiki/Parmastega