Loxomma and Urocordylus enter the LRT
Two basal tetrapod taxa known since the time of Huxley (1871)
enter the LRT today, Loxomma (Fig 1) and Urocordylus (Fig 2). Both are similar to taxa already in the LRT, so it’s no surprise where they nested (Fig 5). That’s why they both waited until now to enter the large reptile tree (LRT, 2338 taxa, subset Fig 5).
So, why now?
Figure 1. Loxomma skull in the 3 views. DGS colors added here.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/loxomma.skull588.jpg?w=143″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/loxomma.skull588.jpg?w=489″ class=”size-full wp-image-92050″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/loxomma.skull588.jpg” alt=”Figure 1. Loxomma skull in the 3 views. DGS colors added here. ” width=”584″ height=”1224″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/loxomma.skull588.jpg?w=584&h=1224 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/loxomma.skull588.jpg?w=72&h=150 72w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/loxomma.skull588.jpg?w=143&h=300 143w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/loxomma.skull588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Loxomma skull in the 3 views. DGS colors added here.
Figure 2. Urocordylus (below) and the skull of another taxon ostensibly within the same genus (above).
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/urocordylus588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/urocordylus588.jpg?w=584″ class=”size-full wp-image-92053″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/urocordylus588.jpg” alt=”Figure 2. Urocordylus (below) and the skull of another taxon ostensibly within the same genus (above).” width=”584″ height=”506″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/urocordylus588.jpg?w=584&h=506 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/urocordylus588.jpg?w=150&h=130 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/urocordylus588.jpg?w=300&h=260 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/urocordylus588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2. Urocordylus (below) and the skull of another taxon ostensibly within the same genus (above). Sauropleura is a relative shown to scale, both at about 0.35x life size.
The reason for their recent entry
is a cladogram in Smithson, Ruta and Clack 2024 (Fig 3) which included these taxa and purported to show “the topology of the tetrapod stem” – as does the LRT (Fig 4) with many more taxa. The authors were nesting a small disarticulated partial skull and pectoral area they named Ossirarus (UMZC 2016.3, Fig 5) from the lowest Carboniferous. Tabular horns are prominent. Skull width about 4cm.
Figure 3. Basal tetrapod cladogram from Smithson, Ruta and Clack 2024. Yellow stripes are taxa duplicated in the LRT, see figure 4.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.cladogram.jpg?w=144″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.cladogram.jpg?w=492″ class=”size-full wp-image-92055″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.cladogram.jpg” alt=”Figure 3. Basal tetrapod cladogram from Smithson, Ruta and Clack 2024. Yellow stripes are taxa duplicated in the LRT, see figure 4.” width=”584″ height=”1216″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.cladogram.jpg?w=584&h=1216 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.cladogram.jpg?w=72&h=150 72w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.cladogram.jpg?w=144&h=300 144w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.cladogram.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 3. Basal tetrapod cladogram from Smithson, Ruta and Clack 2024. 64 taxa were tested by 275 characters. Yellow stripes are taxa duplicated in the LRT, see figure 4. The balance are known from relatively few elements in many cases.
If you want to see how taxa are related to one another,
keep adding taxa. Most traditional clades are retained in the LRT, but some are not due to taxon exclusion. When your cladogram has 2338 taxa you will no longer have to add random outgroup taxa because your cladogram will tell you which taxa are the closest to your new untested taxon.
Figure 4. Subset of the LRT focusing on basal tetrapods. The phylogenetic spread essentially matches the cladogram presented by Smithson, Ruta and Clack in figure 3.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/earlytetrapods2025.jpg?w=43″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/earlytetrapods2025.jpg?w=148″ class=”size-full wp-image-92061″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/earlytetrapods2025.jpg” alt=”Figure 4. Subset of the LRT focusing on basal tetrapods. The phylogenetic spread essentially matches the cladogram presented by Smithson, Ruta and Clack in figure 3.” width=”584″ height=”4036″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/earlytetrapods2025.jpg?w=584&h=4036 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/earlytetrapods2025.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 4. Subset of the LRT focusing on basal tetrapods. The phylogenetic spread essentially matches the cladogram presented by Smithson, Ruta and Clack in figure 3.
When added to the LRT
Ossirarus (Fig 5) nested not with fingered Ossinodus, but with the more primitive finned Elpistostege. However, let’s keep that nesting tentative due to the large size difference, allometric changes during ontogeny and the paucity of traits to measure in little Ossirarus.
Unlike Ossirarus, bigger Ossinodus lacks large temporal horns and the postorbital is nearly the size of the jugal.
Figure 5. Ossirarus diagram from Smithson, Ruta and Clark. DGS colors added here.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.diagram588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.diagram588.jpg?w=584″ class=”size-full wp-image-92059″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.diagram588.jpg” alt=”Figure 5. Ossirarus diagram from Smithson, Ruta and Clark. DGS colors added here. ” width=”584″ height=”360″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.diagram588.jpg?w=584&h=360 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.diagram588.jpg?w=150&h=92 150w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.diagram588.jpg?w=300&h=185 300w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/03/ossirarus.diagram588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 5. Ossirarus diagram from Smithson, Ruta and Clark. DGS colors added here.
Wikipedia – Ossirarus reports
this taxon is ‘four-limbed stem-tetrapod” even though no pelvic elements are known and the manus is missing. “The specimen was presumed to be an adult from the presence of interdigitated sutures.” Hard to see that interdigitation in the diagram (Fig 5)… and the bones were scattered, which usually does not happen with interdigitation.
“A unique characteristic of Ossirarus is the presence of a large, pointed tabular horn.”
Actually... not so unique. Many taxa have that trait.
References
Smithson TR, Ruta M and Clack JA 2025. On Ossirarus kierani, a stem tetrapod from the Tournaisian of Burnmouth, Berwickshire, Scotland, and the phylogeny of early tetrapods. Fossil Record 27 (3) 2024, 333–352 | DOI 10.3897/fr.27.126410
Source: https://pterosaurheresies.wordpress.com/2025/03/22/loxomma-and-urocordylus-enter-the-lrt/
