With questions arising
about the phylogenetic nesting of the fish-like paratetrapod Colosteus with Osteolepis, today several putative members of the Colosteidae were added to the large reptile tree (LRT, subset Fig. 1). According to Wikipedia, clade members should include Colosteus, Deltaherpeton, Greererpeton and Pholidogaster.
Figure 1. Subset of the large reptile tree focusing on basal tetrapods, updated with 9 new taxa. Members of the putative Colosteidae are highlighted in yellow.
As you can see
(Fig. 1) only one putative member of the Colosteida. Pholidogaster nests with Colosteus. Deltaherpeton nests with Eryops the temnospondyl. Greererpeton nests between temnospondyls and the Neotetrapoda with Ichthyostega at its base.
A new clade
The Paratetrapoda is here defined as Colosteus, Osteolepis, their last common ancestor and all of their descendants. Derived taxa developed tetrapod-like limbs by convergence. The Neotetrapoda is here defined as Ichthyostega, Homo, their last common ancestor and all their descendants. This is the clade that leads to all other tetrapods.
Figure 2. Greererpeton in situ with skull colors added.
(Romer 1969, Smithson 1982, Godfrey 1989; Early Carboniferous, 320 mya; 1.5 m in length). Godfrey thought it nested closer to Proterogyrinus than to Ichthyostega. Here Greererpeton nests between temnospondyls, like Sclerocephalus and Ichthyostega. The skull was flattened with orbits on top of the skull. The lacrimal contacted the naris. The torso included some 41 presacral vertebrae. The pectoral girdle was robust. The limbs were small. The powerful tail was the chief organ of locomotion.
Figure 3. Deltaherpeton skull with colors added.
(Bolt JR and Lombard RE 2010; Viséan, Early Carboniferous; Fig. 3) nests with Eryops among the temnospondyls and appears to have a fused nasal/frontal.
Figure 4. Colosteus holotype drawing of the fossil in situ from Hook 1983 compared to the closely related Osteolepis.
(Newberry 1856, Hook 1983; Westphalian, Late Carboniferous, 305 mya; 1m in length; AMNH 6916; Fig. 4) was originally considered a fish (Pygopterus) and renamed by Cope 1869. Here Colosteus nests with Osteolepis and Pholidogaster (Figs. 5, 6) as a paratetrapod convergent with traditional tetrapods. The skull was ovate, the vomers and dentaries had fangs, the fins had transformed to tiny four-fingered limbs. The lacrimal did not reach the external naris. The scales remained large and rhomboid-shaped. Pectoral girdle had not yet evolved an external scapula and coracoid.
Figure 5. Pholidogaster skulls compared to Colosteus and Osteolepis. Panchen reconstruction on left includes a premaxilla that is too wide. At right revised width to fit premaxilla tracing, pectoral girdle and in situ lacrimal and cheek bones.
(Huxley 1862, Panchen 1975; Visean, Early Carboniferous, 340 mya; Figs. 5, 6) was originally considered a labyrindont and an anthracosaur, but here nests with Osteolepis and Colosteus (Fig. 5) among the Paratetrapoda, a clade that developed limbs independent of the Tetrapoda.
The new skull reconstruction (Fig. 5) is narrower than in Panchen 1975 to match the premaxilla and pectoral girdle. The premaxilla carried a lateral fang and the dentary had a corresponding slot for it.
Figure 5. Pholidogaster in situ and with post crania reconstructed based on the Osteolepis bauplan. The long straight ribs are actually neural spines that are elevated here. Small bones, like those found in Osteolepis and Eusthenopteron are retained at the bases of unpreserved dorsal and anal fins. The interclavile extends below the chin. It appears unlikely that this taxon had a neck.
The vertebral column included small bones that were basal to both dorsal fins and anal fin. The long straight unpaired bones once thought to be ribs are here identified as tall slender neural spines. The tail was little different from that found in Osteolepis, including the slight upturn, like a shark’s tail.
The interclavicle and clavicles extended beneath the mandibles. No scapula or coracoid was visible. Those were tiny elements medial to the coracoid and cleithrum. The fingers did not ossify. The pelvis is well ossified with an acetabulum dorsal to the pubis. The hind limb includes metatarsals and a few digits.
The ossified scales that covered the body in Osteolepis and Colosteus are not present here.
Pholdogaster has been known for over 150 years
and if it had only been reconstructed with the present precision I think its fish-like affinities would have been discovered earlier. It’s 150-year-old specific name ‘pisciformes’ points obviously to its fish-like affinities, which were recognized then, but have received less attention in recent studies. It appears unlikely that any paratetrapod had a movable neck.
we have tetrapods crawling on shore and leaving footprints in the Middle Devonian, millions of years before Acanthostega and Ichthyostega in the latest Devonian. These famous taxa now appear to be conservative relicts retaining fish-like traits, rather than liberal land pioneers inventing tetrapod-like traits.
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