Surprised to find this:
Acheloma (Cope 1882; Dilkes and Reisz 1987; Early Permian, 275 mya; aka Trematops), a trematopsid amphibamid lepospondyl basal tetrapod had a confluent antorbital fenestra and naris. Bolt 1974 considered this a “very elongate external naris” and then considered two hypotheses for its origin and use:
- as a nasal salt gland (rather improbable, but still possible, according to Bolt)
- to transfer of forces away from the antorbital bar (Bolt’s preferred hypothesis)
Bolt also noted
that earlier papers referred this morphology to a confluent antorbital vacuity, but dismissed the notion by saying, “There is no evidence that any labyrinthodont, including the ancestors of trematopsids, possessed such an [completely separate] antorbital vacuity.” IMHO, this convergent trait need not have been completely separate to qualify as an antorbital vacuity/fenestra. As Bolt noted, in nearly every case, there is a slight constriction in this vacuity marking the end of the naris and the beginning of the antorbital vacuity (Fig. 1). A nasal flange descends inside the vacuity.
we looked at the antorbital fenestra in other tetrapods here.
Perhaps of interest to this discussion
is the relatively large diameter palatal teeth on the vomers, palatines and ectopterygoids (Fig. 1). Bolt also found evidence for a nasal flange in related Doleserpeton and Tersomius, but not in unrelated Seymouria and Eryops.
Olson 1941 had this odd explanation:
The anterior part was for smelling, the longer posterior part was for respiration and the reason for this was the internal naris lies beneath only the posterior part. Bolt noted the shortest route was not always the only route in tetrapods. Air passages can be quite complicated.
The odd otic notch
that likely housed an eardrum in related taxa, is long and narrow in Acheloma. Dilkes and Reisz (1987) noted, “The shape of the otic notch, however, argues against an impedence-matching hearing system because the vibrational properties of the postulated tympanum would be profoundly different from one with the same surface area but circular in outline.”
Acheloma cumminsi was originally considered a temnospondyl, but here nests between Dendrerpeton and Cacops within the lepospondyls with many traits convergent with temnospondyls, like that large wide skull and large overall size. The related Acheloma dunni (Fig. 1) had giant palatal teeth.
As promised earlier:
lepospondyl traits of Acheloma and Cacops not present in temnospondyls from the character list of the LRT. Let me know if you see errors here:
- Ventral naris chiefly maxilla in lateral view
- Prefrontal separate from postfrontal
- Preorbital length of skull sub-equal to postorbital length of skull
- Naris shape in lateral view < 2x longer than tall
- Palatine exposure on the external skull below orbit.
- Squamosal posterior rim is a ‘big curve’
- Squamosal descends to ventral skull
- Mandible tip straight, does not rise
- Cervical centrum longer than tall
- Cervical neural spines not taller than centra
- Pleurocentra larger than intercentra
- Two sacral vertebrae
- Sacral spines not > acetabulum depth
- Anterior chevron shapes, not wider proximally
- Anterior caudal neural spines not higher than centra
- Clavicle shorter than scapula
- Humerus not ‘L’-shaped
- Manual metacarpals 1-3 align
- Longest metacarpals: 2, 3 and sometimes 4
- Longest manual digit: three and four
- Manual unguals sharp pointed
- Metacarpal 5 absent – except in Cacops. Acheloma has 5 carpals.
- Posterior ilium not longer than anterior ilium
- Pubic apron wide
- Longest metatarsals: 3 and 4
- Pedal 3.1 not > p2.1
- Overall size not > 60 cm in length
Acheloma, Broilleus and Cacops to Eryops adds 24 steps at present. Shifting those three + Dendrepeton and Tersomius adds 17 steps at present. Shfting those five + the three members of the Amphibamus clade adds 35 steps at present.
On a side note:
Having a fifth finger on basal tetrapods (no matter how you count them, 1-4 or 2-5) is rare after Acanthostega partly because a complete manus is rare in basal tetrapods and partly because many taxa have only four fingers. Proterogyrinus, Seymouria, Cacops and basal reptiles all have five fingers preserved. Presently that’s a discontinuous list, but those five fingers could be homologous. If you know of any other related examples, let me know. I need that data.
Bolt JR 1974. Osteology, function, and evolution of the trematopsid (Amphibia: Labyrinthodontia) nasal region. Fieldiana: Geology 33(2): 11-30.
Cope ED 1882. Third contribution to the history of the vertebrata ofthe Permian Formation of Texas. Proc. Phil. Soc., 20: 447-461.
Dilkes DW and Reisz R 1987. Trematops milleri identified as a junior synonym of Acheloma cumminsi with a revision of the genus. American Museum Novitates 2902.
Olson EC 1941. The family Trematopidae. Journal of Geology 49:149-176.