780
Journal of Paleontology 92(5):768–793 Table 1. Characters utilized in phylogenetic analysis.
1. Shell profile. 0 biconvex; 1 convexo-concave; 2 resupinate; 3 concavo-convex or planoconvex; 4 dorsal valve strongly arcuate, gibbous. 2. Outline. 0 semilelliptical; 1 transverse; 2 auriculate or alate. 3. Geniculation. 0 absent; 1 present; 2 strong defining disc and trail. 4. Radial ornamentation. 0 multicostellate or ramicostellate; 1 equally parvicostellate; 2 unequally parvicostellate. 5. Concentric fila. 0 absent; 1 present. 6. Rugae. 0 absent, 1 present on posterolateral areas; 2 strong on the entire surface. 7. Ventral fold. 0 absent; 1 present. 8. Dorsal sulcus. 0 absent; 1 present. 9. Ventral interarea. 0 apsacline; 1 catacline to procline. 10. Dorsal interarea. 0 anacline to orthocline; 1 catacline to hypercline 11. Chilidium. 0 absent; 1 short; 2 large. 12. Delthyrial cover: 0 absent; 1 separated into delthyrial plates; 2 pseudodeltidium. 13. Coverage of pseudodeltidium. 0 absent; 1 apical; 2 nearly complete. 14. Dental plates. 0 absent; 1 short recessive; 2 long well defined. 15. Ventral subperipheral rim. 0 absent; 1 low discontinuous; 2 strongly thickened; 3 defining a disc and trail. 16. Dorsal platform. 0 absent; 1 weak or discontinuous; 2 strong complete; 3 coalescing septules forming curved ridges. 17. Ventral muscle field. 0 short confined delthyrial cavity; 1 long and narrow; 2 small bilobed; 3 large subquadrate or subpentagonal; 4 large bilobed. 18. Ventral muscle bounding ridges. 0 absent; 1 present. 19. Dorsal muscle field. 0 indistinct; 1 quadripartite, orthoid. 20. Dorsal median ridge or myophragm. 0 absent or short; 1 prominent, long and rounded; 2 high bladelike. 21. Cardinal process. 0 simple ridgelike; 1 prominent or bulbous. 22. Ventral mantle canal system. 0 indistinct; 1 impressed on the valve margin; 2 strongly impressed on the entire surface. 23. Shell structure. 0 impunctate; 1 pseudopunctate.
Ahtiella clade appears to have occurred by cladogenesis because A. coloradoensis, a conservative species closely related morphologically to Monorthis, persisted in the Central Andean Basin until the late Darriwilian. Perhaps the main conflict posed by the phylogenetic hypothesis of Figure 7 is that A. quadrata is shown as a basal member of the clade because the Welsh species retains some traits ancestral to A. zarelae and A. famatiniana n. sp. A possible explanation is that A. quadrata originated from a Gondwanan ancestor and then migrated along Gondwanan shelves to reach the colder Avalonian waters. However, to date no records of morphologically related forms are known from Gondwana, which could be due either to a lack of extensive sampling in the still poorly known Bolivian and Peruvian sectors of the Central Andean Basin or to the absence of this species in the Andean region. An alternative interpretation is that A. quadrata evolved independently in the Avalonian paleocontinent from a local species of Monorthis. The possibility of parallel evolution at the generic level as well as its possible causes were discussed in a previous paper (Benedetto, 2008) to account for the nearly simultaneous record of the genus Productorthis in Baltica and the Famatinan volcanic arc, with underlying strata in both regions of the ancestral genus Panderina Schuchert and Cooper, 1931. In our case, the Welsh species M. menapiae of ‘lower Arenigian’ (Floian) age (Bates, 1969) closely resembles the Famatinan species M. transversa and could be a potential ancestor of A. quadrata and, eventually, of the ‘anomalous’ species A.
Table 2. Character state distribution for taxa included in phylogenetic analysis. 1 2
Hesperonomiella
A.lirata
A. tunaensis 0 01
3 4567 8910 0
0 0 0
A. paucirugosa 22 21 01 1 A. argentina 22 22 11 1 12 2 2 2
A. jaanussoni 42 22 12 0 A. quadrata 22 10 0
M. transversa 22 00 00 1 42 22 12 0
01 0
A. famatiniana A. zarelae
A. coloradoensis
22 01 01
2 1 1
1 0
0 0
0 0
01 01
1
12 10 1 0 0
1 1
0
0 1 0 1 1 1
01 1
1 11 0 0
1 0 1 1 1
0 1
concava. It seems likely that cases of parallel evolution in brachiopods, and thus the existence of paraphyletic genera, might be more frequent than previously thought. In summary, available evidence from the Gondwanan
material supports, contrary to previous assumptions, that the basal plectambonitoid Ahtiella could have evolved from the hesperonomiid orthoid Monorthis transversa, and that A. famatiniana n. sp. and A. zarelae are not only the earliest species of the genus, but also are morphologically intermediate between M. transversa and the more derived species of Ahtiella from the Darriwilian of Cuyania and Baltica (Fig. 8).
Some macroevolutionary implications.—There is a general consensus that the peculiar strophomenide shell architecture and concomitant anatomical and physiological changes were adaptations (key innovations) allowing invasion of a new eco- logical niche or adaptive zone, which was essentially the acquisition of an ambitopic or permanent liberosessile life strategy linked to the colonization of low-energy, offshore marine environments (Bassett, 1984). Over time, such mor- phological changes became so marked that they led to the recognition of a separate higher taxon, the order Strophome- nida, which together with other groups was lumped into the large and quite heterogeneous class Strophomenata. Although higher taxa are often viewed as artificial, nonmonophyletic, subjective entities, a number of quantitative studies have
11
0 0 1 2 1 1 0 1 0 1 0
12 13 14
0 01 01 0 0 1 1
0 2 1 1
12
2 11 20 31 1 2 1
? ? 01
? 1 ? ? 1 3
3
01 00 21 1 ?
011 21 31 01 1 4 1
15 0
16 0
17 18 19
0 01 00 00 01 2 22 31 31 0 ?
01
2 21 33 00 01 2 22 31 41 0 2 22 31 31 0 3
20 0 01 1
21 0
0 0 01
22 0
01 2
23 0
0
1 00 1 1 00 1 1 00 ? 1 1
1 0 3 10 1 1 0
012 1 01
1 01 ? 0
01 0 ?
? 1 1
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