During most of the Permian (roughly 278–252 million years ago), vast areas of the South American continent were flooded by an epeiric sea with restricted connections to the Panthalassic Ocean. Nearly 1 100 000 km² of the Brazilian territory area is re presented by the intracratonic Paraná Basin. From the Cisuralian (Artinskian, 290.1-283.5 million years ago) to Lopingian (Wuchiapingian, 259.1-254.14 million years ago) stages this basin was filled with a thick (approximately 1400 m) succession of mixed carbonate- siliciclastic rocks of thePassa Dois Group. This unit is divided from the base to the top into the Irati, Serra Alta, Teresina/Corumbataí and Rio do Rasto formations. Throughout the deposition of the Passa Dois Group the basin experienced variable salinity (hypersaline, brackish, and hyposaline) and dissolved oxygen (dysoxic, anoxic) regimes, mixed siliciclastic and carbonate sedimentation, under climatic changes (humid to arid conditions). Given these environmental conditions, the shallow benthic environments of the Paraná epeiric sea were probably stressful for most of the fully marine macro-invertebrates and were mainly colonized by endemic Bivalves. These Molluscs radiated in the basin especially after the Artinskian Irati Anoxic Event. Distinct Bivalve assemblages are known in all units of the Passa Dois Group reaching their maximum abundance and diversity in the Teresina Formation and in the basal part of the Rio do Rasto Formation. A unique feature of this Molluscan long-lived fauna is the dominance of bivalves in all assemblages, whereas Gastropods are rare or even absent in almost all fossil assemblages. The unique exception are some benthic fossil assemblages in the Rio do Rasto Formation where some Hydrobiid Gastropods are recorded.
The Passa Dois Group Bivalve fauna is taxonomically diverse, and in some clades (e.g., Pachydomidae and Astartidae) the extensive radiations were accompanied by high degrees of specialization of life habits. These resulted in relatively high genus and species diversity, with some forms showing a puzzling array of morphologies. This is the case, for example, of the pachydomid genera Anhembia, Ferrazia, and also Leinzia. taxonomic position of some genera remains unclear, mainly due to the poor knowledge of their internal morphology. This is the case of Leinzia, a genus erected in 1949 based on specimens found in the Rio do Rasto Formation cropping out eight kilometers north of Mallet County, State of Paraná, southern Brazil. The genus is monotypic, with Leinzia similis as the type species. Similar forms may occur in coeval Permian strata in Uruguay, Yaguari Formation, and Paraguay, Independencia Formation. Due to its unusual morphology and the poor preservational quality of available specimens, the taxonomic position of Leinzia still remains obscure.
In a paper published in the journal Acta Palaeontologica Polonica on 21 April 2020, Marcello Simões of the Institute of Biosciences at São Paulo State University, Vitor Guerrini of the Institute of Geosciences and Exact Sciences at São Paulo State University, Suzana Matos, also of the Institute of Biosciences at São Paulo State University, and Rosemarie Rohn, also of the Institute of Geosciences and Exact Sciences at São Paulo State University, based on a detailed review of the topotype specimens described by Josue Camargo Mendes in 1949 and description of newly found specimens of Leinzia from the Serrinha Member, Rio do Rasto Formation, we shed light on the taxonomic position of this genus. Contrary to previous authors, the data gathered by Simões et al strongly suggest a Pachydomidae affinity for Leinzia.
The external ornamentation of Leinzia resembles that of some Spinicaudatan Crustaceans, under the name Acantholeaia, leading some authors to describe tiny specimens of Leinzia (and other Bivalves) as true Spinicaudata. However a 1971 study by Bruce Runnagar and Norman Newell rejected that possibility because Leinzia shells have fine growth striae, small ligamental nymphs, well-developed teeth, and a non-rectilinear hinge. They also synonimized a second species Acantholeaia regoi with Leinzia similis (i.e. concluded that the two species were in fact the same, and declared that as Leinzia similis was described first, that this was the valid species name), although some subsequent studies have retained Acantholeaia regoi.
A study by Luis Ferreira-Oliveira in 2007 re-examined the original material from which 'Acantholeaia' was described and noted that in addition to the observations made by Runnegar and Newell, the irregularly spaced growth lines typically observed in Bivalve Mollusc shells are also visible between the comarginal striae of the shells ascribed to Acantholeaia. Furthermore, the gross morphology of the shells resemble that of Holdhausiella and Leinzia. In other words, the tiny bivalved shells belong to Bivalve Molluscs rather than Spinicaudata.
Although the study of Runnegar and Newell provided increased understanding of the Permian endemic bivalves of the Passa Dois Group, the systematic relationships of Leinzia remained obscure. This is due to the poor knowledge of the external and internal morphology of Leinzia shells, and the fact that the specimens illustrated by these and other authors differ greatly from the original material
described by Mendes.
described by Mendes.
Karl Holdhaus described the first specimens of the species in 1918, placing them in the genux Solenomorpha. Holdhaus was apparently unaware of the curious rostrum present in the anterior cardinal margin of the shells. Mendes was the first author to describe this rostrum. Based on this character, he placed them in the new genus Leinzia and referred them to the superfamily Pterioidea. However, five years later he changed his opinion, because Leinzia clearly lacks numerous pterioid shell characters, declaring the taxonomic position of the species to be uncertain.
Based on the pronounced anterior elongation (rostrum) of the Leinzia shells, Norman Newell tentatively assigned Leinzia to the Grammysiidae in 1969. Subsequently, he and Bruce Runnegar referred Leinzia to an incertae familiae of Crassatellacea, having apparently been influenced by earlier authors that noted the hinge similarities of Leinzia similis and Terraia altissima. Runnegar and Newell offered a detailed description of the hinge of Terraia altissima, which is very complex, as follows: 'hinge with large triangular cardinal tooth in right valve and a second obscure tooth between large tooth and ligament nymph; left valve has, correspondingly, obvious triangular socket bordered posteriorly by narrow tooth and socket; anterior and posterior dorsal margins of left valve function as lateral teeth by fitting beneath edges of right valve; obscure posterior lateral tooth present in right valve but no corresponding tooth below anterior lateral socket.' They also mentioned that as 'the affinities of Terraia are not well understood, Leinzia is even more difficult to classify'. Simões et al. consider these comparisons to be problematic, as they are mainly based on poorly preserved specimens, especially from Leinzia.
Subsequent authors have tentatively listed Leinzia in the Sanguinolitidae, Megadesmidae, or Pholadomyida. Unfortunately, no explicit reason has been given for assigning Leinzia to Sanguinolitidae or Pholadomyida. The placement of the genus in the family Megadesmidae was based upon studies of the hinge in Leinzia, which is clearly distinct of that of Terraia altissima.
In summary, the uncertainties about the taxonomic affinities of Leinzia are mainly related to poor knowledge of several shell characters, including the (i) rostrum, (ii) hinge architecture, and (iii) arrangement of the preserved muscle scars.
Simões et al. examined Mendes’ (1949) topotype specimens from the middle part of the Serrinha Member of the Rio do Rasto Formation at a locality eight kilometers north of Mallet County, State of Paraná, southern Brazil. Mendes' material consists of three mudstone slabs (DGP/7-85, 86, 88) containing at least eight specimens of Leinzia similis. The specimens are not individually numbered in the slabs. These slabs would have been part of a shell pavement with densely packed valves. The slabs are stored in the palaeontological collection of the Institute of Geosciences of the University of São Paulo. In addition to Mendes’ material, four specimens of Leinzia similis were also collected in mudstone layers from the Serrinha Member, cropping out on the BR-373 road, near Prudentópolis city in the State of Paraná. These additional specimens are also deposited in the palaeontological collection of the São Paulo State University. The Mendes’ and the new additional fossils material are both represented by molds, mainly composite ones. In some specimens the plastic deformation of the shell is clearly visible, but signs of shell breakage by compression are missing. The shells of Leinzia similis were almost certainly aragonitic originally and probably thin.
(A) Location of the studied area in the southern part of the Paraná State, south Brazil. The outcrop belt of the Passa Dois Group is in grey. (B) Map of the Paraná State, Brazil (frame indicates the southern part where studied area is located). (C) Geological map showing the main geographic area of occurrence of Leinzia similis. (D) Columnar section of the Rio do Rasto Formation; note the constrained stratigraphic distribution of Leinzia similis in the Serrinha Member. Abbreviations: C, calcareous mudstone; CS, coarse sandstone; G, gravel; FS, fine sandstone; MS, medium sandstone; S, siltstone. Simões et al. (2020).
Leinzia similis has a posteriorly elongated (length to height ratio 2.22 to 4.16), compressed, and non-gaping shell. The anterior shell margin is straight, with a well-defined rostrum. The hinge of the right valve has a large, anteriorly inclined, blunt triangular tooth beneath the umbo, and there is a corresponding socket in the left valve; true lateral teeth absent. Shell ornamentation comprises regularly spaced comarginal rugae, which are also present in the rostrum, but absent in the posterior umbonal carina. The anterior adductor muscle scar is small and reniform; the anterior pedal retractor scars are well defined and separated from the anterior adductor; the anterior pedal protractor scar is attached to the dorsal edge of the anterior adductor scar. Other muscle scars and palial line unkown.
Pachydomid Bivalve Leinzia similis, composite molds, Rio do Rasto Formation, Guadalupian (Permian), Paraná Basin, Brazil. (A) DGP/7-88, left valve (A₁) with well-defined, forwardly inclined, triangular socket (arrowed); right valve (A₂), note the deformed anterior rostrum margin of the shell (arrowed); splayed open valves (A₃). (B) DZP-20417, right valve, note the well-developed triangular blunt tooth (arrowed). (C) DZP-20416A, right valve showing well defined escutcheon (arrowed). (D) DGP/7-86, a slightly deformed specimen with articulated valves and broken anterior margin, note the straight slightly curved posterior margin of the shells. Scale bars 5 mm. Simões et al. (2020).
The right valve hinge of Leinzia similis with its large, forwardly inclined triangular blunt tooth beneath the beak and corresponding socket in the left valve is similar to that of various Permian Pachydomid ivalves. However, the gross morphology of the rostrate shell of Leinzia similis differs from all other known Pachydomidae. The general shape, hinge, and muscle scars of Anhembia, the other rostrum-bearing Pachydomidae shell of the Passa Dois Group are distinct from those of Leinzia similis. The anterior muscle scars of Leinzia similis are also similar to those found in the Pachydomidae (i.e., Cowperesia, Pyramus, Astartilla, and Plesiocyprinella), for example. The hinge and external ornamentation of Leinzia similis resemble those of Cowperesia anceps and, in addition, both shells are also laterally compressed. However, the general shell shape of Leinzia similis is completely distinct from that of Cowperesia anceps. Yet, the shells of Cowperesia anceps have a small, well-defined palial sinus and siphonal gape, which are absent in Leinzia similis. The posteriorly elongated shells of Leinzia similis with a well-marked posterior carina resemble that of Holdhausiella elongata. Notably, the cardinal margin of some shells of this species have an anterior prolongation, but not a well-defined rostrum. Yet, the external ornamentation of Holdhausiella elongata has irregularly spaced growth lines, which are clearly different from the regularly spaced comarginal rugae in Leinzia similis. Yet, the hinge of Holdhausiella elongata is edentelous and therefore different from that of Leinzia similis.
Pachydomid bivalves from the Guadalupian (Permian), Teresina and Serra Alta formations, respectively, Paraná Basin, Brazil. (A) Holdhausiella elongata, DZP-20441, silicified shell, left valve, note the pointed anterior margin of the shell. (B) Anhembia gigantea, DGP/7-91, composite mold, left valve, (B₁) general view; (B₂) detail of a well-developed rostrum. Scale bars 5 mm. Simões et al. (2020).
Although the external ornamentation of shells of Huabiella compressa, from the Permian Gai-As Formation, Huab Basin, Namibia, are similar to that of Leinzia similis, small lamellose or spinose projections along the posterior dorsal margin are only recorded in the Namibian shells. Unfortunately, the anterior margin, hinge and muscle scars are still unknown in Huabiella compressa. Therefore, a more precise comparison between Leinzia and Huabiella is currently not feasible. Finally, the hinge and gross morphology of Terraia altissima and other Terraiin Bivalves (Terraia aequilateralis, Terraia curvata, Terraia bipleura) are completely distinct of Leinzia similis and, therefore, the comparison among them is groundless.
The shells of Leinzia similis were originally thin and the specimens are preserved as compressed composite molds. Consequently, the original gross morphology of the shells was subject to distortion and plastic deformation during diagenesis. Based on the examined collection and specimens in the literature three main characters are susceptible to distortion due to taphonomic processes, as follows: (i) the anterior rostrum, (ii) the dorsal margin, and (iii) the umbonal carina. Indeed, in the specimen DGP/7-88 the anterior rostrum is deeply slanted due to compressional plastic deformation of the umbonal area of the shell. Therefore, a much pronounced taphonomically produced anterior rostrum was generated. In the specimen DZP-20417, the cardinal margin of the shell is also deformed and broken, causing substantial modification of the original shell outline. Thus, the anterior margin of the shell of these specimens is obviously distinct of that of non-distorted ones Yet, the rostrum is lacking (broken?) in most of the specimens available in the examined collections as well as in the individuals assigned to Leinzia similis by previous authors.
Pachydomid Bivalve Leinzia similis, Rio do Rasto Formation, Guadalupian (Permian), Paraná Basin, Brazil. (A) DGP/7-88, right valve, detail of the cardinal margin showing a strongly deformed rostrum (arrowed). (B) DGP/7-85, external mold, left valve; (B₁), the comarginal rugae on the anterior rostrum; (B₂) general view of the shell with well-preserved anterior rostrum. Scale bars (A), (B₁) 1 mm; (B₂) 5 mm. Simões et al. (2020).
The dorsal margin of the shell is the other character that is apparently distorted in compressed specimens. Indeed, in non-deformed individuals or less compacted ones the dorsal margin is slightly encurved, whereas in compressed examplars it is straight. The same also occurs with the shape and width of the umbonal carina that is also slightly encurved in those specimens that were subject to lateral compression.
During the late Permian distinct groups of endemic Bivalves from the Passa Dois Group in Brazil and coeval strata in Uruguay as well as in the Huab Basin, Namibia, developed small, posteriorly elongated, rostrate shells with rugose ornamentation. Traditionally, these shells (except those from Namibia) were all assigned to Leinzia. However, detailed examination of the original collection of Josue Camargo Mendes indicates that the shells of Leinzia are distinct from those specimens described by Runnegar and Newell and some subsequent studies. Since Simões et al. rule out the hypothesis that taphonomic processes distorted the original gross morphology of the shells of Runnegar and Newell, they suggest a taxonomic revision before splitting them from Leinzia. However, it should be remembered that the hinge of some specimens illustrated by Runnegar and Newell seems distinct from those in the type specimens of Leinzia similis.
According to Simões et al.'s observations, the hinge of Leinzia similis has close resemblance with that described both in Mendes. The hinge of the right valve of Leinzia similis with a subcardinal, triangular blunt tooth and corresponding socket in left valve, and no lateral teeth is extremely similar to that present in some Permian endemic Pachydomid bivalves (i.e., Cowperesia) of the Passa Dois Group, Brazil. In this context, Leinzia similis with its unique set of curious shell characters may correspond to a highly derived Pachydomid Bivalve.
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