A new and already extinct Thaumastus from shell mounds (sambaquis) in Paraná, Brazil (Mollusca, Eupulmonata, Orthalicoidea)Abstract
Thaumastus teixeirensissp. nov. is a newly discovered species found exclusively within archaeological shell mounds (sambaquis) located on Teixeira Island in Paranaguá Bay, Paraná, Brazil. This species is primarily distinguished by its small size and short, conical spire, along with a blunt, axially striated protoconch. Notably, its complete absence in surrounding habitats suggests that the species is currently extinct.
Keywords:
Gastropoda; Archaeology; Morphology; Conchology; Taxonomy
INTRODUCTION
Out of the 38 species of Megaspiridae within the genus Thaumastus Martens, 1860, with the type species (OD) being Bulimus hartwegi Pfeiffer, 1846 (MolluscaBase, 2024), 18 are found in Brazil (Simone, 2006; Birckolz et al., 2016). This genus is exclusively distributed across South America and is chiefly distinguished by its substantial size and fusiform shell, characterized by an elongated spire and striated, wide protoconch.
Shell mounds, known locally as ‘sambaquis’, are relatively common along the Brazilian coast, particularly in the southeastern and southern regions. The sambaquis are artificial formations made up, for the most part, of mollusk shells that were used both in the construction of these monuments and as food for prehistoric people (hunters-fishermen-gatherers) (Gernet & Birckolz, 2011). In addition to their significant archaeological value, sambaquis also provide rich biological information (e.g., Souza et al., 2010; Rodrigues et al., 2011; Fontenelle et al., 2014; Gernet et al., 2019). By analyzing the shells accumulated within these mounds, researchers can gain precise insights into the past local malacofauna, which may differ from the present-day species composition.
The present paper reveals one of these discoveries: a new species of Thaumastus with a uniquely characterized shell, markedly distinct from any other known species. Unfortunately, efforts to find similar living specimens, or even empty shells, in that region were fruitless, strongly suggesting that the species is currently extinct.
MATERIAL AND METHODS
The specimens were discovered through visual prospecting within the remaining portions of the sambaqui. Collection procedures were conducted manually, eliminating the necessity for excavations. Subsequently, the specimens were sanitized and deposited in the Malacological collection of the Museum of Zoology at the University of São Paulo (MZSP). Photographs were captured using a digital camera either directly or connected to a dissecting microscope. Measurements were acquired using a digital caliper. Conchological terminology adheres to Simone (2011), and taxonomic classification follows MolluscaBase (2024).
RESULTS
Systematics
Thaumastus teixeirensis new species
(Figs. 1-15)
https://zoobank.org/718D3070-9F14-4765-AFCB-827207801BED
Thaumastus teixeirensissp. nov. some shells of type material. (1-4) Holotype MZSP 166000 (L 46.0 mm). (1) Frontal view. (2) Right view. (3) Dorsal view. (4) Detail of apex, profile, scale = 2 mm. (5-6) paratype MZSP 167077#1 (L 43.1 mm), frontal and dorsal views.
Types: holotype MZSP 166000. Paratypes, MZSP 167077, 2 fragmentary, 3 complete shells from type locality.
Type locality: BRAZIL. Paraná: Paranaguá, Sambaqui da Ilha do Teixeira, Ilha do Teixeira, Paranaguá Bay, 25°29′59.88″S, 48°38′38.73″W [Marcos V. Gernet & Fabiano Pinheiro col., 20.vi.2022].
Etymology: The specific epithet is derived from the local of occurrence, the Teixeira Island.
Diagnosis: Shell relatively small, spire proportionally short, acuminate, slightly longer than aperture. Protoconch relatively blunt, axially and uniformly striated, of 3 whorls; slightly mamillated.
Description: Shell outline oval, of ~ 46 mm; spire ~ 54% of shell length. Transverse section circular (Figs. 10, 14). Color axial reddish-brown bands, of irregular width; with perisutural white, narrow stripe, becoming middle, white, narrow, spiral stripe in last whorl, interrupting axial brownish bands (Figs. 1-3, 6, 9, 11, 13). Protoconch relatively blunt, of ~ 3 whorls; first whorl dome-shaped, other 2 whorls weakly increasing, relatively plane, suture shallow; entirely and uniformly sculptures by axial, numerous, dense striae (Fig. 4); limit with teleoconch clear, slightly prosocline, ~ 15° in relation to longitudinal shell axis (Fig. 4); slightly mamillated. Teleoconch of ~ 3.5 whorls, each whorl plane, suture shallow, increasing successively up to rounded last whorl. Last whorl ~ 68% of total length. Peristome elliptic, ~ 45% of shell length, ~ 60% of shell width; slightly prosocline, ~ 15° in relation to longitudinal axis (Figs. 2, 8, 12); outer lip deflected, rounded, simple, interior edge amply rounded; inner lip concave in middle, superior half slightly convex, covered by thick callus not exceeding peristome; inferior half almost straight, with narrow flap covering umbilicus (Figs. 1, 7, 11); oblique columellar fold between superior and inferior halves of inner lip (Figs. 1, 2, 5, 8, 12). Umbilicus opened, but narrow (Figs. 10, 14, 15).
Thaumastus teixeirensissp. nov. some paratype shells. (7-9) paratype MZSP 167077#2 (L 40.0 mm), frontal, right and dorsal views. (10-15) paratype MZSP 167077#2 (L 35.2 mm). (10) Anterior-slightly left view. (11) Frontal view. (12) Right view. (13) Dorsal view. (14) Anterior view (W 20.5 mm); (15) detail of umbilicus, anterior-slight right view, scale = 1 mm.
Habitat: So far know in shell mound (sambaqui).
Distribution: So far only known to type locality, in the Ilha do Teixeira, see more details in the Discussion.
Material examined: Types, being one entire shell (holotype), and the paratypes MZSP 167077, 2 fragmentary shell, being 1 almost complete (paratype #1) (Figs. 5-6), with outer lip missing only; the other (#2) being only part of the last whorl, plus 3 practically complete shells (Figs. 7-13, 2 of them).
Measurements (in mm): Holotype MZSP 166000 (Figs. 1-4): 46.0 by 22.4; paratype MZSP 167077#1 (Figs. 5-6): 43.1 by 20.6 (broken); #2 (Figs. 7-9): 40.0 by 20.6; #3 (Figs. 11-15): 35.2 by 18.8.
DISCUSSION
The generic attribution of Thaumastus teixeirensis sp. nov. is based on several distinguishing features, including the striated and relatively wide protoconch, shell coloration with axial bands, a pale spiral stripe dividing the last whorl, a weakly deflected peristome, a relatively long spire, and the presence of a columellar fold.
Thaumastus teixeirensis has no congeners that can be confused with it. Its relatively small size, conical shape with a blunt tip, and relatively short spire are unique set of characteristics. All other known species (Simone, 2006; Birckolz et al., 2016) have a proportionally longer spire, exceeding 1.5 times the length of the last whorl, whereas T. teixeirensis has the spire and the last whorl in almost identical proportions. Among those congeners with a shorter spire, T. teixeirensis differs from T. hebes (Strebel, 1910), T. nehringi (Martens, 1889), and T. tiradentensis Pena, Coelho & Salgado, 1996, by its slender shell, more pronounced sculpture, smaller aperture, and flatter whorls. It differs from T. magnificus (Grateloup, 1839) in its significantly smaller size (T. magnificus is almost twice the size of T. teixeirensis) and in having a slender shell with an opaque surface (not glossy) and non-uniform coloration.
The Ilha do Teixeira is situated within the Paraná Coast, encompassing two large estuarine complexes known as Paranaguá Bay and Guaratuba Bay. Located in the Paranaguá Bay area, specifically within the Guarapirocaba cove, the island (25°29′26″S, 48°38′52″W) (Fig. 16), named in honor of Francisco Teixeira, was once a significant agricultural locality. It spans approximately 1.47 km² in territorial extension (Maack, 1969). Recent ongoing studies have revealed that the island serves as a habitat for several living endemic species of landsnails; however, T. teixeirensis is only represented by aged shells found within shell mounds. No specimens, whether alive or as fresh empty shells, have been collected from the island or the surrounding region, indicating the probable extinction of the species in the area.
Detail of the Paraná coast, Brazil, showing Paranaguá Bay, and the location of the Ilha do Teixeira, the type locality of Thaumastus teixeirensis (adapted from Gernet et al., 2020).
The type locality of the new species is locally known as “Sambaqui da Chegada”, it gets its name because it is located in the inner cove of Teixeira Island, almost in front of the mooring docks where people and goods disembark. In addition of being an area of intense movement, a cement walkway was recently built causing its complete destruction. This prehistoric archaeological site, where specimens of T. teixeirensis sp. nov. were found, is located in the domain of the Dense Ombrophilous Forest (Atlantic Rainforest), presenting different phytophysiognomies according to climatic, topographic and pedological variations, being classified as a Forest subformation Dense Ombrophilous of Lowlands (IBGE, 1992). This subformation is characterized by a very typical floristic and structural composition composed of different communities along the altitude gradient and pioneer formations. This ranges from herbaceous vegetation to arboreal remains rich in epiphytes, which can present discrete variations in composition due to the drainage and fertility conditions of its soils, with the conditioning factor being essentially edaphic (IPARDES, 1995; Gernet et al., 2020). This phytogeographic unit is characterized by indisputable environmental importance, with high biological diversity and a large number of endemisms. Thus, the possibility of endemic species in Teixeira Island, including T. teixeirensis, is not an anomaly.
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