Tiny Oyster Shells and New Fossil Species Rewrite Germany’s Aalenian Story

Deep in the wooded valleys of the Zollernalb, a ridge of the Swabian Alb in southwest Germany, streams have been quietly cutting through 170-million-year-old rocks for centuries. Now those rocks are talking back. A detailed new study of the Aalenian stage of the Middle Jurassic, published in the journal Zitteliana, has documented a surprisingly rich fauna of ostracods and ammonites from several sections around the Hohenzollern, including three brand-new ostracod species and fifteen more that are probably new to science. The findings overturn a long-standing assumption that ammonites are vanishingly rare in the Upper Aalenian of this region and deliver one of the most finely resolved pictures yet of life on a shallow Jurassic sea floor.

The research grew out of fieldwork on the overlying Wedelsandstein Formation, when the team extended its survey downward into the Achdorf Formation and the uppermost Opalinuston Formation. What they found was unexpected: beds that earlier workers, notably Hahn in 1975, had described as nearly barren of ammonites turned out to yield a diverse assemblage of these iconic spiral-shelled cephalopods. The microfaunal samples proved equally rewarding, packed with ostracods, the tiny bivalved crustaceans whose shells, often less than half a millimetre long, preserve an exquisite record of changing marine environments.

The technical scope of the study is formidable. The team analysed the lithostratigraphy and biostratigraphy of sections at Balingen-Zillhausen, Hechingen-Boll, Jungingen and Hechingen-Beuren, spanning the interval from the topmost Opalinuston Formation at the end of the Lower Aalenian to the basal Wedelsandstein Formation at the dawn of the Bajocian. Forty-one samples taken in 2018 and 2019, supplemented by material collected as far back as 1988, yielded approximately 4,100 ostracod specimens. After careful counting, the researchers identified 108 species, with 329 specimens remaining undetermined. Three new species were formally described: Cytheropterina crassicostata, named for its coarse, rounded ribs; Eucytherura eberti, honouring the palaeontologist Martin Ebert; and Eucytherura foveolata, distinguished by the characteristic pits dotting its lateral surface.

The new species are small but technically distinctive. Cytheropterina crassicostata, at just over half a millimetre long, bears a subtriangular carapace with broad, softly rounded ribs and a smooth underside to its wing-like extension, setting it apart from related forms like Cytheropterina bicuneata. Eucytherura eberti, barely a third of a millimetre, shows broad, smooth ribs running longitudinally across a weakly reticulate surface, with a row of small elongate pits above the muscle-scar field. Eucytherura foveolata carries a fine median rib flanked by shallow pits roughly fifteen micrometres wide, with four rhomboidally grouped pits in the mid-dorsal area providing a particularly striking signature. Ten additional taxa were figured but left in open nomenclature because they were found only in numbers of one to five specimens and could not be confidently assigned to any known genus.

What makes the ostracod record especially powerful is its sensitivity to change. The assemblages shift markedly at several key horizons: at the base of the Lower Aalenian Comptum Subzone, at the boundary between the Bradfordensis and Gigantea subzones, at the Bradfordensis-Concavum zonal boundary, and most dramatically at the Aalenian-Bajocian transition, where the Concavum Zone gives way to the Discites Zone. A minor turnover also occurs above the Calceola-Bank within the Concavum Zone. In the Concavum Zone alone, 34 species appear for the first time, and the total number of genera climbs from seven in the Bradfordensis Zone to twenty in the Gigantea Subzone and twenty-nine in the Concavum Zone. The cytherurid family shows the most striking development, with five new species appearing in the Gigantea Subzone and fourteen in the Concavum Zone.

The ammonite story is equally compelling. The Upper Aalenian faunas of the region are dominated by the Graphoceratidae, a family whose evolution unfolded as a gradual chronocline from the earliest Aalenian into the earliest Bajocian. Rather than representing a parade of distinct genera, the graphoceratid ammonites form a plexus of morphologies in which individual forms sometimes persist across long intervals while the overall character of successive faunas shifts steadily. The researchers found Graphoceras and Brasilia morphologies coexisting in single beds, accompanied by specimens blending characters of both. The team determined the ammonites conventionally as morphogenera and morphospecies, but concluded that all graphoceratids from the cavatum horizon probably represent a single palaeobiospecies whose variation changed through time.

The beds themselves tell a story of shifting sea-floor conditions. The Inopernabank, named after the bivalve Inoperna sowerbyana, contains large Pholadomya in life position and marks the uppermost Gigantea Subzone. The Konglomeratbank, a grey marly limestone packed with pebbles, and the Calceolabank, named for the masses of the small oyster Gryphaea calceola embedded chaotically in its lower part, both belong to the Concavum Subzone. The Calceolabank in particular yielded well-preserved ammonites including Graphoceras cavatum, G. concavum, G. fallax, G. decorum, G. pulchrum and G. formosum, alongside Brasilia decipiens and the microconch Ludwigella arcitenens. The overlying Rostrote Kalkbank, with its rusty-red weathering crust, completes the cavatum biohorizon, which the team formally introduced for the basal Concavum Zone of the region.

Correlating these faunas with other areas reveals a connected Jurassic world. Thirty-three genera and species from the study are also known from the English, Scottish and Irish Aalenian and Lower Bajocian, indicating a marine connection across the Paris Basin. The cavatum biohorizon of the Zollernalb can be matched directly with the cavatum biohorizon of Dorset in southwest England, where Chandler had previously documented the evolution of the Graphoceratidae at Horn Park Quarry. Species such as Cytherella apostolescui, Eocytheridea lacunosa and Procytherura multicostata tie the German sections to coeval faunas from England, France, Spain, Switzerland and even Argentina, where Aalenian ostracods have been described from the Fastnet Basin and the Neuquén Basin respectively.

The macrofauna adds further texture to the picture. Gryphaea calceola, first described from the vicinity of Jungingen, which is therefore considered its type locality, occurs in masses in the Calceolabank but as isolated valves in chaotic arrangement rather than in life position, suggesting storm reworking. Small specimens in other beds probably represent ecophenotypes that never reached maximum size under less favourable conditions. Byssate bivalves like Mytiloceramus polyplocus, recorded only from the Rostrote Kalkbank, may have lived attached to driftwood or empty ammonite shells in a pseudoplanktonic lifestyle. A single tooth of the shark Asteracanthus personati, found in the Zillhausen Member of the Starzel river, stands as the only vertebrate remain from the studied sections, while tanaidacean arthropod remains, once misidentified as tiny salamanders by Friedrich von Huene, occasionally surface in the Upper Aalenian claystones.

The integrative power of combining ammonite and ostracod biostratigraphy is the study’s greatest legacy. Where ammonites are absent or rare, ostracods can carry the stratigraphic signal, and vice versa, allowing significantly improved correlations between sections and across regions. The Heiligenbach section and the Geisingen clay pit together yielded 115 species from the Upper Aalenian, thirty-one of them recorded in both sections, a diversity far exceeding what was previously known for the interval in southwest Germany. The discovery that the lower boundary of the Levata ostracod zone must be shifted downward, and that Kinkelinella triangula appears directly at the base of the Lower Bajocian, refines the regional zonal scheme. For a stage that has long lived in the shadow of its more famous Toarcian and Bajocian neighbours, the Aalenian of the Zollernalb is finally getting its due.

Subject of Research: New records of ostracods and ammonites from the Aalenian (mainly Concavum Zone) of the Zollernalb (Swabian Alb, SW Germany)

Article Title: New records of ostracods and ammonites from the Aalenian (mainly Concavum Zone) of the Zollernalb (Swabian Alb, SW Germany)

Article References: Wannenmacher, N., Dietze, V., Franz, M., & Schweigert, G. (2021). New records of ostracods and ammonites from the Aalenian (mainly Concavum Zone) of the Zollernalb (Swabian Alb, SW Germany). Zitteliana, 95, 1-55. https://doi.org/10.3897/zitteliana.95.56296

Image Credits: AI Generated

DOI: 10.3897/zitteliana.95.56296

Keywords: paleontology, ostracods, ammonites, Jurassic, stratigraphy, Germany, fossils, records, Aalenian, mainly, Concavum, Zone