Ontogenetic disparity in early planktic foraminifers

Abstract

Abstract. Changes in morphology during ontogeny can have profound impacts on the physiology and biology of a species. Studies of ontogenetic disparity through time are rare because of the lack of preservation of developmental stages in the fossil record. As they grow by incremental chamber accretion and retain evidence of growth in their shell, planktic foraminifera are an ideal group for the study ontogenetic disparity through the evolution of a higher taxon. Here, we quantify different developmental stages in Jurassic foraminifers and infer the evolutionary implications of the shape of these earliest representatives of the group. Using a Zeiss Xradia micro-CT scanner, the development of Globuligerina bathoniana and Globuligerina oxfordiana from the Bathonian sediments of Gnaszyn, Poland, and Globuligerina balakhmatovae and Globuligerina tojeiraensis from the Kimmeridgian Tojeira Formation of Portugal was reconstructed. Disparity is low through the early evolution of planktic foraminifers. The number of chambers and range in surface area per unit volume are lower than in modern specimens. We interpret this morphology as an indication of opportunistic behaviour. The low morphological plasticity during the juvenile stage suggests that strong constraints on the juveniles, described in the modern ocean, were already acting on Jurassic specimens. The high surface area per unit volume in these developmental stages points towards the need to satisfy a higher metabolic demand than in the adult specimens. We are interpreting the lower chamber numbers as indicative of short life cycles and potentially rapid reproduction, both of which may have allowed these species to exploit the nutrient-rich waters of the Jurassic Tethys Ocean.

Jurassic planktic foraminifera from the Polish Basin

“…The stratigraphical distribution of the studied taxa from the Polish Basin closely matches the biostratigraphical scheme of Gradstein et al (2017b). In the studied area, the earliest forms recorded are G. bathoniana and G. balakhmatovae, which occurred in the Morrisi Zone (middle Bathonian) of Gnaszyn (see also Pazdrowa, 1969;Bielecka and Styk, 1981;Hart et al, 2012;Smoleń, 2012;Kendall et al, 2020) and Kawodrza (Fig. 6).…”

“…Globuligerina oxfordiana (Grigelis, 1958) (Loeblich and Tappan, 1987;Hudson, 2009;Gradstein et al, 2017a); see earlier discussion. Subsequently, several sites from extra-Carpathian Poland were studied in terms of Jurassic planktic foraminifera (Kraców region, by Olszewska and Wieczorek, 1988;Ogrodzieniec, by Hudson, 2009, andHart et al, 2012;and Gnaszyn, by von Hillebrandt, 2012, Smoleń, 2012, and Kendall et al, 2020. Moreover, planktic foraminifera were recorded in microfossil assemblages from boreholes in northern Poland (Chrzanów, by Bielecka, 1960;Kcynia, by Bielecka and Styk, 1964;Pasłęk, by Bielecka and Styk, 1966; Bartoszyce, by Styk, 1966, andBielecka, 1974;Grzybnica, by Bielecka and Styk, 1981;and Wolin, by Bielecka and Styk, 1981) where, apart from G. bathoniana from Wolin (Fig.…”

“…These were thought to represent open-ocean environments in which a 99 : 1 P : B ratio indicates planktic ooze deposition above a Jurassic aragonite compensation depth (ACD), although the actual water depth is impossible to estimate. The palaeoenvironmental evidence of Hudson et al (2009) and Hart et al (2012), coupled with the morphological analysis of Kendall et al (2020), is suggestive of a surface-water dwelling habitat for most Jurassic planktic foraminifera.…”

“…In a recent study of Globuligerina spp. from Poland and Portugal, Kendall et al (2020) -using micro computed tomography (micro-CT) analysis -suggest that this morphology reflects short life cycles and potentially rapid reproduction in nutrient-rich coastal environments. In the Czorsztyn Limestone Formation (Oxfordian) of the Pieniny Klippen Belt of Poland, Hudson et al (2009) described thin sections full of simple, globigerina, forms (often with a maximum of four chambers in the final whorl) and very few associated benthic taxa.…”

“…The first planktic foraminifera from the area of Poland are actually recorded from the Bathonian: G. bathoniana and G. oxfordiana (e.g. Pazdrowa, 1969;Smoleń, 2012;Hart et al, 2012;Kendall et al, 2020).…”