Kazacharthrans are exclusively fossil crustaceans restricted to the Triassic and possibly Jurassic period and were first described by Tchernyshev in 1940. Kazachartha is supposed to represent the sister group to Notostraca (tadpole shrimps). Little is known about the morphology of most kazacharthrans, and in general only few publications are available. We present here a specimen with kazacharthan-related traits, which we compare to the co-occurring species Notostraca minor (formerly Triops cancriformis minor), a notostracan representative. Both specimens come from the Museum Terra Triassica in Euerdorf, Lower Franconia, southern Germany and were documented using cross-polarized light and autofluorescence settings to achieve well contrasted, high-resolution images. Key morphological features of the kazacharthran-like specimen are the very broad shield without a dorsal midline and spines, a broad and elongated posterior trunk and a square-shaped telson with lateral bulges. Also some details, like different eye structures, mandibular and cervical groove, intestine, and the paired shell glands are visible. Comparison to Notostraca minor revealed differences in these morphological features between the two specimens. We suggest an interpretation of the presented specimen near Kazacharthra due to the described features and discuss the importance of the specimen for branchiopodan phylogeny and biogeography.
Introduction
Kazacharthra is an exclusively fossil group of branchiopod crustaceans whose representatives only occurred in the Triassic and possibly Jurassic periods (Olesen, 2009). Little is known about Kazacharthra and interpretations about this group can only be made in comparison with related extant groups. The group is generally assumed to be the sister group of Notostraca (tadpole shrimps) due to many morphological similarities, but there are only a few publications available with barely any morphological details (see below). Fossils known so far originate from China, Kazakhstan, Kyrgyzstan, Mongolia and Turkmenistan (Voigt, 2007), with two exceptions, one supposed occurrence in South America (Martins-Neto et al., 2001) and one specimen from Germany, showing kazacharthran-like affinities, but which has not been described in detail (Voigt et al., 2008). Kazacharthrans were first described by Tchernyshev (1940); about 25 species are known (e.g. Novozhilov, 1957; Novozhilov, 1959; Badamagarav, 1985; McKenzie et al., 1991; Olesen, 2009). The habitats of kazacharthrans were likely similar to those of extant notostracans and included shallow lakes and ponds. The assumptions on the feeding habits of kazacharthrans range from feeding on small organic particles from the mud to predatory or even cannibalistic behavior (McKenzie and Chen, 1999). New findings of kazacharthrans can always provide new insights into this group.
Morphologically, kazacharthrans are characterized by a broad and rather flat and thin shield without a dorsal median ridge, in contrast to Notostraca (e.g. McKenzie and Chen, 1999). The shield has a convex upwards shape with eye structures on the dorsal side. The kazacharthran labrum is relatively small, but compared to the front of the head and the head shield in general it has about the same size as the labrum of extant notostracans (Fryer, 1988). The shield has been described to house a pair of shell glands.
In general, the kazacharthran body is organized like that of extant notostracans, into head, anterior trunk, posterior trunk and telson. Anteriorly, the head bears a pair of small antennulae. These are simple and not subdivided. Further posteriorly, the head bears a pair of antennae. The mandibles are similar to those of modern notostracans, large with strong muscles. Additionally, the head region has paragnaths, maxillulae and maxillae. The kazacharthran anterior trunk includes at least 8 and possibly as many as 11 segments (McKenzie et al., 1991) each bearing a pair of appendages. The posterior trunk region consists of a maximum of 31 segments with lateral spines and dorsal ridges. The anterior segments of the posterior trunk region also bear at least one pair but up to several pairs of appendages (Olesen, 2009; Hegna and Ren, 2010). The posterior segments of the posterior trunk are legless. The posterior trunk is followed by the telson with caudal spines and the anal opening. A furca is present similar to extant notostracans (Chen et al., 1996; McKenzie and Chen, 1999).
Figure 1.
Comparison of specimen 1 (kazacharthran-like specimen), and specimen 2 (Notostraca minor). A–C, SMTE 5930-2-12, Neubrunn quarry, preserved in dorsal aspect; A, cross-polarized light; B, autofluorescence; C, as A, with color-marks (online version only); D, E, SMTE 6030-1-73-2, Eltmann quarry, preserved in ventral aspect; D, cross-polarized light; E, as D, with marks and color-marks (online version only). Marks: sh, shield (light blue); el, antero-medial elevation on the shield (light green); ce, compound eyes (light red); me, median eye (dark purple); le, “lateral eyes” of McKenzie and Chen (1999) (orange, see discussion); do, dorsal organ (red); w, W-shaped structure posterior to the elevation (light purple); pts, posterior trunk segments (green and blue); ts, telson (cyan); fr, furcal rami (yellow).
Here, we report a specimen with kazacharthan-related traits, from the Triassic of Germany which co-occurs with the notostracan species Notostraca minor. Differences in morphology and the importance of the specimen for branchiopodan phylogeny and biogeography are discussed.
Material and methods
Material
Two specimens were the basis for this study, one specimen with supposed kazacharthran affinities (specimen 1, SMTE 5930-2-12; Figure 1A–C, Neubrunn Quarry) and one specimen of the triopsid Notostraca minor (specimen 2, SMTE 6030-1-73-2; Figure 1D, E, Eltmann Quarry); both are part of the collection of the Museum Terra Triassica in Euerdorf. Specimen 1 was found in the collection during an earlier study on the triopsids from these deposits, but no other similar specimen was found, although the entire collection was searched thoroughly.
Figure 2.
Details of specimen 1, (kazacharthran-like specimen) SMTE 5930-2-12. A, overview; small white arrows mark intersegmental membrane. B, detail of eye structures, surrounding elevation, mandibular and cervical groove. White arrows mark the mandibular and cervical groove. C, detail of posterior trunk segments. White arrows mark intersegmental membrane. D, detail of the head shield. Black arrows mark compression lines in the shield due to postmortem flattening. E, detail of telson. Small black arrows mark depressions of telsonal setae.
Geological settings
The specimens stem from the Upper Triassic (Carnian, ca. 237–227 Ma) of Lower Franconia, southern Germany, near Würzburg, from the so-called “Coburger Sandstein” (Hassberge Formation). This stratum is composed of sandstone with alternating claystone and siltstone layers, also including thinner layers of dolomite (for further details, also concerning the formation, see Freyberg, 1965; Hornung and Aigner, 2004; Figure 3). Water level and size of the freshwater bodies varied depending on the semi arid to arid climate strongly influenced by monsoons.
A comparable geological setting was described by Hornung and Aigner (2004) for the quarry “Schönbachsmühle”. In all quarries, a series of nine types of lithofacies and seven architectural elements occurs, characterized by different sedimentological and petrophysical features. Specimen SMTE 6030-1-73 from the Eltmann quarry, was found in a dried-out channel of an abandoned river arm. High waters washed up large amounts of dissolved particles, which rapidly deposited numerous notostracan and spinicaudatan branchiopod crustaceans as well as fishes; also, some few insect and plant remains sedimented together with the fauna at this place. Specimen SMTE 5930-2-12 was found in fine-grained claystone-siltstone sediment, together with spinicaudatan branchiopods and insect and plant remains in the Neubrunn quarry.
Documentation and image processing
The specimens were documented with macrophotography using a Canon EOS Rebel T3i equipped with a Canon Macro Lens MP-E 65 mm. For illumination, a Canon macro twin flash MT 24 EX was used. The light was cross-polarized to reduce reflections (e.g. Schaarschmidt, 1973; Bengtson, 2000; Kerp and Bomfleur, 2011; Haug et al., 2011, 2012). Due to the three-dimensional surface of the fossils and the limited depth of field, one image was not sufficient to get a sharp image. Therefore, several images in different focus layers were taken and fused into one sharp image using the freeware Combine ZM. Due to the large size of the specimens and the limited field of view, adjacent stacks were stitched together using the photomerge function of Adobe Photoshop CS2 (Haug et al., 2009). Adobe Photoshop CS2 was used to optimize brightness and contrast, and the filter “unsharp mask” was used to reduce blur.
Specimen 1 was also documented using a Keyence BZ-9000 fluorescence microscope, utilizing the autofluorescence capacity of the fossil (excitation wavelength: 543 nm, TRITC; e.g. Rötzer and Haug, 2015). The objective used had a 2 × magnification, resulting in a final magnification of about 20 ×. The generated image stacks were processed as outlined above.
Additionally, different structures were color-marked in both specimens using Adobe Photoshop CS2, based on the images taken under cross-polarized light.
Results
Specimen 1
Cross-polarized light.—Specimen 1 is preserved in dorsal view with shield, posterior trunk and telson. The kidney-shaped head shield has a posterior notch and is wider than long, with length to width ratio of about 2/3. The anterior margin is well preserved, while the posterior margin is not demarcated clearly. The edge seems to be smooth with no spines visible; a pronounced dorsal midline is missing. The shield has a dorsally convex shape. Compression lines in the shield surface are visible due to the postmortem flattening process. Anterior appendages are not visible (Figures 1A, C, 2A, E).
Antero-medially on the shield, elevated eye structures occur. The anterior part of the elevated area shows a small, dark, roundish structure, the supposed median eye. In the posterior part, a similarly shaped and sized structure is present, the supposed dorsal organ. Between these two structures, slightly laterally from the midline, two larger dark areas are present, presumably compound eyes (but see below). Posterior to the elevation a W-shaped structure is apparent. The two posterior bends of the “W” each form an angle of approximately 90–100°. Its ends are drawn out laterally and terminate in small, dark areas on each side (see also discussion below). The area posterior to the W-shaped structure is characterized by folds, the mandibular and cervical groove. It is four times wider than it is long and is about one-fourth of the width of the shield (Figures 1A, C, 2A, B, E).
Due to the dorsal preservation of the specimen, the anterior trunk, which is located underneath the shield with at least 8 and possibly as many as 11 segments (McKenzie et al., 1991), is not visible. Also the thoracic limbs are concealed by the shield. The anterior trunk is followed by the posterior trunk with a length to width ratio of about 5/1, which consists in kazacharthrans of a maximum of 31 segments (e.g. McKenzie and Chen, 1999). In our specimen, 29 of these posterior trunk segments are preserved. The missing two may not have been developed or, which seems to be more likely, are overhung by the shield. Additionally, the number of segments in notostracans is highly variable and thus can only be of limited confidence (Korn and Hundsdoerfer, 2006).
In some cases, connecting membranes between the segments are visible, which were important for posterior trunk flexibility (McKenzie et al., 1991) (Figures 1A, C, 2A, C, D). At the posterior end, a slightly set-off structure is apparent, the telson. It is roughly square-shaped, with bulged lateral sides (Figures 1A, C, 2A, E). Also two small, brighter structures are recognizable, which may be equivalent to structures described for kazacharthrans addressed to as ‘boundaries between intrasegmental ornamentary structures’ by McKenzie et al. (1991; Figure 2E). Olesen (2009) suggested that these are small depressions of telsonal setae (a character for representatives of Phyllopoda). The posterior part of the telson and the furcal rami are not visible due to the fact that the specimen is not entirely preserved.
Autofluorescence.—The autofluorescence image displays some additional information. Besides the shape of the shield, the posterior trunk shape is better visible in some areas. Due to the autofluorescence, inner structures show a better contrast to the rest of the specimen. The presumed paired shell glands in the shield are very prominent, taking up nearly the entire shield. Also, the intestine is strongly contrasted and is visible along the complete posterior trunk (Figure 1B).
Specimen 2
Specimen 2 appears to be preserved in ventral view and preserves shield, thoracic segments (anterior trunk), posterior trunk, telson and furcal rami. The shield has a length to width ratio of about 1/1. The shield margin is nearly entirely preserved, but is missing some posterior parts. Hence, spines in the posterior part of the shield are not visible. Also, the dorsal midline is not identifiable, due to the very prominent intestines and remains of the thoracic appendages covering the respective area. The shield is bent convex dorsally and shows compression lines due to postmortem flattening (Figure 1D, E). Dorsal structures, such as eye structures or the elevation around them are not visible due to the supposed ventral orientation of the specimen.
The 11 thoracic segments and its appendages are not very well preserved. The specimen shows remains of anterior thoracopods, and also remains of a series of posterior thoracopods are visible. 28 segments are visible in the posterior trunk (abdomen). The exact number cannot be revealed due to the overlying structures and slightly incomplete preservation. The posterior trunk segments are followed by the telson which is not entirely preserved. It is square shaped, with the lateral sides straight, but notched. Furcal rami arise from these notches, though only one ramus is preserved in this specimen (Figure 1D, E). The posterior part of the telson lying between the furcal rami is also slightly bent anteriorly.
Discussion
In the following, we will discuss the observed characters of specimen 1. We will especially emphasize the differences from specimen 2, a representative of the co-occurring species Notostraca minor and discuss why specimen 1 is not conspecific.
Shield shape.—The most striking character of specimen 1 is the shape of the shield. With the length to width ratio being 2/3, its broad shape would be typical for a representative of Kazacharthra. In comparison, the shield of specimen 2 is as long as wide. In extant notostracans, the shield is often even longer than wide and has an elongate elliptic form. The missing pronounced dorsal midline, which would be visible in a dorsally preserved notostracan, also hints to a closer relationship to Kazacharthra instead of Notostraca (Trusheim, 1938; McKenzie and Chen, 1999). In this aspect the Chinese fossil Jeholops hongi is also of interest. The single known specimen has a shield that appears quite similar to the here described specimen and has consequently also been discussed to share certain kazacharthran-like features (Hegna and Ren, 2010).
Eyes.—There are two different hypotheses regarding kazacharthran eyes. The first one states: Kazacharthrans are interpreted to possess, similar to notostracans, a pair of compound eyes resting on an elevation. Anteriorly between these compound eyes, a supposed median eye is located, and slightly posterior to the compound eyes the supposed dorsal organ is situated (McKenzie et al., 1991). This hypothesis was rejected by McKenzie and Chen (1999). They reinterpreted the structures and stated that the location of the assumed compound eyes was in fact a structure they named the eye slit pore and that kazacharthrans possessed a pair of more laterally positioned compound eyes instead. They also pointed out that there is a W-shaped structure in this region, connecting these more laterally positioned eyes and the dorsal organ. According to this theory, the small dark areas at the ends of the W-shaped structure in specimen 1 could represent these eyes. Unfortunately, McKenzie and Chen (1999) did not explain this interpretation in more detail, which is at odds with all known about branchiopod morphology. We cannot finally decide if the theory of McKenzie et al. (1991) or that of McKenzie and Chen (1999) is correct. Yet, we propose that it is more parsimonious to assume that kazacharthrans had a similar, more median positioning of the eyes as notostracans given their supposed sister group relationship, supporting the hypothesis of McKenzie et al. (1991, see also Tchernyshev, 1940).
Mandibular and cervical groove.—Remains of the mandibular and cervical groove are visible in specimen 1. The anterior mandibular groove and the posterior cervical groove are structures known from both Notostraca and Kazacharthra (McKenzie et al., 1991).
Posterior trunk.—The posterior trunk of specimen 1 is elongated with a length to width ratio of 5/1. Noticeable is the size of the posterior trunk itself. It is longer and also much broader in specimen 1 than it is in specimen 2, in comparison to the shield. Twenty-nine posterior trunk segments occur in specimen 1. In Notostraca, the number of posterior trunk segments is variable, up to 33 (Fayers and Trewin, 2002); Kazacharthra show up to 31 segments (e.g. McKenzie and Chen, 1999 for Almatium gusevi). In specimen 2, 28 segments were found. This cannot serve as a point for a sound phylogenetic interpretation. However, the elongate and broad shape of the posterior trunk of specimen 1 slightly points towards kazacharthran affinities.
While the specimen resembles the single specimen of Jeholops hongi in shield shape it differs strongly in trunk morphology. In J. hongi the trunk is rather short, which is in contrast to the very elongate trunk in specimen 1. Although the holotype of J. hongi is comparably small it is unlikely a larva or early juvenile, which could explain a rather short trunk.
Telson.—The telson shape differs between specimen 1 and 2. The almost squared shape, with bulged lateral sides in specimen 1 is kazacharthran-like and differs strongly from the squared shape with lateral notches in specimen 2 and other notostracans (e.g. Novozhilov, 1957; Novozhilov, 1959; Badamagarav, 1985; McKenzie et al., 1991).
The interpretation of the structures present on the telson as ‘recognizable boundaries between intrasegmental ornamentary structures’ would point to a similarity to the specimen described by McKenzie et al. (1991). Yet, we think, in concordance with Olesen (2009) that these structures more likely represent small depressions of telsonal setae. Telsonal setae, however, are a feature of all representatives of Phyllopoda and therefore cannot support a delineation of these two specimens. Hence only the shape can be used in this context.
Considering the characters of specimen 1, we suggest a closer relationship of the specimen to Kazacharthra than to Notostraca. However, we cannot make a more precise phylogenetic interpretation as to whether specimen 1 represents an ingroup of Kazacharthra or a possible sister species.
Consequences
The specimen despite its incompleteness plays an important role in branchiopodan biogeography and evolution. It is the first reliable report of a kazacharthran-like branchiopod from Europe (the specimen from Germany depicted by Voigt et al., 2008, their figure 3C, D, has not been described or discussed in detail) and one of the very few reports outside of Asia (e.g. McKenzie and Chen, 1999; Martins-Neto et al., 2001; Voigt et al., 2006). This may point to a wider geographic distribution of kazacharthrans than previously assumed. Besides the kazacharthran-like features, the specimen also has certain similarities with notostracans (see also Hegna and Ren, 2010 for a similar case), which enhances its possible importance for phylogenetic questions. We would need more specimens in better preservation to address open questions in detail and to achieve a more reliable phylogenetic interpretation.
Acknowledgements
We would like to thank all the people supporting the Museum Terra Triassica and all the private collectors who provided the examined specimens. Tom Hegna (Western Illinois University, Macomb, Illinois, USA) and one anonymous reviewer provided helpful comments on the manuscript. We are grateful to J. Matthias Starck for interesting discussions and support of this study. Furthermore, we thank all the people spending their time programming open source or open access software that was used during this study, such as Open Office and Combine ZM.
