Pseudoscapholeberis enigmatica, Flössner & Fryer, 2016
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publication ID |
https://doi.org/ 10.1080/00222933.2015.1134693 |
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persistent identifier |
https://treatment.plazi.org/id/9264A008-3378-FFE2-FD5D-8658289B6220 |
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Felipe |
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scientific name |
Pseudoscapholeberis enigmatica |
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gen. et sp. nov. |
Pseudoscapholeberis enigmatica gen. et sp. nov.
General form of female, as seen laterally, showing similarities to the extant Scapholeberis . Carapace almost square in lateral aspect. Ventral margins straight. Postero-ventral extremities drawn out to a more or less triangular prolongation whose ventral margin is inclined slightly dorsally. Posterior margins with row of probably soft setae extending over entire length. Ventral margins with row of short, robust setae extending over much of length.
Head similar to that of Scapholeberis , and with anterior ‘horn’ in vicinity of compound eye as in some representatives of that genus. No depression between carapace and head.
Postabdomen elongate, somewhat bilaterally compressed, and – unique within the Anomopoda – segmented: its distal half, or rather more, consisting of four segments. Proximal region undivided. With stout, curved terminal claws, and with ventral armature of probably five very long marginal spines on each side.
Type locality
Quarry of Cadeuil , Charent-Maritime, France .
Age
Lower Cretaceous. Upper Albian or Lowermost Cenomanian. (c.100 My.) Holotype: a parthenogenetic female, embedded in amber: IGR.CDL-2.108 housed in the amber collection of the Geological Institute and Museum, University of Rennes 1.
Diagnostic features
The segmented postabdomen alone is diagnostic of the new species – and genus. This attribute indeed sets it apart from all other known members of the Anomopoda , extant or fossil; nor is such known in the distantly related Ctenopoda.
Known only from the holotype; a first instar female, embedded in amber of Early Cretaceous age.
General description
The right side of the fossil reveals its general form ( Figures 1 View Figure 1 , 2A, C View Figure 2 ). Its trunk has been forced ventrally within the carapace chamber and its limbs have been pushed outside it and lost. The robust postabdomen has been pushed largely outside that chamber, which facilitates observation of this remarkable structure. Only moderately bilaterally compressed, it has twisted rather more than 90° and presents its topographically upper surface to the observer. Rather more of the left side of its armature has been preserved than of the right.
Carapace almost square in lateral aspect: its height being only a little less than its length. Maximum height near middle. Dorsal margin gently curved. Postero-dorsal angle obtuse. Anterior and posterior margins almost straight. Ventral margins straight: each drawn out posteriorly into a broad, tapering projection that is densely pigmented, probably by melanin, postero-ventrally.
Ventral margins each with a single row of robust fringing setae, of similar length throughout the straight portion, then diminishing in size posteriorly on the posteroventral projection. ( Figures 1 View Figure 1 , 2A View Figure 2 ). Their exact nature cannot be elucidated in the holotype, because of the thick layer of amber through which they have to be viewed, but they appear to be flattened [as they are in different ways in the various species of Scapholeberis – see illustrations in Scourfield (1894) and Dumont and Pensaert (1983)] and to overlap. An interpretation of what can be seen only vaguely is given in Figure 2B View Figure 2 . That some such arrangement existed is an indication of both the affinities and probable habits of the extinct species. That such setae, irrespective of their detailed structure, should be present on the straight ventral margins of the carapace suggests that, as is the case in extant species of Scapholeberis , they had water-repellent properties, and can confidently be taken as indicating association with the surface film. Posterior margins with a row of ‘soft’, translucent setae; much less robust than those of ventral margins.
Head – pushed backward between the carapace valves in the fossil ( Figures 1 View Figure 1 , 2A View Figure 2 ) – shown in outline in Figure 2C View Figure 2 as interpreted after being ‘pulled forward’ to probable location in life. Antero-dorsal margin ( Figures 1 View Figure 1 , 2A View Figure 2 ) more or less straight or gently curved. With short, curved, ‘horn-like’ spine, strikingly similar in form and location to that often present in the extant Scapholeberis mucronata , to which P. nana shows considerable similarity. Eye, some of whose lenses are preserved, large, densely pigmented, probably by melanin, and located in antero-ventral region – which is a significant indicator of habits. Antennules possibly represented by projection(s) posterior to eye, but not located with certainty. Probable location in life indicated in Figure 2C View Figure 2 . What, almost certainly, is the masticatory region of a mandible that has been pushed posteriorly, can be seen through the carapace ( Figures 1 View Figure 1 , 2A View Figure 2 ).
Antennae lost, and trunk limbs evidently pushed ventrally outside carapace valves and lost – though one long seta near anterior end ( Figure 2A View Figure 2 ) is presumably from a lost limb.
Postabdomen well preserved, its distal half, or thereabouts, having been pushed beyond the ventral carapace margins and therefore easy to observe. It is much the most remarkable structure of the animal to have been preserved. Two preliminary statements are necessary. The postabdomen in anomopods is, almost invariably, most easily observed in lateral view. That of the fossil has twisted and presents its topographically dorsal aspect ( Figures 1 View Figure 1 , 2A, E View Figure 2 ). A lateral reconstruction is provided ( Figure 2F View Figure 2 ) with the proviso that shape and dimensions are only estimates. Only the dorsal view presents an accurate representation of the actual specimen.
Unlike that of all known extant anomopods, the postabdomen of P. nana is segmented, and very clearly so. Basal region comprising rather less than half the total length, distal to which are four shorter segments. Segments 1–3 simple, of approximately similar length; distal segment (4) conical, tapering distally: with cuticular annulations, difficult to see, which perhaps granted a modicum of flexibility.
Armature robust, and very distinctive ( Figure 2A, E–G View Figure 2 ). Terminal spines stout – relatively more robust than in any living anomopod – and much curved. Curvature greater than in any extant species. (As the claws are of sclerotized chitin it is assumed that their shape was retained during fossilization, a conjecture that appears to be supported by the more slender marginal setae of the carapace that are apparently undistorted.) Marginal spines few; almost certainly five on each side, but extremely long, and robust, though slender. Spines of basal portion arise from its distal extremity (right hand spine lost). Similar spines arise from similar locations on segments 1 (missing on both sides but almost certainly present in life), 2 and 3, and also about midway along the distal segment, suggesting that it may consist of two incipient, or even fused, segments.
To facilitate comparison with other anomopods, a lateral view of the postabdomen as deduced from the dorsal aspect is given in Figure 2F View Figure 2 , with the proviso that its shape is conjectural, as is the exact orientation of the spines. The ventral spines are relatively longer than those of any extant anomopod. As necessitated by their length they are robust. That the postabdomen operated in the same manner as it does in many extant anomopods, which use their unsegmented equivalent for levering against the substratum, is self-evident. As in all anomopods, its protrusion would facilitate the discharge of faeces outside the carapace valves beyond the region where collected material enters the median chamber.
That the postabdomen is segmented is completely unexpected. In all known anomopods there is no trace of such segmentation. Even more remarkable is to find segmentation in a species that not only displays typical anomopod attributes but shares many similarities with members of the extant daphniid genus Scapholeberis . Indeed, had its postabdomen remained unknown, its affinities with this genus, based on similarities of the head and carapace, would have been deemed to be close.
Maximum size unknown. First instar only yet found. Its dimensions as in Figures 1 View Figure 1 and 2 View Figure 2 .
Suggested way of life
The nature of the postabdomen indicates that P. enigmatica spent part of its time on the bottom – other evidence suggests probably in shallow water – in areas with soft, flocculent deposits, or at least with a thin film of such material. Other anatomical details, and comparison with species of Scapholeberis , however, also suggest association with the surface film (see below). The two very different ways of life are not incompatible. Like species of Scapholeberis and Megafenestra , P. enigmatica displays attributes that indicate the exploitation of both.
The function of the stout, re-curved terminal spines of the postabdomen is problematic. These are very different from their small homologues in Scapholeberis and, unless they changed considerably in older instars, which seems improbable, clearly could not have served to clear excess accumulations of material from the food groove as do the terminal claws of many anomopods. Indeed, although paired terminal claws are almost universal in the order, such robust examples with re-curved terminal regions have no parallels among extant species. We suggest that they were used to assist the postabdomen when it levered the animal forward by pressing them against firm elements of the substratum. This would be more effective among soft materials than would the tips of claws such as those of Scapholeberis and many other genera. That this was feasible is indicated by the dorsal view of the postabdomen ( Figures 2A, E View Figure 2 ), which also shows how the long marginal spines could be extended laterally, would not impede such a role by the terminal spines, and could perhaps apply their distal portions horizontal to a surface, so providing another 10 widely separated points of contact for this purpose. The ridging of the distal segment suggests that it may have been slightly flexible – an attribute that, if possessed, may have made a small contribution to levering.
Pseudoscapholeberis shares with the extant Scapholeberis (seven species) and the clearly related Megafenestra (two species) straight ventral margins of the carapace valves, such as are found in no other daphniid. In both extant genera these margins are modified, but in very different ways, so as to form flanges that are much wider in Megafenestra than in Scapholeberis ( Fryer 1991) . Although the flanges differ much in their armature, in both cases they serve to enable the animal to suspend itself beneath the surface film. The narrow flanges of Scapholeberis bear a row of highly modified setae: the much broader margins of Megafenestra bear less elaborate setae. The key feature that has been exploited in different ways to similar ends by the extant genera is the straight ventral margins of the carapace valves. This strongly suggests that Pseudoscapholeberis also exploited it in a similar manner. In several respects the fossil bears more similarity to Scapholeberis than does Megafenestra which, as both extant genera frequent the under surface of the surface film – as do no other daphniids – suggests that Pseudoscapholeberis did likewise.
Of extant species of Scapholeberis we suggest that Scapholeberis microcephala may give the best indication of the probable habitat of P. enigmatica . It frequents small spaces among Sphagnum at the margins of often very small bog pools ( Flössner 2001). Here very shallow conditions prevail. Such habitats, designated as mesobryophytal by Hüsmann (1961), appear to have been present in the Cenomanian Araucarian woodlands of western Europe in early Cretaceous times. It so happens that S. microcephala is one of the three species of Scapholeberis in which a curved frontal spine is sometimes present on the head, as it is in P. enigmatica . The other two such species are S. mucronata and Scapholeberis armata . Although the trunk limbs of P. enigmatica remain unknown, we assume that, as variants of the anomopod plan, they were probably similar to those of species of Scapholeberis , but the unexpected nature of the postabdomen cautions against speculation.
An element of complexity is involved in that, although representatives of several genera of the anomopod family Chydoridae have straight ventral carapace margins, of these only Dadaya macrops has exploited this attribute to permit suspension from the surface film. Members of this family are, however, basically benthic, and typically collect food by scraping material into the median chamber, whereas daphniids do so by filtering particles from suspension. In chydorids straight carapace margins have, not unexpectedly, been exploited to permit intimate association with such situations as the surfaces of submerged plants, where the habit has given rise to such specialized exponents of this way of life as Alonella exigua , Peracantha truncata and Graptoleberis testudinaria ( Fryer 1968) . Here specialization for gliding over plant leaves achieves a level of extreme perfection in G. testudinaria , which employs elaborately modified carapace margins and highly specialized setae borne there, as components of a device that enables it to reduce the pressure within the carapace chamber, to attach itself by suction, and to glide over such surfaces. Other chydorids exhibit intermediate stages along the route to such specialization. However, although the extinct P. enigmatica had long, close-set setae on the ventral carapace margins that conceivably served a role similar to their homologues in Graptoleberis , or of less specialized exponents of such association with surfaces; as a daphniid it presumably collected particulate material from suspension. Its segmented postabdomen – which is unique among anomopods – was clearly used for pushing or levering.
Although its postabdomen is unique in displaying segmentation, and although the difference between a segmented and an unsegmented postabdomen is great, other attributes of the fossil species are so obviously those of an anomopod that we have no hesitation in not only assigning it to that order, but in suggesting affinity with Scapholeberis .
How might the segmented postabdomen be interpreted?
Its similarity to Scapholeberis notwithstanding, Pseudoscapholeberis differs strikingly, not only from this genus and from all other members of the Daphniidae , but from all other extant representatives of the order Anomopoda . Within this morphologically diverse and highly successful group, the postabdomen displays enormous diversity in form and armature. The most divergent of any extant species is the macrothricid Neothrix armata , which lacks not only marginal armature (being provided with only an array of fine spinules), but also the otherwise omnipresent terminal claws ( Gurney 1927; Fryer 1974). These attributes are clearly the result of reduction and simplification. By comparison segmentation of the postabdomen in Pseudoscapholeberis involves anatomical divergence. In no other anomopod are there signs of even incipient segmentation. It is tempting to regard this as an example of the retention of – or, more probably, reversion to – a much more ancient and primitive arrangement. Such a condition does not even conform to the current definition of the Anomopoda , yet other attributes of the fossil indicate not only its placement in that order but affinities with the extant genus Scapholeberis ! It is informative that a molecular phylogenetic study of the related and more speciose genus, Daphnia ( Colbourne and Hebert 1996) revealed that it was differentiated as such> 200 My ago, and that some extant species have been distinct for> 100 My (c.130 My in the case of Daphnia longiremis and Daphnia salina ). As it clearly did not arise within the Daphnia lineage, the ancestor of Scapholeberis must have parted company from that of Daphnia > 200 My ago.
Although the nature of the trunk limbs of the fossil species is unknown, its striking similarity in several respects to the extant species of Scapholeberis is so great that it is difficult to believe that they are not related, or indeed that the shared similarity of the straight ventral margins of the carapace – unique to them and the related Megafenestra among the Daphniidae – are not associated in each case with the habit of hanging suspended by these margins from the surface film. Furthermore, the presence in the fossil of a backwardly-directed, curved spine on the head which, among living anomopods, has a counterpart only in Scapholeberis , seems to reinforce the evidence for affinity. It would be a remarkable coincidence were such a structure to have arisen in just one extant genus and in an extinct species that existed in Early Cretaceous times, 100 My ago, that bears other striking resemblances to it. We therefore incline to the view that the Scapholeberis -like attributes of the fossil indicate true affinity rather than convergence. How then does one assess the significance of the strikingly divergent postabdomen of the extinct Pseudoscapholeberis ?
That a segmented postabdomen was present in what otherwise appears to be not only a typical anomopod but a species that displays striking similarities to a distinctive genus that has several extant species, is particularly intriguing. That it represents a survival of a once widespread primitive condition in the evolutionary line in question seems improbable as the Anomopoda is an ancient group, of which Scapholeberis , to which the fossil bears striking similarities, has no claim to be a primitive representative. Although entirely speculative, one possibility is that the condition represents an extremely ancient atavism that for some reason appeared in a Scapholeberis- like form in early Cretaceous times or earlier. An unsegmented postabdomen is clearly a very ancient attribute of the Anomopoda ( Fryer 1995) – as it is also in the now greatly different, but probable sister group, the Ctenopoda; and indeed in the more distantly related and extremely ancient Spinicaudata and Laevicaudata. If these are constituents of the same monophyletic group, as Taylor et al. (1999) advocate in their ‘robust phylogeny’, which confirms a long-held belief based on other grounds, they must be of enormous antiquity. The postabdomen is unsegmented in all known fossils of these orders, but may have been derived, long before the Cretaceous period, by the fusion of several segments – an event of which we have no fossil evidence.
Of anomopods, the morphologically diverse Chydoridae is clearly a very ancient family. Indeed, a study of sequence variation in two mitochondrial genes by Sacherová and Hebert (2003) indicated that its major lineages originated as much c. as c400 My ago, i.e. in Devonian times, yet extant species show no trace of segmentation in the postabdomen. However, in both the Anostraca and Notostraca, two ancient groups of the Branchiopoda with living representatives; and in the Devonian lipostracan Lepidocaris – and the Cambrian Rehbachiella – the telson (the equivalent of the anomopod postabdomen) is borne at the end of an abdomen of several segments. An atavism, which happened to be functional, could have arisen in a member of the group that gave rise to the extant genus Scapholeberis , or even in Scapholeberis itself if that genus had become distinct by that time.
The segmented nature of the postabdomen of the fossil – a member of a group that otherwise displays derived, rather than primitive, attributes – perhaps suggests that the completely unsegmented postabdomen of all extant anomopods was derived by the fusion of several segments at the posterior extremity of the trunk, and not from an unsegmented region as has probably long been tacitly assumed.
The segmented postabdomen of Pseudoscapholeberis bore stouter terminal spines than do extant species. Their structure was such that they could not have served to remove excess food from the food groove, as they do in many extant anomopods. This indicates that this function, important in many extant species, may not have been acquired in early anomopods. Observations on species of Scapholeberis to ascertain whether they make little use of the terminal claws for clearing the food groove may be informative.
The ventral spines of the postabdomen of P. enigmatica were relatively much longer than those of any species of Scapholeberis , and indeed relatively longer than those of any extant anomopod. Comparison with those of the macrothricid Ilyocryptus , and especially of Ilyocryptus sordidus , in which they are particularly long, are informative. Although very different in shape from that of P. enigmatica , the postabdomen has a ventral ridge that bears about eight long spines on each side, which help it to lever the animal over the bottom or among detritus, as was clearly the case in P. enigmatica . Particularly intriguing is that, notwithstanding this almost certain ability, the extinct species had straight ventral carapace margins, remarkably similar to those of extant species of Scapholeberis , with which they also share other attributes. These not only indicate relationship: they suggest that they shared the habit of suspending themselves beneath the surface film.
Hence, P. enigmatica appears to have had two well-defined special attributes – evident from its anatomy – which adapted it for two very different ways of life. However, its postabdomen played no role in its exploitation of the surface film, and its straight carapace margins were equally of no obvious relevance to its ability to lever itself over the bottom or among detritus. Indeed, the latter conceivably mitigated the success of the former. Although entirely speculative, we suggest that this possible conflict of interests may have played a part in the failure of P. enigmatica , or relatives with similar attributes, to survive when several specialists exploited either one or other of these very different niches.
Some final reflections
A striking, and initially perplexing, feature of the fossil is its very small size. What might initially and erroneously be taken to be an egg-bearing female is only about c.65 µm in length, and therefore very much smaller than the adult of any known extant anomopod. The smallest extant member of the order is the chydorid, Alonella nana , females of which have a length of c.200–280 µm. The supposed egg is, however, an artefact, and the fossil is a first instar juvenile, probably lost prematurely from the parental brood pouch as a result of entrapment of the parent in resin. Postero-dorsally there is a residual accumulation of the nutrient material that had fuelled the development of the embryo and neonate-to-be ( Figures 1 View Figure 1 , 2A View Figure 2 ).
Tantalizing, and misleading, is that, on its right side the fossil displays indistinct creases in the tissues preserved. These give the superficial impression that the individual is not only mature but that it may even be carrying a particularly large ovum – which would presumably have had a counterpart on the other side. This interpretation we firmly reject. That no extant daphniid produces such a relatively large egg is noteworthy, but is not the reason for rejection. An egg membrane is thin and delicate. What might be mistaken for the contour of the egg postero-ventrally is no such membrane. It is a ridge – or fold – of chitin of uncertain origin that has nothing to do with any egg. What gives the impression of the rounded anterior portion of an egg may include the detached rudiment of a mandible, but this is uncertain.
That the fossil was subjected to considerable distortion during preservation is selfevident, not only by its presence outside the parental brood pouch from which it has been forced prematurely, but from the loss of the antennae, the pushing of the head posteriorly into the carapace chamber, and the complete loss of the rudimentary trunk limbs that must have been present at this stage. The size of the adult is of course unknown, but the very small first instar suggests that it was smaller than S. mucronata or even S. microcephala .
Because the fossil resembles Scapholeberis in several, sometimes striking ways it is tempting to assume that it is related, perhaps even closely, to that genus. However, the postabdomen of P. enigmatica has no parallel in any extant anomopod. This cautions against assuming that the trunk limbs necessarily follow the basic anomopod plan, though we strongly suspect that they do. Indeed we think it probable that they displayed features characteristic of the Daphniidae , as well as unique attributes.
It is worth noting that, as Oleson (2009) has pointed out, the fossil record of the ‘Cladocera’ – which embraces a wider assemblage than the Anomopoda – has until now contributed little to our understanding of the evolution of these branchiopods. While recent discoveries by Butterfield (see Introduction) may yet change this, it is difficult to see how Pseudoscapholeberis does much more than reveal that, in the distant past, the posterior end of the trunk of some anomopods was segmented. Beyond noting that this was unpredictable, and has left no known legacy among extant species, little can be said at present.
As a final conundrum we note that, while most structural features of the fossil are suggestive of affinities with Scapholeberis – which is a member of the Daphniidae – the segmented postabdomen is unique, and stands in marked contrast to the unsegmented postabdomen of all extant anomopods. At present we consider it unwise to speculate on how this may best be interpreted in a phylogenetic context. The finding of another individual with preserved trunk limbs would be extremely informative.
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