Contributions to Zoology, 72 (2/3) (2003)
Morphometric analysis of extant Nephrops norvegicus (Linné, 1758) and Solenocera membranacea (Risso, 1826) applied to systematic palaeontology of fossil decapod crustaceans
Giuseppina Damiano , Alessandro Garassino , Stefano Scali , Giorgio Teruzzi
Keywords: Crustacea, Decapoda, morphometric analysis
The study of fossil decapods have shown the need for reliable statistics to document the value of the characters considered. Morphometric data, however, are usually absent in scientific papers on living decapod crustaceans since neontological systematists normally analyze internal organs and tissues not available in fossils. A series of morphometric data for two extant species have been processed statistically in order to obtain numerical models that could be applied to fossil species showing morphological affinities.
For over twenty years, the Dipartimento di Paleontologia degli Invertebrati (Museo civico di Storia naturale, Milano), has been carrying out systematic studies of Mesozoic and Cenozoic decapod crustaceans, both from Italy and abroad. We have encountered difficulties in the study of highly diverse faunal assemblages, such as the ones recorded from Solnhofen or Lebanon, and problems experienced in highlighting morphological features useful in species discrimination have shown the need for statistics to document the value of the characters considered. For this reason we have initiated a research program on extant decapod crustaceans that have morphological traits in common with many fossil species, such as Solenocera and Nephrops, in order to find out whether morphological characters, e.g., carapace grooves or rostral spines, are diagnostically valid in samples of fossils, or not, but also to integrate into systematic paleontology new elements, e.g., sexual dimorphism and ontogenetic stages, that are rarely evident in fossil specimens.
The vagaries of preservation of decapod crustaceans, above all fragile characters such as spines, cephalic appendages, and/or thoracic and abdominal appendages, make systematic assignment of such animals occasionally difficult. Paleontological studies based exclusively on morphologically discriminant criteria often lead to arbitrary choices, e.g., what characters to use to classify any particular species.
The present study considers extant species so as to obtain a large data set of significant morphometric data that can be used to elaborate a systematic paleontology at the lower levels of classification (genera, species). Suitable morphometric data are usually absent in scientific papers on living decapod crustaceans since neontological systematics normally analyses internal organs and tissues not available in fossils (Holthuis, 1991, 1993). A series of morphometric data for extant species have been processed statistically in order to obtain numerical models which could be applied to fossil species showing morphological affinities.
The sample used for the present study comprises 274 specimens of N. norvegicus and 302 specimens of S. membranacea from the central Adriatic Sea. Fossils considered are all housed in the Museo di Storia naturale (Milano), and include twenty specimens of Ifasia madagascariensis (van Straelen, 1933) from the Lower Triassic of Madagascar, and six species of decapod crustaceans from Hakel and Hadjula (Lower-Middle Cenomanian, Lebanon), viz., Eryma cretacea Roger, 1946, Homarus hakelensis (Fraas, 1878), Odontochelion cretaceum (Roger, 1946), Microchela rostrata Garassino, 1994, Carpopenaeus callirostris Glaessner, 1945 and Hakelocaris vavassorii Garassino, 1994.
All morphometric data have been analyzed statistically (descriptive, discriminant and principal components analyses).
The morphometric analyses of N. norvegicus and S. membranacea confirm the occurrence of sexual dimorphism, while descriptive analyses of N. norvegicus and the U test of Mann-Whitney highlight that males are larger than females, while the reverse is true for S. membranacea. In an earlier study we compared S. membranacea with I. madagascariensis; significant morphological differences were found. In particular, the length of the rostrum and the length of the chelae of the first pereiopods are greater in the juvenile stages of I. madagascariensis than in S. membranacea. Discriminant analysis of S. membranacea has confirmed the importance of five characters to discriminate the sexes. The same analysis isolated ten characters to discriminate the sexes in I. madagascariensis.
We subsequently compared N. norvegicus and Late Cretaceous macruran decapod crustaceans from the Lebanon. Principal components analysis applied to all Lebanese species grouped these in Natantia and Reptantia, according to their morphological characteristics, thus confirming customary paleontological classification. Principal components and discriminant analyses applied to H. hakelensis and E. cretacea showed the importance of the length of the carapace and that of the cutting chela for discrimination between the sexes. This statistic analysis also enabled us to highlight ontogenetic stages in these two fossil decapod species.
The statistical analyses applied to the fossil samples have proved highly efficient for keeping species apart and for documenting sexual dimorphism and ontogenetic stages. However, their application to fossil samples presupposes a certain sample size (should consist of at least twenty specimens), and rests on the importance of comparison between fossil species and extant ones that share morphological affinities.
In future, this program will analyze other Recent species that show morphological affinities with fossil taxa so as to create a data base which is as complete as possible. This should then become a valid tool in our study of fossil decapods.
Holthuis LB. 1991. FAO Species Catalogue. Volume 13. Marine lobsters of the world. FAO Fish. Syn. 125: 1-292.
Holthuis LB. 1993. The recent genera of the caridean and stenopodidean shrimps (Crustacea, Decapoda): with an appendix on the order Amphionidacea. Leiden (Nationaal Natuurhistorisch Museum).