DISCUSSION ON THE ORIGIN AND PALEOGEOGRAPHY OF THE BRACHYURA

Trabajo recibido el 13 de febrero de 1978 y aceptado para su publicación el 28 de febrero de 1978.

RESUMEN

DISCUSSION

The question concerning the origin of the Brachyura as a group continues to be a debatable subject whose answer still remains uncertain. Unfortunately, no decisive records exist of a possible ancestral stock. Nevertheless, this has not kept some authors from postulating hypotheses regarding the origin of the true crabs. Steveie (1971) has succinctly discussed three of the major groups of such hypotheses which support the contention that the recent brachyurans have arisen from ancestors that perhaps had either Macrouran, Anomuran, or Pemphicoid characters. According to Steveie, among the stronger advocates of a Macrouran origin were Husley, Bouvier, and Boas; each of these authors argued that the Brachyura may have arisen from the Astacidae, the Nephropsidae, or the Thalassinidae, respectively. The German carcinologist Ortmann, in contrast, was in favor of deriving the Brachyura from an ancestral forra with Anomuran affinities between the Paguridae and the Galatheidae. For Ortmann, the Dromiacea represented either the end product in the evolution of the Anomura or the most primitive of the Brachyura. Other authors such as Van Stralen, Beurlen, and Glaessner, based on paleontological evidence, were of the opinion that the Brachyura had originated from the Triassic Pemphicoidea approximately 225 million years ago.

At any rate, the evolutionary process that led the ancestral brachyurans to achieve a high level of morphological organization is thought to have started during the Jurassic Period and by the Cretaceous had produced the true crabs as we know them today. This evolutionary process, commonly referred to as brachyurization or carcinization, was probably induced by the forces of natural selection which acted in favor of the reduction of the long abdomen as the main locomotory organ in ancestral forms and promoted the development of stronger abulatory legs.

As a first step in the progress of such modifications, the abdomen, given its tender structure, was constantly being folded under the cephalothorax to, protect it from damage. This new position of the abdomen finally caused its degeneration and led to the differentiation of the pereiopods to achieve a more efficient locomotion; simultaneously, the cephalothorax became increasingly broad and depressed. In the course of the brachyurization process most of the appendages underwent further specialization accompanied by modification of their respective segments and thus, from the fusion. of the thorasic sternite arranged in parallel, a rigid sternal plastron resulted that permitted the peculiar sideways locomotion.

The subsequent differentiation of the other parts of the body, including internal systems, led to the structural and functional integration of a more complex level of brachyuran organization. The fusion of the thoracic: segments, for instance, contributed to the integration of the central nervous system from a ventral ganglia into a great ganglionic: mass that improved the coordination of movements on the one hand, and created a rather complex pattern of behavior on the other.

Apparently at the beginning, the great majority of the crabs were shallow-water inhabitants; however, once a new level of structuro-functional organization was attained they acquired a remarkable adaptative radiation (Glaessner, 1957). They were then able to exploit successfully a variety of habitats ranging from the terrestrial to the fresh-water, the littoral and sublittoral to the deep-sea environment. In the process of adapting themselves to new habitats and new particular ways of life, the crabs underwent further differentiation or in the words of Russell (1962), an adaptive specialization that caused the emergence of subgroups or Tribes within the Brachyura; in some instances such differentiation had a dramatic effect as in the case of the Hapalocarcinidae that live in cavities or galls of living parts of corals, whereas in others, some ancestral characters were essentially retained as ocurred in the Homolodromiidae. Even though at present it is feasible to recognize the progressive differentiation from fossil to recent forms, the high diversity of the Brachyura in conjunction with the oecurrence of convergent and regressive evolutionary processes preclude the establishment of clear pathways of evolution.

From the paleogeographic standpoint, the fossil records of the crabs appear to indicate that the ancient brachyuran fauna had a widespread dispersion over much of the coastal plains of the Tethys Sea. During the Jurassic Period when the Brachyura started to evolve, the Tethys Sea was a seaway that separated two megacontinents, Laurasia in the north and Gondwana in the south, thus allowing free communication with the Indo-Pacific, the Mediterranean, and the Central Atlantic: (Ekman, 1953). The prevailing conditions in this particular geological period were such that no major climatic: or topographic: barriers existed and consequently the degree of provinciality was reduced to a minimum. Most zoogeographers concur that these events were instrumental in the integration of a circumtropical marine biota, fairly homogeneous in composition, known as the Tethys fauna. The significant affinity in terms of families and genera among the brachyurans that inhabit the present tropical regions of the world constitutes valid prove of their common faunal origin during the Tethyan Sea period.

In the course of the late Jurassic and through much of the Cretaceous the continental masses started to split up and gradually migrated until they reached their present position.

According to Valentine (1973) the new position of the landmasses brought about fundamental changes in the ecological stage of the biosphere during the Mesozoic and the Cenozoic. The former homogeneity of the Tethyan fauna was abrupt1y disturbed by the formation of new topographic barriers that interrupted the free passage between the Mediterranean and the Indian Ocean as well as that between the Atlantic and the Eastern Pacific. Furthermore, the fragmentation of the continents caused the progressive enlargement of deep oceanic basins which impeded the dispersion of shallow-water elements over distant areas. The overall effect that these sets of circumstances had upon the distribution of the faunas was reflected in a trend of increasing provinciality recorded by paleontologists during the late Cretaceous and Cenozoic; perhaps the significant diversification in the number of fossil brachyuran species registered towards the Tertiary (Rogers, 1953) can be ascribed to the divergent development that the Brachyura fauna underwent once new latitudinal (temperature regime) and longitudinal (landmasses-deep sea basins) provinces were established.

A major trend in the evolution of the marine climate through the Cenozoic, especially towards the late Tertiary, was the progressive cooling of the poles that provoked prolonged periods of glaciation. Concomitant lowering of the temperature conditions in most parts of the world oceans coupled with the changes in sea level (Transgression-Regression) due to the trapping of water in the form of ice at the poles proved to be detrimental to the tropical biota; large scale extinction of the earlier tropical fauna is believed to have occurred in the course of the Miocene and Pliocene (Ekman, 1953). The one-time widely dispersed fauna was then confined to more stable environments near the equatorial region where it remained until temperature conditions improved again in the late Pliocene and Quaternary Period.

The extinction of tropical elements was followed by a flourishing of temperate forms that were formerly represented only at high latitude.

In a way, this shift of biota can partially be detected in the list of fossil Brachyura offered by Rathbun (1936) for the Atlantic and the Gulf Coastal Plain. A good number of the brachyuran genera and species reported from the Cretaceous later became extinct in the succeding periods; such was the case of the calappid crab Necrocarcinus a thriving element in the Cretaceous that dissapeared after the Eocene, whereas members of the family Raninidae seemed to have endured the change in conditions and still occur in the waters of the Coastal Plain. During the Eocene there is an assorted group of brachyuran families reported by Rathbun most of them represented by species no longer in existence. Unfortunately the fossil material from the Oligocene and Pliocene is too scarce, but from the Miocene Rathbun listed nearly fourteen species that are also recent.

According to Steveie (1971), Beurlen was the first to point out that the evolution of the Brachyura was related to paleogeographic changes occurring on the earth's surface. He postulated that the resulting environmental conditions during the periods of Transgression promoted the diversity of groups whereas the Regression in turn created stress conditions that accelerated the extinction of some species and forced others to seek refuge in either the land, the fresh-water, or the deep-sea habitats. In reference to the invasion of the deep-sea, Beurlen added that this process has taken place at least on three different occasions since the Jurassic, and a fourth is probably in progress at the present as the bathymetric range of the Majidae and Leucosiidae indicates.

In opposition to Beurlen's statement, Madsen (1961) asserted that the initial immigration to the deep-sea by brachyuran elements took place when they were abundant and widely dispersed in the shallow habitat, and, rather than seeking refuge, these elements were in search of new habitats to colonize; while these elements became completely adapted to the deep-sea life, their shallow-water relatives disappeared, due to the ecological deterioration of their original habitat.

I am greatly indebted to Gilbert L. Voss, Rosenstiel School of Marine and Atmospheric Science (RSMAS), University of Miami, for critically reading the manuscript.

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