La période MP 29–MN 2 (et peut-être MN 3) se caractérise donc par une diminution de la taille et une baisse de la diversité des booïdes ; ils sont alors représentés essentiellement par des érycinés. De plus, les faunes ne sont, ni riches (exceptée celle de Coderet ; Oligocène terminal), ni nombreuses. Le début du Miocène avait été qualifié de Dark Period des booïdes [21] ; nous étendons ici ce qualificatif à toute la période MP 29–MN 2 (MN 3 ?) ; nous y incluons donc la fin de l'Oligocène.

Le changement entre MP 28 (dont la faune peut être qualifiée de « normale ») et MP 29 (début de la Dark Period) est brutal. Seuls de petits booïdes, montrant tous des adaptations à la vie fouisseuse, ont survécu. Ce changement est certainement lié à l'aridification et au refroidissement qui se sont développés pendant l'Oligocène et se sont accélérés vers la fin de la période [12,16,18]. Toutefois, les modifications climatiques ont certainement été plus ou moins progressives, alors que le changement de faune a été abrupt. Le changement entre MN 2 (MN 3 ?) et MN 4 est lui aussi très marqué. Une faune riche et diversifiée succède à une faune très pauvre, très peu diversifiée, et aucun booïde de la Dark Period n'a survécu. Ce changement résulte probablement du retour de l'humidité et du réchauffement du début du Miocène, ce dernier ayant atteint un maximum durant MN 4 [1,17]. Ces modifications climatiques ont probablement eu un effet direct sur les booïdes de la Dark Period, mais avec, en plus, l'établissement de la connexion terrestre entre l'Afrique et l'Eurasie ; elles ont aussi amené en Europe une vague d'immigrants, incluant des colubroïdes, qui a certainement contribué à l'élimination des derniers booïdes de la Dark Period. Pendant la Dark Period, divers serpents (dont les plus anciennes vipères) sont arrivés en Europe. Pourtant, bien que des niches écologiques se soient certainement trouvées vides, aucun booïde n'a participé à ces immigrations ; les causes qui ont empêché la pénétration des booïdes en Europe à cette époque restent inconnues.

Les faunes de booïdes de la période MP 29–MN 2 (MN 3 ?), ou Dark Period, se distinguent nettement des faunes précédentes et suivantes. Parmi les faunes du Tertiaire, elles se reconnaissent aisément. Il convient de noter qu'un phénomène similaire, et peut-être en partie contemporain, le 'Cricetid Vacuum', a touché les rongeurs [2]. Cependant, il a été de durée beaucoup plus courte que celui qui a affecté les booïdes, puisqu'il ne s'étend que sur la moitié supérieure de MN 3 [4]. Toutefois, en raison de la concomitance partielle de ces deux phénomènes, cette période de transition Oligocène/Miocène mérite une certaine attention.

After the Grande Coupure, the snake faunas of the beginning of the Oligocene (zone MP 21) are very poor. They include only undescribed non-erycine booids of small size and perhaps the tropidophiid Dunnophis. After MP 21, immigrations (probably of Asiatic origin) and probably local evolution progressively increase the faunas. Among immigrants, are the Colubridae which first appear in Europe in MP 22. Apart from the Colubridae, the post-MP 21 Oligocene includes rare Aniliidae and Tropidophiidae, but mainly Boidae; the snake faunas are of Euro-Asiatic nature [8]; they are not as rich and diverse as those of the Eocene (which were of Euro-American origin). Up to MP 24 the size of all European booids is small (the centrum length of vertebrae, i.e. the reference measurement, is not over 5 mm). From MP 25 to MP 28, large booids are present in Europe. Most of them are assigned to the boine Bavarioboa [21]. In MP 25 and MP 26, the centrum length of the largest vertebrae approaches or exceeds 6 mm. In MP 28, it reaches 7.3 mm in Bavarioboa crocheti. But, in the last two zones of the Oligocene (MP 29 and MP 30), only small snakes occur. Non-erycine Boidae are no longer present, the Booidea being represented by only an erycine boid (Bransateryx), which is the predominant taxon, and by the tropidophiids Platyspondylia and Rottophis. In the largest vertebrae of booids from MP 29 and MP 30, the centrum length does not exceed 4.6 mm (Fig. 1).

In the first two zones of the Miocene (MN 1 and MN 2), as in the last two zones of the Oligocene, only small snakes are present; the centrum length of booids does not reach 5 mm (Fig. 1). Moreover, the faunas are neither rich nor diverse; they are known only from France and Germany. In France, booids have been found at Paulhiac, Pyrimont (MN 1), Marcoin, and some localities of the area of Saint-Gérand-le-Puy (Poncenat, Chavroches, Montaigu-le-Blin) (MN 2). In Germany, only Weisenau (MN 1) and Ulm-Westtangente (MN 2) have produced Booidea [5] and [21]. These booids from MN 1 and MN 2 are represented by almost only trunk vertebrae that look like those of Erycinae, more specifically Bransateryx. Astonishingly, caudal vertebrae, i.e. the most reliable elements in Erycinae, have been found only at Paulhiac. Szyndlar and Rage [21] referred the booids from Paulhiac, Chavroches, Montaigu-le-Blin and 'Saint-Gérand-le-Puy' to Bransateryx vireti and Szyndlar and Böhme [18] assigned the form from Weisenau to the latter species. But, the absence of caudal vertebrae in these localities (except Paulhiac) casts doubts on the referral of these booids to the Erycinae because Szyndlar and Böhme [19] shown that erycine-like trunk vertebrae may be present in non-erycine Booidea. Consequently, B. vireti is identified at Paulhiac, but the booids of the other localities cannot be securely referred to this species and even to the Erycinae. The booids from Poncenat (unpublished) and Weisenau pose the same problem, although at Poncenat rare skull bones appear to support assignment to Bransateryx [9]. The booids from Marcoin and Ulm-Westtangente remain indeterminate [21]. Finally, Pyrimont yielded a single vertebra that cannot be identified within the Booidea (unpublished). But, whatever the precise taxonomic assignment of the Booidea from MN 1 and MN 2, it should be noted that they are small and not numerous.

The zone MN 3 represents a peculiar problem, because it is not possible to establish definitely whether large booids are present in this level. At Kimi (Island of Euboea, Greece) was found a boid described as Python euboicus by Roemer [15]. The age of the locality appears to be MN 3, but this cannot be definitely accepted [21]. The single specimen that represents P. euboicus is probably lost, but from the illustration and text it may be deduced that this snake was large (centrum length over 10 mm [21]). On the other hand, Bavarioboa, a genus of generally large boids present in the Oligocene, is back in MN 3. But, from this zone, Bavarioboa is known only in the Czech locality Merkur-North (Bavarioboa sp. [7]), where it is represented by a rather small form whose centrum length does not reach 5 mm. Other booids from MN 3 are small indeterminate Boidae [21].

MN 4 is characterized by a rich and diverse fauna of snakes. The presence of Bavarioboa and Python in this zone marks the return of large booids in Europe. Several localities from MN 4 yielded Bavarioboa hermi (Petersbuch 2, Germany, and Dolnice, Czech Republic), Bavarioboa sp. and cf. Bavarioboa [21]. Python europaeus has been found only in France, at Béon 1 and at Vieux-Collonges (in fact, the age of the latter site is transitional between MN 4 and MN 5). In B. hermi, the centrum length reaches 6.8 mm, whereas it amounts to 11.1 mm in P. europaeus. Moreover, in an indeterminate booid from Ivancice (Czech Republic) this measurement reaches 6.8 mm. Aside from large forms, MN 4 produced small booids: erycine Boidae and the Boinae or Tropidophiidae Falseryx.

MN 5 retains large booids. The largest ones are Bavarioboa sp. from L'Isle d'Abeau (France) and Python europaeus from the old level of La Grive (France). In these two snakes, the centrum length reaches 6.6 mm and 10.8 mm respectively. Astonishingly, the presence of small booids from MN 5 cannot be confirmed. The 'old collections' of La Grive include the small erycine Albaneryx depereti that might come from the old level (MN 5) of the locality, but this stratigraphic provenance remains doubtful. A. depereti is the only possible small booid from MN 5.

After MN 5, large Booidea are no longer found in Europe. Only small species persist beyond the MN 5/MN 6 transition: the enigmatic Falseryx is perhaps present in MN 7+8 [21] and the Erycine have survived up to the present.

The period MP 29–MN 2 (MN 3 ?) appears somewhat strange as far as booids are concerned. Szyndlar and Rage [21] termed 'Dark Period' the beginning of the Miocene (MN 1-MN 3). In fact, the end of the Oligocene (MP 29–MP 30) is part of this peculiar period. On the other hand, it remains uncertain as to whether or not MN 3 represents the last zone of the Dark Period, the beginning of the revival of snakes in Europe, or a transitional period.

Localities bearing snakes from the Dark Period are not numerous, the snake fauna of each locality (except Coderet, see below) is neither rich nor diverse, and all booids are small. From MP 29 to MN 2, the faunas of booids are almost exclusively comprised of the erycine boid Bransateryx. The only non-erycine booids are the Tropidophiidae Platyspondylia lepta from the French localities Dieupentale (MP 29) and La Colombière (MP 30) and Rottophis atavus from Rott (MP 30; Germany) [21]. Whatever the booid taxon, from MP 29 to MN 2, the centrum length of the largest specimens is not over 5 mm. It should be added that, except Vipera, all non-booid snakes are also small.

Coderet (MP 30; France) appears to be somewhat dissonant among the localities from the Dark Period, because it has produced numerous specimens. However, as in the other localities, its fauna is not diverse; it includes hundreds of bones (nearly only vertebrae) referred to Bransateryx vireti, the only booid from the locality, and few (less than ten) vertebrae of Coluber cadurci (Colubridae). In other words, the composition of the snake fauna from Coderet is similar to that of the other localities of the Dark Period.

These assemblages of small booids from MP 29 to MN 2 are sandwiched between richer and more diverse faunas in which large forms are present.

Two of the identified taxa from the Dark Period (Bransateryx vireti and Platyspondylia lepta) are survivors from older Oligocene levels, whereas Rottophis atavus is known from a single locality. It should be noted that, aside from small size, the vertebrae of all booid taxa from the Dark Period show adaptation to fossorial life (Bransateryx [5]; Platyspondylia [13]; Rottophis [19]).

The changes between MP 28, during which a 'normal' fauna inhabited Europe, and MP 29, i.e. the outset of the Dark Period, is abrupt. The depauperate fauna of small snakes from MP 29 markedly contrasts with the rich and diverse assemblage from MP 28, which includes large species. It is worth noting that only small fossorial booids survived to the MP 28–MP 29 limit. This event appears to be connected to palaeoenvironmental changes. Aridification, initiated during the Eocene, increased throughout the Oligocene and the aridity was certainly pronounced by the Latest Oligocene, a period that was moreover relatively cold [12], [16] and [18]. Aridification is supported by the predominance of Bransateryx; as living Old World erycines, this snake probably lived in desert-like environments. This is probably the cause that led to extinctions and/or withdrawals of several taxa and left only small and fossorial booids after MP 28. However, the climatic change was relatively gradual while the faunal change was abrupt.

From MP 29 to MN 2 (and MN 3 ?), the booid fauna did not evolve notably despite a climatic change at the beginning of the Miocene. At that time, a humid tropical to subtropical climate was restored. However, the tropidophiids that were rare in the Oligocene part of the Dark Period did not survive beyond the Oligocene–Miocene limit. Therefore, in the Miocene part of the Dark Period, only the erycine Bransateryx (and indeterminate forms) represented the Booidea. Although most niches were certainly free, no newcomer reached Europe and increased the booid assemblages (and no endemic evolution took place). Nevertheless, snakes were able to enter Europe, since vipers first appeared there in MN 1, probably coming from the east. The factor(s) that precluded the arrival of booids during the Dark Period cannot be determined.

The fauna from MN 3 (the composition of which cannot be accurately established) set aside, a marked change occurs between the faunas from MN 2 and MN 4. That of MN 2 is typical of the Dark Period, whereas the fauna from MN 4 displays a marked renewal. As far as snakes are concerned, the transition between MN 2 and MN 4 represents an important faunal change [6] and [22]; this event marks the end of the Dark Period. The booid fauna from MN 4 deeply differs from the preceding one because it does not include booids inherited from the Dark Period and because new forms invaded Europe at that time. The erycine Bransateryx, the only booid from the MN 1–MN 2 period, definitely died out before MN 3 [21]. In MN 4, erycine boids were represented by the extant genus Eryx [23]. Bavarioboa, which withdrew from at least western and central Europe before MP 29, was back and frequent in MN 4. It should be noted that Bavarioboa re-appeared before MN 4, in a single locality (Merkur-North, MN 3), but it was represented there by a small form (Bavarioboa sp. [7]). Pythonine Boidae (Python europaeus) appeared in Western Europe at Béon 1 (Rage and Bailon, in progress). The small and enigmatic Falseryx petersbuchi (Boidae or Tropidophiidae?) completed the booid fauna.

This diversity of booids from MN 4 fits into the renewal that affected the whole fauna of snakes, but also anuran amphibians [14] and probably lizards. As far as snakes are concerned, in addition to the above booid newcomers, elapids and large vipers of the 'oriental group' of Vipera first appeared in Europe, while colubrids became largely diversified during MN 4 times. These newcomers eliminated the last booid (Bransateryx) that was present during the Dark Period.

As for the beginning of the Dark Period, this faunal event is probably connected to palaeoenvironmental changes. Humidity and heat were back by the beginning of the Miocene. The warming reached a maximum by the end of the Early Miocene [1], i.e. by MN 4 times [17]. The environmental changes certainly affected directly the last booids present during the Dark Period. But, in addition, the arrival in Europe of new snake taxa certainly contributed to the elimination of these booids. These newcomers probably reached Europe owing to the new conditions. Moreover, the terrestrial connection between Eurasia and Africa that was established during the Early Miocene might have been the cause of the arrival of part of the immigrants in Europe. However, the geographic origin(s) of the MN 4 invaders remain(s) unknown. It was only demonstrated that the elapid that was present at Vieux-Collonges (MN 4/5; France) is probably closely related to Naja romani, a member of the Asiatic assemblage of Naja [20]. But this does not rule out a possible African origin of part of the invaders.

MN 5, that is poorly documented, appears as a continuation of MN 4 since large representatives of Bavarioboa and Python were present. After MN 5, European booids have undergone a progressive decay. Apart from one non-erycine booid (Falseryx?) from MN 7+8, only erycines have been reported from post-MN 5 levels [21]. This has certainly resulted from both competition with colubroids and climatic degradation.