Paleoenvironments of Laetoli, Tanzania as Determined by Antelope Habitat Preferences

Abstract

We examined the fossil remains of antelope (Mammalia: Bovidae) postcrania recovered from the Upper Laetolil Beds and the overlying Upper Ndolanya Beds in northern Tanzania. We used analyses of the ecomorphology of extinct antelopes to determine their habitat preferences. The most common postcranial elements - the phalanges, astragali, and distal radii - were examined. A total sample of 446 specimens was analyzed. Changes in the relative proportion of habitat preferences through time are suggestive of temporal fluctuations in habitat availability. As has been noted elsewhere, the mammals from Laetoli do not show the expected distribution of body sizes, so this paper also examined how such size biases, assumed to be taphonomic in origin, might have affected the reconstructions proposed here. We conclude that, on the basis of habitat preferences of the antelopes recovered from Laetoli, there is evidence for the continuous regional presence of woodland and forest throughout deposition of the Upper Laetolil Beds. Antelopes preferring forest and heavy cover habitats dominate the assemblage. Antelopes that locomoted principally in open and light cover habitats are largely in the minority, with a combined frequency never exceeding around 25% of the total in each stratigraphic unit of the Upper Laetolil Beds. The relative proportions of heavy cover and forest preferring antelopes in particular are likely affected by body size biases caused by taphonomic processes active during the deposition of the Upper Laetolil Beds, which favoured the preservation of the postcrania of smaller antelopes. During the formation of the Upper Ndolanya Beds, the proportion of antelopes preferring more open habitats is greatly increased, although forest-preferring antelopes are still present. The conclusion that forest and woodland were always present throughout the sequence is robust.

Appendix: Measurements and Ratios Used in This Study

Distal Radius

LDARTML:

log(mediolateral breadth of distal articular surface)

LDMAXAP:

log(maximum anteroposterior diameter, distal end)

LDCENTAP:

log(anteroposterior length of central facet, distal end)

LDCENML2:

log(mediolateral breadth, most constricted point of the medial facet, distal end)

LDLATAP:

log(anteroposterior length of lateral facet, distal end)

LDLATML:

log(mediolateral length of lateral facet, distal end)

LDTUBML:

log(distance between tuberosities, dorsal side of distal end)

LDISI3:

ldmaxml/ldcresap = log(maximum mediolateral breadth, distal end)/log(anteroposterior length of distal crest)

LDISI8:

ldcentap/ldlatap = log(anteroposterior length of central facet, distal end)/log(anteroposterior length of lateral facet, distal end)

LDISI24:

ldmaxap/ldtubml = log(maximum anteroposterior diameter, distal end)/log(distance between tuberosities, dorsal side of distal end)

LDISI25:

ldtubml/ldcresap = log(distance between tuberosities, dorsal side of distal end)/log(anteroposterior length of distal crest)

Astragalus

TARSMLT:

Tarsal articulation M-L

TAMAP:

Depth of medial portion of tarsal articulation

MINLEN:

Minimum astragalus length

TUBTIBA:

Tibial articulation to fibular tuberosity

LENRAT11:

Flangeml/medlen*100 = ML across flange of medial trochlea/medial length

LENRAT21:

Timap/medlen*100 = Length of tibial articulation, medial side/medial length

PCFRAT24:

Pcflap/tubml*100 = posterior calcaneal facet, lateral length/mediolateral width at tuberosity

TICRAT2:

Ticap/tilap*100 = tibial articulation, minimum length/tibial articulation, lateral length

SIRAT3:

Lcfdsi/tubtiba*100 = distal lateral calcaneal facet, depth/tibial articulation to fibular tuberosity

TARAT5:

Tarsmlm/Tarsmlt*100 = tarsal articulation ML, medial side/tarsal articulation ML

TARAT9:

Tarsmlm/tamap*100 = tarsal articulation ML, medial side/depth of medial portion of tarsal articulation

Proximal Phalanges

LPARTBR:

log of the proximal articular breadth

LMAP:

log of midshaft anteroposterior diameter

LDMAXAP:

log of maximum anteroposterior dimension of the distal end

LDMINML:

log of the minimum mediolateral dimension of the distal end

LPROX1:

lmaxlen/lminlen = log(maximum length)/log(minimum length)

LPROX2:

lpmaxht/lpmaxbr = log(maximum height of proximal end)/log(articular breadth of the proximal end)

LPROX7:

ldmaxml/ldminml = log(maximum mediolat­eral breadth of distal end)/log(minimum mediolateral breadth of distal end)

LPROX12:

lminlen/lpmaxht = log(minimum length)/log(maximum height of the proximal end)

LPROX18:

lminlen/ldminap = log(minimum length)/log(minimum anteroposterior dimension, distal end)

LPROX20:

ldmaxml/lmml = log(maximum mediolateral breadth of distal end)/log(midshaft mediolateral diameter)

LPROX21:

lmaxlen/lmml = log(maximum length)/log(midshaft mediolateral diameter)

Intermediate Phalanges

LMAXLEN:

log(maximum length)

LMML:

log(midshaft mediolateral diameter)

LDARHMAX:

log(maximum height, distal articular surface)

LINT1:

lmaxlen/lminlen = log(maximum length)/log(minimum length)

LINT4:

lmap/lmml = log(midshaft anteroposterior diameter)/log(medshaft mediolateral dia­meter)

LINT5:

ldmaxml/ldminm = log(maximum breadth, distal end)/log(minimum breadth, distal end)

LINT8:

ldarhmax/ldminml = log(maximum height, distal articular surface)/log(minimum breadth, distal end)

LINT11:

lmaxlen/lLS = log(maximum length)/log (supe-rior length)

LINT17:

lpmaxbr/ldmaxml = log(maximum breadth, proximal end)/log(maximum breadth, ­distal end)

LINT21:

lmaxlen/lmml = log(maximum length)/log(midshaft mediolateral diameter)

Distal Phalanges

LMAXLEN:

log(maximum length)

LPMAXBR:

log(maximum breadth, proximal end)

LLENSUP:

log(superior length)

LMAXHT:

log(maximum height)

LMPSB:

log(breadth of the plantar surface at its midpoint)

LDIS8:

llensup/lpartht = log(superior length)/log(proxi-mal articular height)

LDIS17:

lpartht/lmpsb = log(proximal articular height)/log(breadth of the plantar surface, at its midpoint)