Material of tesseraspids (Tesseraspidiformes) is reported from the uppermost Severnaya Zemlya Formation
(Lochkovian, Lower Devonian) of the Severnaya Zemlya archipelago, in the Russian Arctic, where it is associated
with other vertebrate remains, including corvaspids, acanthodians, and large but rare specimens of
osteostracans. The tesseraspid material is not abundant, and most often preserved as a “patchwork” of bony
platelets (tesserae), except for a few partly articulated specimens. We redescribe the holotype of Tesseraspis
mosaica Karatajūtė-Talimaa, 1983, whose head carapace is preserved as a flattened tube of adjacent tesserae.
This material is compared to the already published tesseraspid taxa, i.e., T. tessellata Wills, 1935, T. toombsi
Tarlo, 1964, T. mutabilis (Brotzen, 1934), T. oervigi Tarlo, 1964 emend. Dineley and Loeffler, 1976, T. denisoni
Tarlo, 1964, and T. talimaae Tarlo, 1965. All species are based upon rare and incomplete material, as no
head carapaces associated with trunk and tail are known, and so, the intraspecific variability is also unknown.
Distinction between “species” is based on the detail of the superficial sculpture of the tesserae of the head carapaces,
which is unsatisfactory. It is concluded that only four of the nominal species can be retained. A review
of all other known tessellated pteraspidomorphs indicates that our knowledge of tessellated heterostracans is
currently insufficient to support a meaningful classification.
The work reports on the development of random three-dimensional Laguerre-Voronoi computational models for open cell foams. The proposed method can accurately generate foam models having randomly distributed parameter values. A three-dimensional model of ceramic foams having pre-selected cell volumes distribution with stochastic coordinates and orientations was created in the software package ANSYSTM. Different groups of finite element models were then generated using the developed foam modeling procedure. The size sensitivity study shows that each of foam specimens at least contains 125 LV-cells. The developed foam models were used to simulate the macroscopic elastic properties of open cell foams under uni-axial and bi-axial loading and were compared with the existing open cell foam models in the literature. In the high porosity regime, it is found that the elastic properties predicted by random Laguerre-Voronoi foam models are almost the same as those predicted by the perfect Kelvin foam models. In the low porosity regime the results of the present work deviate significantly from those of other models in the literature. The results presented here are generally in better agreement with experimental data than other models. Thus, the Laguerre-Voronoi foam models generated in this work are quite close to real foam topology and yields more accurate results than other open cell foam models.