est potential tetrapod trackways suggest a
mode of locomotion more similar to that
of salamanders (5). If the locomotion seen
in mudskippers is a more adequate model,
it needs to be shown how alternating,
hindlimb-driven (salamander-like) locomo-
tion evolved later on (6, 7). More interme-
diate fossils are needed (8), but alternative
modern analogs should also be considered
(9). Moreover, the mostly aquatic Late Devo-
nian stem tetrapods with highly limited ter-
restrial capabilities probably encountered
challenging inclined sandy or muddy banks
when moving onto land. This is the issue
that McInroe et al. address in their interdis-
ciplinary study. Drawing from diverse lines
of argumentation and using the crutching
model as their starting point, the authors
propose that stem tetrapod tails may have
improved the robustness of locomotion on
challenging substrates, in a similar fashion
as seen in mudskippers.
Both in robots and mathematical models,
parameter combinations can systematically
be varied to access the sensitivity of the result to variation in individual parameters (2,
10)—something that is not possible when
only living organisms are used as analogs.
Bioinspired robots are increasingly used
as heuristic tools to investigate animal
adaptive behavior (11). Similarly, computer
animation has great power for generating
hypotheses on motion using digital mod-
els (10). However, both approaches face
the same problem: how to decide which of
the many possible modes of locomotion is
the most realistic (10). To validate a model,
it needs to be shown that its predictions
closely match experimental data of the bio-
mechanics measured in modern analogs.
Only if this is achieved should the model be
used to infer characteristics of a fossil (12).
Using bioinspired robotics as well as ani-
mations or even sophisticated musculoskel-
etal modeling (13) to elucidate function in
extinct organisms has further advantages.
Optimization criteria can be used to find
plausible parameter combinations without
user interference (10, 11). The results be-
come less dependent on subjective opinion,
and future insights can more readily be in-
corporated into existing models. It is this
integrative methodology, exemplified by
McInroe et al.’s study, that fosters increas-
ingly interdisciplinary analyses of
function in fossils.
Whether or not mudskippers’ tail
use is an adequate modern analog
for stem tetrapods remains debatable. It can only be confirmed by
identifying morphological correlates
that are present both in modern
analogs and in fossils. McInroe et al.
do not resolve this issue, but rather
contribute to a more general understanding of the mechanism of coordinated tail use during crutching
locomotion on soft substrates. The
authors were interested in the overarching principle of tail use, and
their models serve as templates. In
contrast, attempts to reconstruct the
function of a structure of a specific
fossil should use its anatomical details as anchors for the model (3, 14).
McInroe et al.’s study shows that
recruiting expertise from different
fields facilitates integrated modeling approaches to problems of form
and function in extinct organisms.
This enables new hypotheses to be
generated and existing ones to be tested in
a transparent and reproducible manner. j
REFERENCES AND NOTES
1. B. McInroe etal.,Science 353, 154 (2016).
2. D.Jablonski, Science 284,2114(1999).
3. S. Pierce et al ., Nature 486, 523 (2012).
4. P. Arnold etal ., J.Anat. 225, 31 (2014).
5. G.Niedzwiedzki et al., Nature 463,43(2010).
6. S. M. Kawano, R. W. Blob, Integr. Comp. Biol .53, 283 (2013).
7. H.M.King et al., Proc. Natl. Acad. Sci. U.S.A. 108,21146
8. J. S. Anderson et al ., PLOS ONE 10, e0125446 (2015).
9. J.A.Nyakaturaetal., Evol.Biol.41, 175(2014).
10. J.R.Hutchinson, S.M.Gatesy,Nature 440,292(2006).
11. A.J.Ijspeert, Science 346,196(2014).
12. P.S.L.Anderson etal.,Biol.Lett. 8, 119(2011).
13. S. L. Delp etal ., IEEETrans.Biomed.Eng.54, 1940 (2007).
14. R. J. Full, D. E. Koditschek, J.Exp.Biol. 202, 3325 (1999).
I thank E. Amson, J. Wölfer, and B. Kilbourne for their insightful critique of this manuscript. Bild Wissen Gestaltung: Ein
Interdisziplinäres Labor is supported by DFG grant EXC 1027.
Coordinated tail use. Mudskippers use their fins and tail to
navigate difficult terrain while out of the water. McInroe et al. use
biomechanical analysis of mudskipper motion, mathematical
modeling, drag measurements in granular media, and bioinspired
robots to investigate whether early land vertebrates also relied on
their tails to move on land.
By Jerry Y. S. Lin
Separation and purification are criti- cal industrial processes for separating components of chemical mixtures, and these processes account for about half of industrial energy usage (1). Gas mix- tures of compounds with very similar
physical properties are particularly difficult
to separate. On pages 137 and 141 of this is-
sue, Cadiau et al. (2) and Cui et al. (3), respec-
tively, show that microporous materials can
be designed to have high adsorption capacity
and selectivity for particular hydrocarbons,
enabling energy-efficient separation.
Traditional gas separation technologies
use distillation, requiring repeated evapo-
ration and condensation of the mixture, or
absorption with a liquid medium that re-
quires cooling and heating a large amount of
nonactive solvent to complete a separation
cycle. Both technologies are energy intensive.
Newer technologies use solid separation me-
dia and are generally more energy efficient.
For example, membrane-based separation
can require 90% less energy than distillation
to separate propylene–propane mixtures (1,
4). The cornerstone of these newer separa-
tion processes is a solid adsorbent or mem-
brane, often made of a microporous material
with a pore size smaller than 0.5 nm and a
large internal pore surface area (>300 m2/g).
In many microporous solid media used
as adsorbents or membranes (5), there is a
trade-off between adsorption capacity (or
permeability) and selectivity for separating
challenging gas mixtures, making it difficult
for the adsorption or membrane process to
achieve high separation efficiency. To improve
adsorption capacity while also maintaining
selectivity, scientists have modified the internal surface properties of porous media to
enhance interaction of the adsorbent surface
with a specific component and thus increase
the adsorption capacity for that component.
For example, the crystalline microporous
zeolite LiX provides good adsorption capacity for nitrogen over oxygen due to Li cation–
sieves for gas
materials enable efficient
separation of similar gases