Deep in southern New Mexico, not far from the city of Las Cruces, lie the Robledo Mountains. Tectonic activity along the Rio Grande Rift pushed up the Robledos during the past 30 million years, but the rocks that make up the mountains are about ten times older. In fact, just about 290 million years ago the most famous rocks of the Robledos were being created as layers of fine-grained mud collected near the shores of an Early Permian sea.
This Permian mud was of a perfect consistency to preserve the imprint of anything that touched its surface—the sharp-clawed tracks of a passing Dimetrodon, ropelike branches fallen from Walchia conifers, impact craters from raindrops—even the footfalls of tiny insects left their marks in the surface. As the mud was buried and hardened into rock, the imprints were preserved within the sandstones and siltstones that today make up the Robledo Mountains. The abundance, diversity, and quality of tracks provide a unique look into an Early Permian ecosystem. And the scientific importance of the site has inspired a grassroots movement campaigning for a Prehistoric Trackways National Monument in the Robledos.
A paper in the latest issue of Palaeontology is a great example of the sort of information coming out of the Robledo Mountains. Nicholas Minter and Simon Braddy describe walking and jumping trackways made by an extinct group of insects and compare their locomotor reportoire to those of their living relatives.
The holotype and paratypes of Tonganoxichnus robledoensis.
Photograph and interpretive line drawing of the trackways.
Scale bar=10mm. This and all subsequent figures from Minter and Braddy 2006.
© Copyright The Palaeontological Association. Reproduced with permission.
The photo and diagram above show the type specimens of the ichnospecies Tonganoxichnus robledoensis. It preserves the imprints made by two small insects as they jumped across the mud, one on the left and one on the right. The trackmakers were travelling from bottom to top in this view, with imprints of legs towards the front and a long abdomen and “tail” trailing behind.
The Paleozoic monuran Dasyleptus.
scale=1mm. From Minter and Braddy 2006.
© The Palaeontological Association
The shape of the tracks is suggests that the trackmaker belonged to a group of insects known as the Archaeognatha, which have simple legs ending in points, long abdomens, and long terminal filaments (”tails”). There are jumping archaeognathans living today called “bristletails” because they possess lateral cerci that give their “tail” a three-pronged appearance. But the Tonganoxichnus tracks show no signs of a bristly tail, and were probably made by an extinct group of archaeognathans called monurans (”one tail”) for what I suspect is an obvious reason.
Trackway of a monuran walking,
then jumping, then walking again.
From Minter and Braddy 2006.
© The Palaeontological Association
Minter and Braddy examined 23 sets of Tonganoxichnus robledoensis trackways and found that the Robledo monurans could travel by means of successive, forward-facing jumps several times their body length. Subtle details in the tracks showed that some of the monurans were pushing off more with their legs, while in others a flexing of the abdomen provided the main propulsive force. There were even a few tracks that suggested the jumper was using both its legs and abdomen to propel itself forward.
A living archaognathan, the bristletail Petrobius brevistylis, has a similar proclivity for jumping, but with a few distinct differences. Petrobius normally progresses by successive, leg-propelled hops about one body-length in distance. But when confronted with a predator, Petrobius switches to a series of confusing, abdomen-propelled jumps that carry it much further, but in an essentially random direction (none of the Tonganoxichnus robledoensis trackways showed any evidence of random jumping).
Petrobius occasionally walks, but only when travelling on the underside of rocks where hopping is problematic. Trackway evidence indicates that the Robledo monurans walked somewhat more regularly. Minter and Braddy note that five specimens of insect-walking trackways (referred to the ichnospecies Stiaria intermedia) either end in a Tonganoxichnus-style jumping trace or begin with a similar landing imprint. One specimen (shown here, travelling from bottom to top) shows where a monuran was walking, jumped a few body-lengths forward, landed on its abdomen, then resumed its stroll. The authors note that similar walking trackways (uninterrupted by jumping traces) are relatively common elements of the Robledo tracksites, suggesting that walking played a greater role in monuran locomotion than it does in their distant, modern relatives.
Minter, N. J. and Braddy, S. J. 2006. Walking and jumping with Paleozoic apterygote insects. Palaeontology 49: 4, 827–835.
Thanks to the Palaeontological Association for granting permission to reproduce figures from the article, and to Nicholas Minter for bringing greater attention to these nifty little tracks. Tip of the toupee to Allan Lerner for sending a copy of the paper my way.
—Matt Celeskey.
File under: Permian, Recent Discoveries.
Comments on record: (2)
