The Cretaceous Age of Chimeras and Other Oddities

Among a clump of conifers, one species of shrubby an-giosperm had crowded out its neighbors, and a number of curious insects had collected on the flowers. Small solitary hymenopter-ans were actively skimming back and forth over the stamens, stopping momentarily to scrape pollen onto bristly hairs covering their hind legs. The branches on those hairs helped retain the grains, although many dislodged and fell on the receptive female flower parts as these primitive bees went about their everyday business. One tiny hirsute body was actually shaking from the exertion of collecting, and this bee stopped momentarily to use the back two pairs of legs to wipe excess pollen from the wing surfaces while employing the cleft claws on the front legs to brace herself. Then the exhausted female flew to the hidden nest tunneled deep within the soil where she deposited the last brood ball of the season.

The mature kauri trees in the forest were each a microcosm teeming with arthropod activity. A tiny brown ant-like stone beetle crawled slowly over the many fissures on a trunk and eventually lowered an extended abdomen, inserted a pointed ovipositor under a loose piece of bark, and deposited several eggs. The small female then resumed her other everyday activities, which included hunting down that favorite food, mites. Scurrying over the tree surface, the ant-like insect froze when a mite crossed nearby, but it was not the preferred kind and she continued to search. Finally an other type of mite appeared, one adapted to the crannies and crevices on the araucarian tree. A quick pounce secured the prey between the vise-like grip of strong front legs, made all the more efficient by their extra segment, a unique character that distinguished this small insect species from all others in the forest. The minute beetle voraciously consumed the small arachnid and then resumed patrolling the bark surface.

Much lower on the same tree, a tick larva waited patiently on a twig for a passing vertebrate. This creature desperately needed a blood meal since stored reserves were nearly depleted. A slow-moving dinosaur approached and the tick suddenly became alert, extending the palps and front legs in anticipation. The outstretched palps were armed with a cluster of claws near their tips, a feature that made this particular tick different. As the dinosaur unknowingly brushed against the twig, the opportunity had come and the tick half-dropped, half-scrambled onto the moving animal, having completed the first important step in procuring a meal.

Up in the leafy crown of the same tree, a group of aphids prepared to abandon their home turf on the foliage and search for other feeding sites. The females were filled with small elliptical eggs that were the stock to establish new colonies. Most had withdrawn the long, narrow stylets through which they took sap from the host tree. These aphids differed from other aphids because they possessed only a single pair of wings instead of the normal four. Some began to test their wings by extending them as far as they could. A barely detectable gust of wind brushed across the outstretched appendages and they became airborne, flitting their way upward. Some landed against sticky pools of resin while others eventually settled on some leaves silhouetted against the morning light.

Further down at the base of the tree, the dim rays of light faintly illuminated a group of small flies viewing the world in a unique way— upside down! Equipped with spines on their hind legs that faced backwards, some females just dug a single prickly leg into bits of moss, while others attached both legs to rough bark fragments. Their clear wings contrasted with the dark bark as they hung mo-

tionlessly. These vampire-like flies, which were equipped with uniquely-shaped serrated mandibles, were slowly digesting the blood they had obtained during the night.

I was astonished when I peered down the microscope lens and saw my first Cretaceous chimera. When studying insects in younger Dominican, Mexican, and Baltic amber, it was commonplace to place the fossil in a modern family and sometimes in an extant genus. But these mid-Cretaceous fossils opened up an exciting, strange new world with creatures bearing combinations of characters never before seen among the living. Besides instilling wonderment, trying to identify these enigmas kept me awake at night since according to established standards, they shouldn't have existed at all. It made me realize that the current classification system has little significance at and below the family level for insect fossils 100 million years old.

These were reminiscent of the mythological she-monster with a lion's head, goat's body, and serpent's tail that the Greeks called a chimera, and that term is adopted here for fossils with morphological features found in two or more present-day groups. And for that very reason, knowing or deciding how to classify them is a conundrum. Vertebrate paleontologists have Archaeopteryx, a strange animal with teeth and feathered wings with claws that appears both bird and reptile. We entomologists have more numerous and just as interesting chimeras, proof that the Cretaceous was an age when species radiated extensively both structurally and behaviorally, and the partitioning of habitats was continually changing.

Because attempting a DNA analysis of these rare fossils necessitates destroying them and therefore would not be feasible, our conclusions are based on their physical features. We are left with speculating on how they arose and why they became extinct. Some of their characters could be the result of convergent evolution, where two entirely different lineages acquire the same features. Others are possibly due to spontaneous mutations followed by adaptive radiations, and a number might be throw-

backs to earlier lineages where inhibitors were mistakenly switched off during development and latent DNA in the genome activated. A few may have the characters of two groups because they are intermediate between two lineages.

My first Cretaceous insect chimera was the little Burmese amber Hapsomela, an ant-like stone beetle with both front legs equipped with 6 instead of the normal 5 segments found in all living insects (fig.16).39 How astonishing that only the front pair of legs had the extra segment while the remaining had the normal number! I kept staring at those front legs, looking at each segment over and over again, not wanting to accept the obvious. Should I just place the specimen back in the container and file it away? No, this was worth presenting to the scientific world even though the facts went against all conventional wisdom. The closest modern groups that have extra-segmented legs are spiders and mites, and this fossil certainly didn't fit there. Why would any insect need additional leg segments? While crustaceans, arachnids, and millipedes have more, modern insects do quite well with their basic five. However, we do know that some ancient insects found extra joints useful because a number of Paleozoic forms had an extra one or two.

I began to contemplate how this chimera arrived on the Cretaceous scene. Did the lineage originate in the Permian some 300 million years ago, continue undetected by paleontologists into the Cretaceous, and then disappear? That would mean the little beetle was one of the terminal descendants of an ancient group. Or was this a throwback, suggesting that insects retained the capacity for making six-segmented legs in their genome. If so, then something switched off some inhibitors and activated a long-dormant segment of DNA, making those segments appear. Maybe this individual was just the result of a single spontaneous mutation that happened to get captured in amber. But what would be the odds of a lone mutant becoming entrapped in resin and ending up under a scientist's microscope?

The tick Cornupalpatum, also from Burmese amber, likewise falls into our category of chimeras. A quick inspection suggested that it was just a typical hard tick (color plate 11E). However, a more detailed examination revealed that the pair of small sensory appendages known as palps had 5 terminal claws.45 While claws are found in that location in some predatory mites, they have never been observed on present-day ticks. Here is a mite character on a tick, leaving us to wonder why the fossil came to have these talons and if their purpose was the same as that in predatory mites, subduing the prey. We are left to postulate whether they helped affix Cornupalpatum to its host, possibly some type of dinosaur, or were used to scarify mucous membranes in preparation for a blood meal.

So far, all Burmese amber aphids examined have only one pair of functional wings, while all other extant and extinct aphids have two pairs.43 So while most characters orient these creatures with extant aphids, their vestigial hind wings aligns them with scale insects, whose males have rudimentary hind wings called hamulohalters (color plate 3E). Here again, we have chimeras with morphological features of two groups. What function, if any, could these stumpy hind wings serve? Flies (Diptera), which also have only two functional wings, vibrate their minute, residual hind wings (called halters) during flight, and some believe they serve as balancing organs. Perhaps these homologous structures on aphids had the same function or maybe they were just useless, vestigial organs on the way to oblivion, like the human appendix.

Since two specimens of the upside-down hanging flies, Da-cochile, have been recovered, they probably were fairly common at the Burmese site. A debate centers around whether this little fly belongs to the moth fly family or primitive cranefly family.4647 The wing shape and venation are like those of primitive crane-flies but the clear wing membrane and absence of a distinct wing lobe are moth fly characters. However, in toto, they actually have more features in common with the former, so they have been classified with the primitive craneflies (Tanyderidae) (color plates 10A, 10B). Problems arise when we attempt to determine what Dacochile was feeding on in the forest. It has well developed mandibles with serrated edges, indicating they were used for slicing or cutting into something, but what? Perhaps skin, maybe that of a vertebrate—in which case, dinosaurs would have been possible hosts.

We have already spoken about the small "protobee," Melit-tosphex, which contains characters found in both modern bees and hunting wasps (color plate 14B). With a body covered in plumose hairs, rows of hairs on the hind legs, and cleft claws, this hymenopteran had features of many bees. But with a pair of apical spurs on the middle legs, it also resembled wasps. Since this insect did not fit comfortably with extant bees or wasps, a new family had to be erected. Although small, this insect was probably quite efficient pollinating flowers, and was definitely a prototype bee.44

While chimeras have characters that resemble those in two or more modern families, there were some insect fossils that were just plain oddities. As far as can be determined from their remains, most of their features were unique. One example, a strange flea-like creature known as Strashila that roamed the earth some 150 mya,104 could not be placed with certainty in a current insect order (fig. 21). We can imagine that this unusual specimen lived a furtive existence, resting under some debris or buried in the plumage of a bird or even a feathered dinosaur. With such huge, protruding eyes, detecting quarry was easy, and when suddenly extended, the enlarged hind legs would have effortlessly propelled the body high into the air and hopefully onto an unwary victim. After burrowing through the feather layer to the skin, the stout beak was inserted into the vertebrate and blood sucking began. The strange, paired appendages extending from the abdomen probably anchored Strashila firmly in place. This aberrant fossil has been categorized with the fleas, and while possessing some flea-like characters, most of its features do not resemble those of any modern insect, earning the creature an appropriate specific name, incredibilis.

Figure 21. The strange Strashila incredibilis lives up to its specific name by being one of the most enigmatic insects from the Mesozoic period.1" Its large hind legs are adapted for jumping and would have grasped the shafts of feathered dinosaurs. Its proboscis could have been used for taking blood.

Figure 21. The strange Strashila incredibilis lives up to its specific name by being one of the most enigmatic insects from the Mesozoic period.1" Its large hind legs are adapted for jumping and would have grasped the shafts of feathered dinosaurs. Its proboscis could have been used for taking blood.

Questions about the nature of chimeras and oddities, such as their lifestyle, why and when they disappeared, and how should they be classified, will challenge paleobiologists far in the future. They certainly represent some of the most intellectually challenging facets of paleontology.

0 0

Post a comment