Counting dinosaurs

Since 1990, there has been almost a doubling (85%) in the known number of dinosaur genera. So our understanding of even who dinosaurs were is changing so much, it's well to think about ways to estimate the total number of genera, that is the generic diversity of dinosaurs, that ever existed. In this box, we introduce several of the ways that diversity can be estimated.

Cladistic estimates

Although we have heretofore emphasized the use of clado grams for reconstructing evolutionary relationships, they can also portray the relative sequence of the appearance of organisms. For animals - such as dinosaurs - with a fossil record, this relative sequence from the cladogram can be compared with the real sequence of appearance that comes from the geological record ofthe same dinosaurs. The cladogram-based sequence ought to compare well with the sequence of strati-graphic occurrence of the fossils themselves.

In addition, the combination of phylogeny and stratigraphy has a lot to say about presence and meaning of gaps in the fossil record. That is, even if an ancestor is not preserved, the cladogram allows us to infer when it must have existed. To understand this, we turn to an example.

Suppose Dinosaur X and Dinosaur Y were each other's closest relative; thus they share a unique common ancestor. If Dinosaur X is known from rocks dated at 100 Ma and Dinosaur Y came from 125 Ma rocks, then this ancestor had to be at least 125 million years old (that is, the age of the older of the two dinosaur species). And if this is true, then there must be some not-yet-sampled history between this ancestor and Dinosaur X - to the tune of 25 million years - all because of phylogenetic continuity calibrated through the use of stratigraphy. Such a 25 million year gap can be referred to as a minimal divergence time (MDT); it can be calculated for any two taxa so long as their phylogenetic relationships and stratigraphic occurrences are known and is an estimate ofthe completeness ofthe fossil record. Lineages must have existed that have so far not left us a physical record (through fossils) of their existence; these are called ghost lineages. MDTs are measures of their duration (Figure B13.2.1).

Ceratopsia counted

It has sometimes been claimed that ceratopsians have one of the best fossil records among all dinosaur groups. How can we test this? There are 32 ceratopsian species known, less than are found in theropods, sauropodomorphs, and ornithopods, yet more than in ankylosaurs, stegosaurs, and pachycephalosaurs. Averaged over their total time on Earth, ceratopsians apparently produced new species at the rate of one every 1.9 million years, as compared with a high of one new species per 1.4 million years for sauropodomorphs and a low of one per 5.6 million years for stegosaurs. By this reckoning, ceratopsians had relatively high rates of speciation.

To estimate the total diversity of a group, however, we turn to ghost lineages. For ceratopsians, MDT values range from 0 to nearly 30 million years, with an average ofjust over 5 million years. These are among the smallest MDTs for all producing "greenhouse" climates. Climates from the Early through mid Cretaceous (to about 96 Ma) were therefore warmer and more equable than today (see Chapter 2).

Who enjoyed these balmy conditions? Representatives of groups including all of our old friends from earlier times as well as a number of new groups of dinosaurs made their appearance. The Early Cretaceous marks the rise of the largest representatives of orni-thopods. Ankylosaurs also became a significant presence among herbivores of the Early Cretaceous times, as did the earliest ceratopsians.

Moreover, the balance of the faunas seems to have changed. During the Late Jurassic, sauropods and stegosaurs were the major large herbivores, with ornithopods represented primarily by smaller members of the group. Now, in the Early Cretaceous (and, in fact, throughout the Cretaceous), ornithopods first make their mark. Sauropods and stegosaurs were still present, but the significance of these groups, particularly stegosaurs, seems to have been greatly reduced. Was the spectacular Cretaceous ascendency of ornithopods due to the feeding innovations developed by the group? The parallel success of ceratopsians in Late

Figure B13.2.1. Ghost lineages and MDTs. Dinosaur X and Dinosaur Y are preserved 25 million years apart. If they are closely related, they both share a common ancestor that is at least as old as Dinosaur Y. Thus an estimate of the minimum divergence time (MDT) of the two lineages is as old as, or older than, Dinosaur Y (125 Ma). The record of that divergence is unpreserved and is therefore called a ghost lineage (curved blue lines on the figure).

Figure B13.2.1. Ghost lineages and MDTs. Dinosaur X and Dinosaur Y are preserved 25 million years apart. If they are closely related, they both share a common ancestor that is at least as old as Dinosaur Y. Thus an estimate of the minimum divergence time (MDT) of the two lineages is as old as, or older than, Dinosaur Y (125 Ma). The record of that divergence is unpreserved and is therefore called a ghost lineage (curved blue lines on the figure).

Dinosauria, which suggests that the fossil record of this group is comparatively pretty well represented. Furthermore, actual ceratopsian species counts are nearly 70% of the total after ghost lineages have been added to the diversity total. On these measures, ceratopsians do indeed have one ofthe best records of all ofthe major dinosaur groups.

Other ways

Sophisticated statistical treatments have opened up other ways to count dinosaurs as well. Dodson (1990) used a published compilation, and simply counted. Fastovsky et al. (2004) used an updated version ofthe compilation used by Dodson (1990), and applied a statistical technique called rarefaction to the data. This technique allowed them to compare different sized samples to determine whether the diversity of dinosaurs actually changed through time, or whether just the samples varied because of preservation.

Another interesting statistical approach was applied by Wang and Dodson (2006), who developed a method for estimating the number of fossils for particular groups that have yet to be discovered! Here they introduced a metric called "coverage," which statistically assesses exactly how closely the known diversity from a locality (that is, what has been collected) conforms to its actual diversity (that is, a complete inventory of what theoretically ought to be preserved in the locality). Using the coverage metric, Wang and Dodson were able to take the total number of currently known dinosaur genera - 527 genera as ofthe year 2006 - and estimate the total number of dinosaur genera that ever existed: approximately 1,850 genera.

Cretaceous time, and the independent invention by that group of similar feeding innovations, suggest that sophisticated chewing didn't hurt. Then, too, the Early Cretaceous witnessed a revolution in carnivorous theropods, most notably, the deinonychosaurs of both North America and Asia.

During the Late Cretaceous (100-65.5 Ma; see Figure 2.7), never before in their history had Dinosauria been so diverse, so numerous, and so incredible. The Late Cretaceous boasted the beefiest terrestrial carnivores in the history of the world (tyrannosaurids), a host of sickle-clawed brainy (and brawny) killers worthy of any nightmare, and herds of horned herbivores, honking hadrosaurids, ankylosaurs, and dome-heads.

Climate seems not to have affected diversity. In fact, although diversity increased, from the mid-Cretaceous time onward, seasonality gently increased. This occurred at the same time as a marine regression, which undoubtedly played a role in the destabilization of climate. At the very end of the Mesozoic, there is no evidence for a sudden drop in temperatures, or any significant modification of climate.

Endemism. Southern continents tended to maintain the veteran Old Guard: a large variety of sauropods, some ornithopods and ankylosaurs, and, in South America, the abelisaurid theropods. In northern continents, however, new, very different faunas appeared. Among herbivores, sauropods were still present, although rare at higher latitudes. Stegosaurs took their final curtain call in the Late Cretaceous (these dinosaurs were already extinct globally with the exception of one equivocal record in India). But in their place lots of new creatures roamed, including potentially migrating herds of pachycephalosaurs, ceratopsids, and hadrosaurids.

Finally there is the magnificent diversity of Late Cretaceous theropods. Nothing shaped quite like tyrannosaurids had ever been seen, or has existed since. Yet, the Late Cretaceous theropod story might be better told in the diversity of smaller forms: oviraptorosaurs, alvar-ezsaurids, dromaeosaurids, troodontids, and therizinosaurs.

Across the Bering Straits? North America and Asia share a rich Late Cretaceous record, including ceratopsians, tyrannosaurids, and ornithomimosaurs. Because of this, there may have been multiple migrations of herds of dinosaurs across a Bering land bridge throughout much of the Late Cretaceous (see Figure 6.31), just as humans are thought to have migrated to North America from Asia many tens of millions of years later.

Out with a whimper or a bang?

And then, in the earliest Cenozoic, it was over. Just like that. While one of the great enigmas of dinosaur paleontology has historically been how the animals went extinct, the problem has begun to yield to concentrated study over the past 30 years. We'll save that story for Chapter 15.

After the ball is over. With the end of the Cretaceous, non-avian dinosaurs disappeared from Earth forever, and it definitely was the end of an Era. Mammals, well entrenched as the dominant terrestrial vertebrates in the Cenozoic, would be no more likely to give up their place in Tertiary ecosystems to dinosaurs than dinosaurs had been likely throughout the 163 million years of their incumbancy to give up their place to mammals!

Was this article helpful?

0 0

Post a comment