Early scientists used a variety of observable characteristics to group organisms into phyla and classes. These characteristics were primarily based on morphological features (physical appearance and structure) and anatomical similarities.

Here are some examples of the observations used:

* External anatomy:

* Body shape: Was the organism spherical, elongated, flat, or segmented?

* Number of limbs: Did the organism have legs, wings, or fins?

* Presence of external features: Did the organism have antennae, scales, feathers, or fur?

* Internal anatomy:

* Skeletal structure: Did the organism have a backbone (vertebrate) or an exoskeleton (invertebrate)?

* Digestive system: What type of digestive system did the organism possess?

* Reproductive system: How did the organism reproduce?

* Physiological characteristics:

* Movement: Did the organism swim, fly, walk, or crawl?

* Feeding habits: Was the organism a herbivore, carnivore, or omnivore?

* Life cycle: What were the stages of the organism's life cycle?

It's important to note that early classifications were primarily based on visible traits, and did not take into account genetic relationships or evolutionary history, which are key factors in modern classification systems.

As scientific knowledge and understanding of evolution progressed, the criteria for grouping organisms became more sophisticated, leading to the development of phylogenetic classification, which considers evolutionary relationships and genetic similarities.